Protecting Manufactured 

Homes from Floods and 

Other Hazards

A Multi-Hazard Foundation and Installation Guide

FEMA P-85, Second Edition / November 2009 

FEMA

Protecting Manufactured 

Homes from Floods and 

Other Hazards

A Multi-Hazard Foundation and Installation Guide

FEMA P-85, Second Edition / November 2009

FEMA

PROTECTING Manufactured Homes FROM FloodS AND OTHER HazardS

A Multi-Hazard Foundation and Installation Guide

i

Preface

The Federal Emergency Management Agency (FEMA) first published Manufactured Home Installation in Flood Hazard Areas (FEMA 85) in 1985. Since then, manufactured homes have become 

better built, and natural hazards like flood, wind, and earthquake (seismic) events are better 

understood. 

To benefit from the advances made in the last 24 years, FEMA 85 has been updated to 

reflect the requirements of the most current codes and standards and to provide a best 

practices approach in reducing damages from natural hazards. While the original version 

of FEMA 85 concentrated on flood and wind events, this version also addresses seismic 

hazards and recommends several multi-hazard resistant foundation designs. Designs are 

included for wood-framed foundations, conventional concrete and masonry pier foundations, and ground anchors. The ground anchor foundations are based on results from a 

series of first-of-its-kind saturated and dry soil anchor tests. The anchor tests were conducted with the support of the U.S. Department of Housing and Urban Development (HUD), 

the Manufactured Housing Institute (MHI), the Systems Building Research Alliance (SBRA, 

formerly the Manufactured Housing Research Alliance [MHRA]), and several ground anchor manufacturers. A detailed example showing step-by-step procedures on how to design 

a foundation for a manufactured home is also included.

This guidance is also valuable to designers of alternate foundations allowed by the HUD 

24 CFR 3285 Model Manufactured Home Installation Standards, especially for homes located in 

Special Flood Hazard Areas (SFHAs) for which certain 24 CFR 3285 foundation designs are 

not applicable (24 CFR 3285.303, Table 1, Note 4, et al.).

The foundation designs discussed in Chapter 10 and shown in Appendix H of this guide are 

but one group of acceptable foundation solutions. They should not be considered mandatory or all inclusive. Alternative foundation systems, designed to resist equivalent loads and 

provide equivalent performance, should be considered equally acceptable.

Limitations of the Guide

This manual has been prepared to assist in protecting manufactured homes from floods and other 

hazards. Builders, installers, architects, and engineers using this guide assume responsibility for the 

resulting designs and the performance during a natural hazard event.

The foundation designs and analyses presented in the guide are based on load combinations contained in the American Society of Civil Engineers (ASCE 7-05) and the 2006 version of the International 

Residential Code® (IRC®).

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Acknowledgments

The Federal Emergency Management Agency would like to acknowledge the significant contributions made by the following individuals in developing the Second Edition of this publication.

John Ingargiola 

FEMA Building Science Branch

Edward Laatsch

FEMA Building Science Branch

Marcus Barnes

FEMA Building Science Branch 

Lois Forster

FEMA Floodplain Management Branch

Brad Loar

FEMA Region IV

Mike Mahoney

FEMA Building Science Branch

Cliff Oliver

FEMA Acquisition Branch 

John Plisich

FEMA Region IV

Mike Robinson

FEMA Floodplain Management Branch

Paul Rooney

FEMA Data and Dissemination 

Management Section 

Juanita Thompson

FEMA Floodplain Management Branch

Phil Bergeldt

Florida Department of Highway Safety and 

Motor Vehicles, Mobile Home and Recreational Vehicle Construction

Mike Blanford

HUD Office of Policy Development and 

Research

Jason McJury

HUD Office of Manufactured Housing 

Programs

Rick Mendlen

HUD Office of Manufactured Housing 

Programs

Kelly Cobeen

Cobeen & Associates

Bill Coulbourne

Applied Technology Council

Deb Daly

Greenhorne & O’Mara

Bill Farish

Clayton Homes

Jeff Inks

Manufactured Housing Institute

Joseph Klein

Dewberry

Emmanuel Levy

Systems Building Research Alliance (formerly 

Manufactured Housing Research Alliance)

Julie Liptak

Greenhorne & O’Mara
iv PROTECTING MaNufaCTuREd HOMEs fROM flOOds aNd OTHER HazaRds 

A Multi-Hazard Foundation and Installation Guide

ACKNOWLEDGMENTS

Thayer Long

Manufactured Housing Institute

Dave Low

DK Low and Associates

Bonnie Manley

American Iron and Steel Institute

Therese McAllister

National Institute of Standards and 

Technology

Ken Morris

Oklahoma Water Resources Board

Mark Nunn

Manufactured Housing Institute

George Porter

Manufactured Housing Resources

Jim Rossberg

American Society of Civil Engineers

Chuck Sanders

former Alabama State NFIP Coordinator

Adrienne Sheldon

URS

John Squerciati

Dewberry

Bill Turney

Florida Manufactured Housing Association

Frank Walter

Manufactured Housing Institute

Mark Weiss

Manufactured Housing Association for 

Regulatory Reform

Jimmy Yeung

Greenhorne & O’Mara

Naomi Chang Zajic

Greenhorne & O’Mara

Brian Zelenko

URS
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Table of Contents

Preface...........................................................................................................................................i

Acknowledgments.................................................................................................................. iii

1 Introduction.................................................................................................................. 1-1

1.1 Purpose and Scope of the Guide............................................................................................ 1-1

1.2 Background.........................................................................................................................1-3

1.2.1 Manufactured Homes in the United States...........................................................1-3

1.2.2 National Flood Insurance Program.......................................................................1-5

1.2.3 Performance of Manufactured Homes in Wind and Flood Events.....................1-6

1.2.3.1 Performance of Pre-1994 Manufactured Homes.....................................1-6

1.2.3.2 Performance of Post-1994 Manufactured Homes....................................1-7

1.2.3.3 Performance of Manufactured Homes During Hurricane Charley

(2004) in Florida.........................................................................................1-8

2 Manufactured Homes............................................................................................... 2-1

2.1 Manufactured Home Characteristics...................................................................................... 2-1

2.1.1 Chassis Support System........................................................................................... 2-1

2.1.2 Integrated Support System.....................................................................................2-2

2.1.3 Envelope Construction...........................................................................................2-2

2.1.4 Double Section........................................................................................................2-3

2.2 Types of Foundation Systems................................................................................................2-3

2.2.1 Typical Foundation Systems...................................................................................2-5

2.2.1.1 Typical Installation.....................................................................................2-5

2.2.1.2 Piers and Ground Anchors........................................................................2-6

2.2.1.3 Perimeter Wall Foundations......................................................................2-7

2.2.2 Proprietary Systems.................................................................................................2-7

2.3 Utilities and Mechanical Equipment.......................................................................................2-8

2.3.1 Utilities Placement..................................................................................................2-8
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2.3.2 Mechanical Access...................................................................................................2-9

2.4 Attachments – Carports, Decks, Porches, and Awnings..........................................................2-10

3 Regulatory Requirements........................................................................................ 3-1

3.1 Introduction to the NFIP........................................................................................................ 3-1

3.2 Identifying and Mapping Flood Hazards..................................................................................3-2

3.3 The NFIP's Community Rating System (CRS)...........................................................................3-8

3.4 NFIP Definitions Related to Manufactured Homes...................................................................3-8

3.5 General NFIP Floodplain Management Requirements for Manufactured Homes.........................3-9

3.6 NFIP Requirements for Manufactured Homes in Riverine and Inland Flood Zones....................3-10

3.6.1 Approximate A Zones........................................................................................... 3-11

3.6.2 Elevation in A, A1-30, AE, and AH Zones............................................................3-13

3.6.3 3-Foot Pier Foundation.........................................................................................3-14

3.6.4 Elevation in Zone AO...........................................................................................3-16

3.6.5 Anchoring..............................................................................................................3-17

3.6.6 Flood Damage-Resistant Materials.......................................................................3-18

3.6.7 Utilities and Mechanical Equipment...................................................................3-18

3.6.8 Enclosed Areas......................................................................................................3-19

3.6.9 Floodways...............................................................................................................3-20

3.7 NFIP Requirements for Manufactured Homes in Coastal Flood Areas......................................3-22

3.7.1 Elevation and Anchoring......................................................................................3-23

3.7.2 Fill..........................................................................................................................3-25

3.7.3 Enclosed Areas and Breakaway Walls...................................................................3-25

3.7.4 Setbacks.................................................................................................................3-26

3.8 Existing Manufactured Homes in Flood Hazard Areas............................................................3-27

3.8.1 Relocation..............................................................................................................3-27

3.8.2 Evacuation.............................................................................................................3-28

3.8.2.1 Manufactured Home Substantially Improved or Returned to a

Different Site or Pad in an Existing Manufactured Home Park............3-29

3.8.2.2 Manufactured Home Placed in a New Manufactured Home Park or

Subdivision................................................................................................3-30

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3.9 HUD Manufactured Home Construction and Safety Standards................................................3-30

3.10 HUD Model Manufactured Home Installation Standards.........................................................3-32

3.11 Model Building Code Requirements.....................................................................................3-34

3.11.1 IRC 2006................................................................................................................3-34

3.11.2 NFPA 5000.............................................................................................................3-35

3.11.3 NFPA 501...............................................................................................................3-36

3.11.4 NFPA 225...............................................................................................................3-36

4 Site and Development Options.............................................................................. 4-1

4.1 Step 1: Compiling Site Information.........................................................................................4-2

4.2 Step 2: Reviewing Basic Siting Information............................................................................4-4

4.3 Step 3: Hazard Analysis and Risk Assessment........................................................................4-4

4.3.1 Flooding...................................................................................................................4-5

4.3.1.1 Accessibility.................................................................................................4-6

4.3.2 Other Hazards.........................................................................................................4-8

4.4 Step 4: Protecting Properties In and Near Hazard-Prone Areas...............................................4-10

4.4.1 Placement Options...............................................................................................4-10

4.4.1.1 Flood-Prone Areas....................................................................................4-10

4.4.1.2 Areas Subject to Landslides.....................................................................4-12

4.4.2 Design and Construction Techniques.................................................................4-12

4.4.2.1 Flood-Prone Areas....................................................................................4-12

4.4.2.2 Dam Failure Inundation Areas................................................................4-12

4.4.2.3 Areas Subject to Landslides.....................................................................4-13

4.4.2.4 Areas Subject to Seismic Events...............................................................4-13

4.4.2.5 Areas Subject to Wind/Debris Hazards..................................................4-13

4.5 Step 5: Deciding on Property Development: Proceed or Reject...............................................4-13

5 Natural Hazards – Design Considerations........................................................ 5-1

5.1 Flood Data........................................................................................................................... 5-1

5.2 Flood Characteristics............................................................................................................5-5

5.2.1 Frequency, Duration, and Rate of Rise..................................................................5-5

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5.2.2 Flood Elevation and Depth....................................................................................5-7

5.2.3 Hydrostatic (Buoyancy) Forces..............................................................................5-8

5.2.4 Hydrodynamic Forces.............................................................................................5-9

5.2.5 Erosion and Scour.................................................................................................5-12

5.2.6 Debris Impact Forces............................................................................................5-13

5.3 Wind .................................................................................................................................5-14

5.3.1 Wind Forces on Structures...................................................................................5-15

5.3.2 Wind Forces in Combination with Flood Forces................................................5-15

5.4 Earthquakes.......................................................................................................................5-16

5.4.1 Design Philosophy.................................................................................................5-16

5.4.2 Design Standard....................................................................................................5-16

5.5 Evaluation of Multi-Hazards................................................................................................5-17

5.5.1 Load Combinations (ASCE 7).............................................................................5-18

5.5.1.1 Strength Design (Load and Resistance Factor Design).........................5-18

5.5.1.2 Allowable Stress Design (also known as Working Stress Design)..........5-19

6 Soils................................................................................................................................. 6-1

6.1 Bearing Capacity.................................................................................................................. 6-1

6.2 Effects of Flood Duration and Frequency on Soil.....................................................................6-2

6.3 Soil Liquefaction..................................................................................................................6-2

6.4 Recommended Soil Testing and Criteria for Manufactured Home Installations...........................6-3

7 Ground Anchors........................................................................................................... 7-1

7.1 Types of Anchors and Installed Configurations........................................................................7-2

7.1.1 Types of Anchors.....................................................................................................7-2

7.1.1.1 Helical Earth Anchors................................................................................7-2

7.1.1.2 Concrete Anchors.......................................................................................7-2

7.1.1.3 Cross Drive Anchors...................................................................................7-2

7.1.2 Anchor Construction and Capacity.......................................................................7-3

7.1.3 Anchor Selection.....................................................................................................7-3

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7.1.4 Anchor Installation.................................................................................................7-4

7.1.5 Anchor Performance..............................................................................................7-5

7.1.6 Anchors and Other Foundation Elements............................................................7-8

7.2 FEMA Anchor Test Program...................................................................................................7-8

7.2.1 Anchors in Saturated Soils.....................................................................................7-8

7.2.2 Anchor Test Results.................................................................................................7-9

7.3 Recommended Ground Anchor Certification, Performance, and Design Values......................... 7-11

7.3.1 Recommended Ground Anchor Certification Performance............................. 7-11

7.3.2 Recommended Ground Anchor Design Values.................................................. 7-11

7.4 Ground Anchors in Seismically Active Areas.........................................................................7-13

8 Foundation Systems.................................................................................................. 8-1

8.1 Introduction......................................................................................................................... 8-1

8.2 Enclosed Foundations...........................................................................................................8-2

8.3 Open Foundations and Breakaway Walls................................................................................8-3

8.3.1 Pier Systems.............................................................................................................8-3

8.3.1.1 Reinforced Pier Systems.............................................................................8-4

8.3.1.2 Unreinforced Pier Systems........................................................................8-5

8.3.2 Pile Foundations.....................................................................................................8-7

8.4 Bracing................................................................................................................................8-9

8.5 Footings............................................................................................................................8-10

8.6 Foundation Materials Selection........................................................................................... 8-11

8.6.1 Wood Foundations................................................................................................8-12

8.6.2 Concrete Foundations..........................................................................................8-12

8.6.3 Steel Foundations.................................................................................................8-13

8.6.4 Masonry Foundations...........................................................................................8-13

8.7 Foundation Selection and Flood Resistance..........................................................................8-13

8.7.1 Flooding Types......................................................................................................8-14

8.7.2 Flood Characteristics............................................................................................8-14

8.7.3 Flood Hazard Zones..............................................................................................8-15

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8.7.4 Proximity to Flood Source....................................................................................8-15

8.7.5 Foundation Selection Guidance..........................................................................8-16

9 Recommended Design Process and Criteria for Manufactured 

Home Foundations in SFHAs.................................................................................. 9-1

9.1 Performance Criteria............................................................................................................ 9-1

9.2 Design Criteria..................................................................................................................... 9-1

9.3 Design Process....................................................................................................................9-2

9.3.1 Step 1: Determine Design Criteria.........................................................................9-3

9.3.2 Step 2: Select a Design Methodology and Assess Load Combinations and

Failure Modes..........................................................................................................9-4

9.3.2.1 Design Methodology..................................................................................9-4

9.3.2.2 Load Combinations....................................................................................9-4

9.3.2.3 Primary Failure Modes...............................................................................9-4

9.3.3 Step 3: Select Foundation Type and Material.......................................................9-5

9.3.4 Step 4: Determine Forces at Connections and on Foundation Components.....9-6

9.3.5 Step 5: Specify Connections and Framing Methods Along with Component

Dimensions to Satisfy Load Conditions.................................................................9-6

9.3.6 Step 6: Note All Design Assumptions and Details on Drawings...........................9-6

10 Recommended Foundations................................................................................. 10-1

10.1 Design Criteria for Recommended Foundations.................................................................... 10-1

10.1.1 Reinforced Masonry Perimeter Foundation Walls..............................................10-2

10.1.2 Wood Framed Perimeter Foundation Walls........................................................10-3

10.1.3 Braced Masonry Pier Designs...............................................................................10-3

10.1.4 Wood H-Frame Designs........................................................................................10-4

10.1.5 Ground Anchor Designs.......................................................................................10-4

10.2 Summary of Recommended Foundations.............................................................................10-5

10.3 Floodwater Velocity Design Considerations for Pier Foundations............................................10-6

10.4 Recommended Foundation Designs for Seismic Areas..........................................................10-7

10.4.1 Concrete Masonry Pier Foundation Designs.......................................................10-7

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10.4.2 Masonry Wall Foundation Designs......................................................................10-7

10.4.3 Wood Framed Foundation Designs.....................................................................10-8

10.5 Design Drawings................................................................................................................10-8

Appendices

Appendix A: References

Appendix B: Sources for Flood Information

Appendix C: Flood Velocity Determination

Appendix D: Definitions

Appendix E: Acronyms and Abbreviations

Appendix F: Example Calculations

Appendix G: Wind Zone Comparisons (HUD's MHCSS and ASCE 7-05)

Appendix H: Pre-Engineered and Prescriptive Foundation Designs

Figures

Chapter 1

Figure 1-1. Basic wind zone map for manufactured housing...................................................1-4

Figure 1-2. Inadequate turnbuckle anchor installed by the homeowner on this pre-1994

manufactured home, coupled with lack of elevation and an unreinforced

foundation system, led to severe damage...............................................................1-7

Figure 1-3. The addition to this manufactured home was destroyed, causing considerable

damage to the rest of the home..............................................................................1-7

Figure 1-4. Reinforced masonry pier foundation system under a manufactured home

installed after 1994 that performed well................................................................1-8

Figure 1-5. Manufactured home in Cudjoe Key, Florida, built and installed after 1994,

survived Hurricane Georges with only minor damage caused by the loss of an

awning. ....................................................................................................................1-8

Chapter 2

Figure 2-1. Traditional chassis system........................................................................................2-2

Figure 2-2. Integrated floor system consisting of steel-reinforced perimeter framing...........2-2

Figure 2-3. Main construction features of a typical manufactured home. .............................2-4

Figure 2-4. One section of a double-section manufactured home being transported. ..........2-5

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Figure 2-5. Typical installation of a manufactured home on masonry block piers

with tie-down straps. ...............................................................................................2-6

Figure 2-6. Typical installation of a double-section manufactured home on a pier and

ground anchor foundation system.........................................................................2-6

Figure 2-7. Perimeter wall foundation under a manufactured home.....................................2-7

Figure 2-8. Utility line damage in unstable soils.......................................................................2-9

Chapter 3

Figure 3-1. Sample DFIRM.........................................................................................................3-4

Figure 3-2(a). Sample FIS Summary of Discharges table...........................................................3-5

Figure 3-2(b). Sample FIS Floodway Data..................................................................................3-5

Figure 3-2(c). Sample FIS Flood Profile.....................................................................................3-6

Figure 3-3. Manufactured home with lowest floor elevated to the BFE................................3-14

Figure 3-4. Manufactured home on reinforced pier foundation 36 inches high.................3-16

Figure 3-5. DFIRM showing the floodway (cross-hatched area in Zone AE)........................ 3-21

Figure 3-6. Manufactured home with the bottom of the lowest horizontal structural

member elevated to the BFE. ...............................................................................3-24

Figure 3-7. Mean high tide line development restriction......................................................3-27

Figure 3-8. Coastal development well-suited to the land: generous setbacks, in combination

with deep lots and avoidance of dune areas, should afford protection from

erosion and flooding for years to come...............................................................3-27

Figure 3-9. Certification plate for manufactured homes built after June 15, 1976............... 3-31

Chapter 4

Figure 4-1. Portions of an FIS and a FIRM................................................................................4-7

Figure 4-2. Floodplain/floodway schematic............................................................................ 4-11

Chapter 5

Figure 5-1. Example of a FIRM..................................................................................................5-2

Figure 5-2. FIS Summary of Discharges table...........................................................................5-3

Figure 5-3. FIS Stream Flood Profile.........................................................................................5-4

Figure 5-4. FIS Floodway Data table..........................................................................................5-4

Figure 5-5. Buoyancy forces acting on a structure....................................................................5-9

Figure 5-6. Failure due to sliding.............................................................................................5-10

Figure 5-7. A manufactured home destroyed by the hydrodynamic forces of flooding.......5-10

Figure 5-8. Failure of a modular home due to high winds. The home lifted off of its

foundation (concrete slab) when the connections failed...................................5-14

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Figure 5-9. A manufactured home that failed during a high-wind event..............................5-15

Chapter 7

Figure 7-1. Cross drive anchor...................................................................................................7-2

Figure 7-2. Single and double helix ground anchors with strap connection and single

helix anchor with a closed-eye connection............................................................7-3

Figure 7-3. Electric anchor drive machine................................................................................7-4

Figure 7-4. Typical ground anchor installation.........................................................................7-5

Figure 7-5. In line ground anchor installation.........................................................................7-6

Figure 7-6. Typical response for an axially loaded anchor.......................................................7-7

Figure 7-7. Typical response for a non-axially loaded anchor used with a stabilizer plate.....7-8

Figure 7-8. Failed foundation system that used masonry piers................................................7-9

Figure 7-9. Three graphics showing the anchor configurations described in Tables 7-2

and 7-3. .................................................................................................................. 7-11

Chapter 8

Figure 8-1. A manufactured home elevated on a perimeter foundation wall.........................8-2

Figure 8-2. Reinforced masonry and concrete piers................................................................8-5

Figure 8-3. Bolted connection between frame and reinforced pier........................................8-6

Figure 8-4. Manufactured home on a pile foundation. ...........................................................8-7

Figure 8-5. Pile driving methods................................................................................................8-8

Figure 8-6. Diagonal bracing...................................................................................................8-10

Figure 8-7. Knee bracing..........................................................................................................8-10

Chapter 9

Figure 9-1. A home that was partially submerged and displaced from its foundation by

hydrostatic forces. ...................................................................................................9-5

Tables

Chapter 3

Table 3-1. Common SFHA Designations for Riverine or Inland Flood Zones.......................3-6

Table 3-2. Common SFHA Designations for Coastal Flood Areas..........................................3-6

Chapter 4

Table 4-1. Information Checklist..............................................................................................4-2

Table 4-2. Partial Listing of Local, Regional, and State Responsibilities................................4-4

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Table 4-3. Flood Hazards...........................................................................................................4-5

Table 4-4. Hazard-Prone Areas.................................................................................................4-9

Chapter 5

Table 5-1. Sources for Information About Past Flood Events.................................................5-6

Table 5-2. Load Combination Nomenclature (ASCE 7) ......................................................5-17

Chapter 7

Table 7-1. Soil Classifications Using a Standard Torque Probe (STP) ..................................7-4

Table 7-2. Dry Site Anchor Data.............................................................................................7-10

Table 7-3. Wet Site Anchor Data.............................................................................................7-10

Table 7-4. Recommended Design Loads – from FEMA Ground Anchor Testing

Program..................................................................................................................7-12

Table 7-5. Recommended Design Stiffness for Tested Anchors............................................7-12

Chapter 8

Table 8-1. Recommended Manufactured Home Foundation Selection for Lake/Pond

Flooding (for very low velocity less than 1 fps).....................................................8-16

Table 8-2. Recommended Manufactured Home Foundation Selection for Riverine Flood

Zones (and maximum flood flow velocity)...........................................................8-17

Table 8-3. Recommended Manufactured Home Foundation Selection for Coastal Flood

Zones.......................................................................................................................8-18

Chapter 9

Table 9-1. Design Standards and Publishers for Building Materials.......................................9-6

Chapter 10

Table 10-1. Summary of Recommended Foundations............................................................10-5

Table 10-2. Design Flood Flow Velocity for Concrete Masonry Unit Foundations................10-6

Table 10-3. Foundation Drawings.............................................................................................10-8

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1	 Introduction

This revised edition of FEMA 85 provides new recommendations for manufactured home foundation design and installation. The initial 1985 edition of FEMA 85 provided guidance on 

installation methods designed to make manufactured homes less susceptible to damage caused 

by flood and wind events. Significant advances in design and construction technologies, mitigation strategies, and regulatory requirements since 1985 have all contributed to the need for 

updated guidance. 

1.1	 Purpose and Scope of the Guide

Like the original guide, this updated version of FEMA 85 focuses primarily on the installation 

and foundation requirements for manufactured homes in floodplains for communities participating in the National Flood Insurance Program (NFIP). However, it also addresses other 

natural hazards such as high winds and earthquakes. 

The purpose of the second edition of FEMA 85 is to provide guidance for the design and construction of alternative foundation systems as described in the HUD Model Manufactured Home 

Installation Standards (24 CFR 3285). It is important to recognize that both the U.S. Department of Housing and 

Urban Development (HUD) and the NFIP require that 

foundation systems for manufactured homes located in 

Special Flood Hazard Areas (SFHAs) prevent flotation, 

collapse, or lateral movement of the structure (24 CFR 

3285.302 and 44 CFR 60.39(a)(3), respectively). 

With its broad and comprehensive scope, this document provides guidance on siting and installing manufactured homes in areas exposed to natural hazards. The guide is organized into 10 

chapters, each covering a different aspect of manufactured homes in the United States, and 8 

appendices.

Chapter 1 provides a historical overview of Federal, State, and local regulations that affect the 

design, construction, and installation of manufactured homes, including guidance on the NFIP 

and HUD installation requirements.

Chapter 2 presents and defines the characteristics of a manufactured home, the types of foundations used, typical installation techniques, and additional design considerations of attachments 

to manufactured homes (e.g., carports, decks, porches, and awnings).

Chapter 3 provides the regulatory requirements pertaining to the installation of manufactured 

homes in flood-prone areas. NFIP, HUD and model building code requirements are discussed 

as well as methods for mitigating manufactured homes (e.g., elevation and relocation). 

The manufactured housing industry 

has adopted the term “community” 

for manufactured home developments. However, the term “park” is 

used in the NFIP and is also used 

in this publication.
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1 INTRODUCTION

Chapter 4 presents issues to consider in the siting of manufactured homes. 

Chapter 5 provides a review of natural hazards that must be considered in site selection, foundation design, and installation of manufactured homes. The discussion covers flooding, including 

the special hazards associated with coastal flooding (e.g., storm surge, velocity flow, and wave 

impact), high winds, and seismic events. The combined effects of multiple hazards are also covered.

Chapter 6 contains a review of the geology and hydrology of soils and their effects on manufactured home foundation systems. Soil characteristics and the behavior of saturated soils are 

discussed. Recommendations are provided for soil testing criteria applicable for manufactured 

home installation.

Chapter 7 presents a review of the use of ground anchors with manufactured home foundations, 

including the results of laboratory and field tests of anchor performance in saturated soils.

Chapter 8 presents different types of manufactured home foundation systems, including their 

performance, installation procedures, maintenance requirements, and possible modifications. 

Systems discussed include pier systems that incorporate ground anchors, braced piers and 

piles, slabs, elevated floors or crawlspaces, and proprietary systems.

Chapter 9 discusses recommended design processes and criteria for manufactured home 

foundations in SFHAs. Recommended design criteria, performance requirements, and best 

practice recommendations are presented. Sites that fall outside of the criteria specified for 

recommended foundations in Chapter 10 can use the design process detailed in this chapter.

Chapter 10 provides design criteria for recommended foundations appropriate for typical installation of manufactured homes in flood zones designated A, AE, A1-A30, A0, or AH on a 

FEMA Flood Insurance Rate Map (FIRM). Criteria are presented regarding maximum flood 

depth, flow velocity, wind speed, and seismic force. The designs are shown in Appendix H.

The appendices include:

A: References 

B: Sources for Flood Information 

C: Flood Velocity Determination

D: Definitions 

E: Acronyms and Abbreviations 

F: Example Calculations

G: Wind Zone Comparisons 

H: Pre-Engineered and Prescriptive Foundation Designs

The foundation designs discussed in Chapter 10 and shown in Appendix H are but one of a 

group of acceptable foundation solutions. They should not be considered mandatory or all 
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INTRODUCTION 1

inclusive. Any modifications to the foundation drawings must be designed and approved by a licensed professional engineer. Alternative foundation systems designed to resist equivalent loads 

and to provide equivalent performance should be considered equally acceptable. 

Flowcharts, checklists, maps, formulas, and drawings are provided throughout the guide to help 

in understanding the issues to consider when installing a manufactured home in a floodplain. 

Examples are presented to demonstrate decisions and calculations designers must make to reduce the potential damage to manufactured homes from natural hazard events. 

1.2	 Background

1.2.1	 Manufactured Homes in the United States

Manufactured homes help fill a demand for affordable housing in many parts of the United 

States. In 2007, the U.S. Census reported that the industry shipped 95,700 homes with an average price of $64,500. Single-section homes had an average price of $35,200, and double-section 

homes had an average price of $73,100. U.S. Census Bureau figures from Census 2007 show that 

74 percent of new manufactured homes were located on private properties and 26 percent were 

located in manufactured home parks. The average floor area of a manufactured home placed 

in 2007 was approximately 1,600 square feet. Approximately 31 percent of the manufactured 

homes placed in 2007 were located in four States (Florida, California, Louisiana, and Texas). 

Since 1976, the NFIP has regulated the installation of manufactured homes in floodplains. Over 

the years, the NFIP has strengthened the regulations by defining existing and new manufactured 

home parks and applying differing standards for each. The standards governing manufactured 

homes continue to improve; Federal, State, and local governments and the manufactured home 

industry strive to institute construction practices and regulations to increase the safety of manufactured homes in natural hazard environments. The following list summarizes regulations, 

programs, and actions that have been developed to improve the resistance of manufactured 

homes to natural hazards:

n	 On July 13, 1994, HUD adopted new structural resistance guidelines for the construction of 

manufactured homes to be placed in HUD Wind Zones II and III (Figure 1-1).

n Section 605 of the National Manufactured Housing 

Construction and Safety Standards Act of 1974 (42 

U.S.C. 5401), as amended by the Manufactured 

Housing Improvement Act of 2000, authorized the 

Secretary of HUD to establish and implement a 

national manufactured housing installation program 

to include (1) installation standards, (2) the training 

and licensing of manufactured home installers, 

and (3) the inspection of manufactured home 

installations. 

States may choose to operate an 

installation program for manufactured homes in lieu of the Federal 

program. The State must implement 

standards that provide protection 

to its residents that equals or exceeds the Model Manufactured 

Home Installation Standards (24 

CFR 3285.1(a)(1). See http: / /

www.hud.gov/offices/adm/hudclips for more information.
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Figure 1-1. Basic wind zone map for manufactured housing.
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n Installation program requirements have been established under 24 CFR 3286, Federal 

Manufactured Home Installation Program effective October 20, 2008. Compliant training and 

licensing programs for installation and inspections are being implemented in all States. 

n The National Fire Protection Association (NFPA) has published three documents on the 

subject of manufactured housing:

n NFPA 501, Standard on Manufactured Housing

n NFPA 501A, Standard for Fire Safety Criteria for Manufactured Home Installations, Sites and 

Communities

n NFPA 225, Model Manufactured Home Installation Standard, which will, in adopted areas, 

govern installation of manufactured homes.

n The National Technology Transfer Act of 1995 included a goal that Federal agencies 

use technical standards developed or adopted by voluntary consensus standard bodies. 

Although HUD has recognized NFPA as a consensus standard developing body, HUD is not 

obligated to use standards developed by that body.

n Several States and localities, including Florida and North Carolina, have strong installation 

standards, which include requiring manufactured homes to meet State and local building 

code requirements; a manufactured home installer education, testing, and certification 

program for HUD homes; and aggressive inspection programs to ensure proper 

installation.

1.2.2 	 National Flood Insurance Program

The U.S. Congress established the NFIP with the passage of the National Flood Insurance Act 

of 1968, as amended. The NFIP is a Federal program enabling property owners in participating 

communities to purchase insurance as a protection against flood losses in exchange for State 

and community floodplain management regulations that reduce future flood damages. Participation by communities in the NFIP is based on an agreement between communities and the 

Federal Government. If a community adopts and enforces floodplain management regulations 

to reduce future flood risks to new construction in floodplains, the Federal Government will 

make flood insurance available within the community as a financial protection against flood 

losses. Participation in the NFIP by communities is voluntary.

Federal flood insurance is designed to provide an alternative to disaster assistance and disaster 

loans for home and business owners. Disaster assistance rarely comes close to covering all of 

the costs to repair and cleanup. While available to qualified victims, disaster loans do not significantly ease the financial burden due to repayment terms. It is important to remember that 

disaster assistance is available only after floods have been declared major disasters by the President of the United States. In contrast, insurance claims will be paid any time damage from a 

qualifying flood event occurs. 

Another important objective of the NFIP is to break the cycle of flood damage. Many buildings 

have been flooded, repaired, or rebuilt, and flooded again. In some parts of the country, this 

cycle occurs every couple of years; people rebuilt in the same flood-prone areas and used the 
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same construction techniques that did not adequately protect the structure. By encouraging 

communities to guide development to lower risk areas, and by requiring the elevation of new 

buildings and older nonconforming buildings that are subject to flood damage, one of the longterm objectives of the NFIP can be achieved: reducing flood damage and losses. Older buildings 

may be removed or replaced, or they may be upgraded or modified with techniques that lead to 

little or no future flood damage.

The NFIP is administered by the Federal Emergency Management Agency (FEMA), which is 

part of the Department of Homeland Security (DHS). 

In order to participate in the NFIP, communities must adopt minimum floodplain management 

requirements that meet or exceed the minimum requirements of the NFIP. Communities can 

adopt NFIP floodplain management requirements through building codes, zoning ordinances, 

subdivision regulations, health and safety codes, and stand-alone floodplain ordinances. 

NFIP regulations have specific floodplain management requirements for manufactured homes 

located in a SFHA. In general, manufactured homes must be elevated and anchored to resist 

flotation, collapse, or lateral movement. At sites having a base flood elevation (BFE) identified 

for Zones A1-30, AH, or AE on the community’s FIRM, NFIP regulations generally require that 

manufactured homes be elevated on a permanent foundation such that the lowest floor is at or 

above the BFE, and be securely anchored to an adequately anchored foundation system to resist flotation, collapse, or lateral movement. At sites having a BFE identified for Zone VE on the 

community’s FIRM, NFIP regulations require that manufactured homes be elevated on pilings 

and columns so that the bottom of the lowest horizontal structural member of the lowest floor 

is elevated to or above the BFE. More detailed information pertaining to the NFIP requirements 

for manufactured homes installed in flood hazard areas is provided in Chapter 3.

1.2.3	 Performance of Manufactured Homes in Wind and Flood Events

1.2.3.1 	Performance of Pre-1994 Manufactured Homes

In 1992, Hurricane Andrew struck Dade County, Florida, destroying 97 percent of the manufactured homes in its path. In 1994, in response to the devastating damage, HUD adopted more 

stringent wind design criteria for manufactured homes installed in HUD Wind Zones II and III. 

Provisions for doors and windows more resistant to wind pressures were required. Although numerous hurricanes have made landfall in the U.S. since 1994, none have produced winds that 

approached those of Hurricane Andrew. In 1998, Hurricane Georges caused damage throughout Monroe County (the Florida Keys), Florida. FEMA dispatched a Building Performance 

Assessment Team (BPAT) to report on the performance of manufactured housing in impacted 

areas. Most of the damage observed occurred to homes installed before Monroe County had adopted the NFIP regulations that required new and substantially damaged manufactured homes 

located in SFHAs be elevated to the BFE and anchored to resist flotation, collapse, or lateral 

movement. 

Most of the flood damage caused by Hurricane Georges to the manufactured homes constructed before 1994 was the result of a lack of adequate elevation, the use of unreinforced piers 
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(dry-stacked blocks) in areas exposed to moving floodwaters, inadequate anchoring, and failure of attached site-built additions (Figures 1-2 and 1-3). Anchoring failure problems included 

poorly attached anchors; lack of corrosion-resistant materials; homes not fastened to their support piers; and improperly attached tie-down straps.

1.2.3.2	 Performance of Post-1994 Manufactured Homes

The lessons learned from Hurricane Andrew in Florida have resulted in manufactured homes 

being built stronger and installed more solidly; thus these homes are able to better resist extreme loading (in particular, flood and wind loads). 

Figure 1-2. Inadequate

turnbuckle anchor

installed by the

homeowner on this pre1994 manufactured

home, coupled with

lack of elevation and an

unreinforced foundation

system, led to severe

damage.

Figure 1-3. The addition

to this manufactured

home was destroyed,

causing considerable

damage to the rest of the

home.
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Figure 1-4 shows the success of the reinforced masonry pier foundation of a manufactured 

home installed after 1994. Although the area experienced moving floodwaters fast enough to 

displace the air conditioning compressor shown under the home and create localized erosion 

and scour, the reinforced masonry piers survived without damage.

The manufactured home shown in Figure 1-5, on Cudjoe Key in Monroe County, was built to 

the 1994 standards. It survived Hurricane Georges with only minor damage.

1.2.3.3	 Performance of Manufactured Homes During Hurricane Charley (2004) in Florida

Based on field observations, the Hurricane Charley Mitigation Assessment Team (MAT) provided the following conclusions in their report as related to the performance of manufactured 

homes:

Figure 1-5.

Manufactured home in

Cudjoe Key, Florida, built

and installed after 1994,

survived Hurricane

Georges with only minor

damage caused by the

loss of an awning. The

older manufactured

home on the lot next

door was destroyed by

high winds and coastal

surge.

Figure 1-4. Reinforced

masonry pier foundation

system under a

manufactured home

installed after 1994 that

performed well.
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“Finally, performance of manufactured housing was also observed to be a function of age of the building and the regulations to which the units were designed, 

constructed, and installed. Widespread damage was observed to manufactured 

housing designed and constructed prior to the 1976 HUD regulations. The performance of units installed between 1976 when the first HUD regulations were 

enacted and the implementation of the 1994 HUD regulations was observed to 

be somewhat improved, but significant improvements in performance were observed in the units designed and installed to the HUD regulations implemented 

after 1994 in response to Hurricane Andrew. Although some instances of structural failure were observed, the newer manufactured housing units typically 

sustained minimal structural damage and remained secured to their foundations 

when installation followed State requirements (e.g., enforced by the Division of 

Motor Vehicles, Department of Highway Safety and Motor Vehicles, etc.) of unit 

tie-downs (anchors) at 5 feet, 4 inches on-center (if no ancillary structures were 

attached to the unit). Much of this improved performance was difficult to observe due to widespread damage caused by the failures of improperly designed 

and constructed attached structures (including screen enclosures, carports, and 

accessory structures). The failure of these attached structures, in many places 

occurring where wind speeds were below the design wind speed for the area, resulted in extensive damage to roof coverings, siding, windows, and doors of the 

manufactured units, and generated significant amounts of debris. Very few manufactured homes had glazing protection and, as a result, numerous unprotected 

windows on units along the path of the eye of the storm were damaged and broken. Had the [HUD design wind speed] Zone II and Zone III homes installed in 

areas where debris protection is required for site-built one- and two-family dwellings been shipped with appropriate glazing protection, these homes [within the 

path of Hurricane Charley] would have been protected from windborne debris.”

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2	 Manufactured Homes

2.1	 Manufactured Home Characteristics

Manufactured homes are one of several types of homes constructed entirely or partially in an offsite factory, transported over roadways, and then placed or assembled on a site-built foundation. 

After the home is in position, utilities (e.g., water, sewer, electric) are connected, ancillary components (e.g., siding, skirting) are installed, and the home is ready for habitation. Factory built 

homes include manufactured homes, modular homes, panelized homes, and pre-cut homes.

Modular, panelized, and pre-cut homes must comply with the same State and local building 

codes as “site stick built” codes. Manufactured homes must meet HUD regulations.

HUD regulations for manufactured homes are contained in 24 CFR 3280, Manufactured Home 

Construction and Safety Standards (MHCSS), and 24 CFR 3285, Model Manufactured Home Installation Standards (collectively referred to as the HUD codes). HUD regulations define manufactured 

housing as:

“..a structure, transportable in one or more sections, which in the traveling mode 

is 8 body feet or more in width or 40 body feet in length or which when erected 

on-site is 320 or more square feet, and which is built on a permanent chassis and 

designed to be used as a dwelling with or without a permanent foundation when 

connected to the required utilities” (24 CFR 3280.2 and 24 CFR 3285.5)

2.1.1	 Chassis Support System

Manufactured homes are constructed on a chassis consisting of main steel beams and cross 

members; fitted axles, leaf springs, and wheels making up the running gear; and a steel hitch 

assembly. After the home is sited, the chassis frame distributes the manufactured home loads to 

the foundation system. The hitch assembly is generally removed for appearance purposes.

There are two general chassis designs. The traditional chassis system consists of two longitudinal steel beams (between 10 and 12 inches deep); steel cross members that span between the 

beams; and steel “outriggers” extend beyond the beams to support exterior walls of the home 

(Figure 2-1). Manufactured homes designed to be placed on perimeter foundation walls often 

are manufactured with shorter outriggers to provide clearance for the site-built foundation 

walls. 
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2.1.2	 Integrated Support System

An alternative configuration relocates the steel main beams to the perimeter of the home, eliminating the cantilever “outriggers” (Figure 2-2).

2.1.3	 Envelope Construction

The manufactured home envelope must be designed to meet MHCSS (24 CFR 3280) strength 

and rigidity requirements. The floor decking material, attached to the floor joists, is usually 

wood composite panels or plywood. Floor joists spanning between steel cross beams generally 

are spaced at 6-inch centers. Exterior wall frames generally are constructed with wood studs, 

and the exterior of the home is generally covered with vinyl, aluminum, or wood siding. Common wood stud dimensions used in HUD Code housing are typically 2 inches wide by 3 inches 

deep, or 2 inches wide by 4 inches deep based on the design vertical and lateral loads. Some designs for manufactured homes located in HUD Wind Zone III require the use of studs 2 inches 

wide by 6 inches deep.

Figure 2-1. Traditional

chassis system.

Figure 2-2. Integrated

floor system consisting of

steel-reinforced perimeter

framing.

Source: Manufactured Home 

Producer’s Guide to the SiteBuilt Market, HUD.
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Interior structural walls must have the structural capacity adequate for their intended use, with a 

minimum capacity to resist a horizontal load of 5 pounds per square foot (24 CFR 3280.305(f)(2). 

Interior wall frames typically use 2-inch by 3-inch wood studs. Interior non-structural walls can 

use 1-inch by 2-inch studs for framing.

The roof and ceiling system is typically constructed with prefabricated scissor trusses or other 

peaked trusses, sheathed with composite roof panels, underlayment, and shingles. Roof trusses 

typically are spaced at 24-inch centers.

Other construction features of manufactured homes include insulation, vapor barriers, gypsum 

wall and ceiling board, exterior roof and wall sheathing, doors and windows, and other finishing 

materials similarly found in site-built and modular homes. Figure 2-3 shows the main construction features of a typical manufactured home, including the envelope construction elements. 

2.1.4	 Double Section

A double-section manufactured home is constructed from two single floor sections and is, therefore, generally twice as wide as the normal 12- to 16-foot wide single-section unit. The design 

and construction of the chassis/floor system is comparable to that of the single-section unit. 

The double section is transported as two separate floor sections (Figure 2-4). The floor sections 

are then attached at the home’s site. The area where the two floor sections come together is 

called the marriage line. Marriage beams, beam joists, and walls are properly aligned and connected at the site. The roof, walls, and floor along the marriage line must be properly aligned, 

sealed, and supported. 

2.2	 Types of Foundation Systems

Typical types of foundation systems used to support a manufactured home located in SFHAs include the following:

n Piers and ground anchors.

n Perimeter wall foundations.

n Proprietary foundation systems that transmit loads from the manufactured home to the 

ground using patented components or assemblies. 

The HUD Codes (24 CFR 3285 Subparts D – Foundations and E – Anchorage Against Winds) 

provide design standards for pier foundations and anchor systems, including ground anchors. 

Alternative foundations must be manufactured and installed in accordance with their listings by 

a nationally recognized testing laboratory, based on a nationally recognized testing protocol, or 

be designed or tested by a registered professional engineer or architect in accordance with accepted engineering practice, and must not take the home out of compliance with the MHCSS 

(24 CFR 3285.301).
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Figure 2-3. Main

construction features of

a typical manufactured

home.
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Additional technical information on foundation systems for installation of manufactured homes 

is provided in Chapter 8 of this guide.

2.2.1	 Typical Foundation Systems

2.2.1.1 Typical Installation

A manufactured home is typically placed on a site that has been stabilized and improved to 

provide adequate support for the home and anchoring system. Site and area improvements 

techniques vary widely across the country. Typical improvement techniques include simple 

ground stabilization (ground compaction), application of gravel, and/or construction of a concrete runner or slab.

Typical manufactured home foundations consist of a system of piers and ground anchors (refer 

to Section 2.2.1.2). Piers are typically placed beneath the two steel beams at a spacing of 8 to 10 

feet along the length of the manufactured home. Frame ties are connected to the steel chassis 

or perimeter beams, and run to ground anchors (Figure 2-5) that are used with tie-downs and 

straps to secure a manufactured home in place. The frame ties and anchors provide lateral support; the piers provide vertical support. 

Although typical manufactured home foundations and installation methods often address wind 

events, many give little consideration to the forces associated with flooding and seismic events. 

They generally are not designed for flood effects such as hydrodynamic and hydrostatic forces, 

buoyancy, erosion, and scour. Potential failure modes observed in a typical installation include:

n Buoyancy, particularly during rapidly rising floodwaters

n Lateral movement, particularly when exposed to moving floodwaters that extend above the 

home’s steel beams

Figure 2-4. One section

of a double-section

manufactured home

being transported.

Source: Manufactured 

Housing Institute.
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n Pier collapse, particularly when homes 

are exposed to wind and moving 

floodwaters simultaneously

n Erosion and scour, particularly when 

homes are exposed to high velocity 

floodwaters 

2.2.1.2 Piers and Ground Anchors

Using both pier foundations and anchor assemblies together is the most common type 

of foundation system (Figure 2-6). Piers support the gravity loads of the home, and the 

anchors resist uplift and lateral loads. Piers 

are installed under the main beams of the 

home sections, under the marriage line of 

multi-section homes, and at locations designated by the home manufacturer. Common 

types of piers include several configurations 

of steel jack stands and stacked, reinforced or 

unreinforced hollow-core concrete masonry 

blocks of various configurations. Ground anchors, a specific anchoring assembly, typically 

consisting of auger-type (screw-in) ground 

anchors, are the most commonly used. The 

anchor is attached to the home’s steel beams 

by steel straps. Chapter 7 discusses ground 

anchors in more depth.

Figure 2-5. Typical installation of a manufactured home

on masonry block piers with tie-down straps.

Figure 2-6. Typical

installation of a doublesection manufactured

home on a pier

and ground anchor

foundation system.
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The advantages of this foundation system are that it adapts easily to the site conditions, does not 

require much dimensional precision, is installed very quickly, and is economical to install. 

2.2.1.3	 Perimeter Wall Foundations

When perimeter foundations are used with a manufactured home constructed with chassis beams, the 

chassis beams provide support for gravity loads, and 

the perimeter walls resist uplift and lateral loads. 

When used with a manufactured home constructed 

with an integral floor framing system, the perimeter 

walls resist uplift, lateral, and gravity loads (Figure 

2-7). With chassis systems, interior piers support the 

chassis, points along the marriage wall, and other areas of concentrated loads. 

Perimeter walls can be constructed with typical building materials (e.g., cast-in-place concrete, masonry, 

or preservative-treated wood); footings are generally 

cast-in-place concrete. Attaching the floor joists to 

the foundation wall provides resistance to horizontal 

and uplift forces.

Some considerations in using this system include the 

following:

n The system must be precisely measured and 

constructed before the home is delivered to the 

site.

n Typically, a crane or roller system will be needed to place the home onto the foundation.

The cost and construction time of this system is greater than the pier and ground anchor foundation system. 

2.2.2	 Proprietary Systems

A proprietary foundation is a patented system manufactured and sold only by the owner of the 

patent or licensee. If a proprietary system is selected for a manufactured home, it must be capable of resisting all design loads of the MHCSS, and those imposed by any site-specific natural 

hazards. (See the checklist on the next page for more information.) 

The Systems Building Research Alliance (SBRA, formerly the Manufactured Housing Research Alliance [MHRA]) has evaluated proprietary and non-proprietary foundation systems 

for manufactured homes. Their publication, Guide to Foundation and Support Systems for Manufactured Homes, provides guidance for deciding among alternative foundation designs that are 

appropriate for a given site and budget. The SBRA is also continuing to conduct research foundation systems developed for use in SFHAs. 

Figure 2-7. Perimeter wall foundation under a

manufactured home.
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2.3	 Utilities and Mechanical Equipment

2.3.1	 Utilities Placement

Care needs to be taken in the placement of utilities (such as water, sewer, and gas services) and 

mechanical systems. Connecting the manufactured home to these utility and mechanical systems requires them to extend from the grade beneath the home up through the floor. Their 

location makes them susceptible to being inundated by floodwaters and damaged by floating 

debris. Therefore, separating these systems to protect them is important. To minimize damage 

to utility lines, they should be placed in waterproof risers located adjacent to the elevated foundation member on the downstream side of the expected (or anticipated) flood flow. 

State and local codes and regulations must also be followed for utility services installations and 

connections. These provisions may require the waterproofing of all connections, the use of 

certain specific waterproof materials, and backflow preventers on water and sewage service. Underground telephone and electric service should also be enclosed in a riser and protected from 

damage in a similar manner. If electrical and telephone service is supplied from overhead lines, 

the service connection to the manufactured home must be located above anticipated flooding.

Special care must be taken in running underground utilities to homes. Differential movement 

between the home and utility can cause failure of the home to utility connection. Failures can 

cause electrical shock hazard or leakage of gas, water, or raw sewage. Similar problems can occur when utilities run through uncompacted backfill around a basement excavation or when 

utilities are subjected to seismic loads. Installing a flexible connection in the line is one way to 

reduce potential line breakage.

It is possible that a home with utility services could be displaced in a design wind or a seismic 

event. By design, ground anchors that are used to hold down many homes will displace on the 

order of 2 to 3 inches. Seismic forces can cause even higher displacements to utility connections than a design wind event (Figure 2-8). Therefore, some flexibility must be provided in the 

Proprietary Foundation System Checklist

n Identify type of natural hazards for the manufactured home site. Follow the steps in Chapter 

9 of this guide.

n Identify natural hazards design loads for the manufactured home. Follow the steps in Chapter 

9.

n Check design loads on engineering drawings or specifications. Design loads must be greater 

than or equal to the loads identified in Step 2 of Chapter 9.

n The drawing must have a professional engineer’s or architect’s seal, ensuring the system was 

designed by a professional.
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utility lines to accommodate for any potential movements. Utility attachments to the manufactured home should be capable of accommodating the anticipated seismic and wind displacement 

of the support and anchorage system.

Propane and fuel oil tanks used to supply energy for heating or other services should be anchored against flotation or elevated above the anticipated flood level. Tank foundations should 

be separated from the home's foundation system. To minimize the potential for debris impact 

and damage, tanks and their supporting foundations should be located on the downstream side 

of homes. Bollards can be used to deflect floodborne debris and reduce the potential of impact 

damage. Additional information on utility systems for manufactured homes in the floodplain is 

provided in Section 3.6.7.

For manufactured homes located in seismic areas, site-installed water heaters should be provided with seismic bracing straps in accordance with the manufacturer’s recommendations. 

Strapping a water heater to a wall to prevent it from falling over is an easy and cost-effective 

seismic mitigation measure. Other site-installed equipment weighing more than 100 pounds 

should be anchored to resist horizontal seismic forces. 

2.3.2	 Mechanical Access

Components of heating and air-conditioning systems installed within manufactured homes 

are generally located above the floor. When the home is elevated to the BFE, interior components are protected. However, components located below the floor remain vulnerable to flood 

damage. In particular, exterior heating and air conditioning compressors, and crossover ducts 

are common components that are damaged by floodwaters.

Figure 2-8. Utility line damage

in unstable soils.
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Compressor units can be elevated by placing them on platforms at the BFE; however, crossover 

ducts require the entire home to be elevated 1 to 2 feet above the BFE to prevent them from 

being damaged during the design flood. 

2.4	 Attachments – Carports, Decks, Porches, and Awnings

Carports, decks, porches, and awnings are often attached after the manufactured home has 

been installed. The HUD installation standard (24 CFR 3285.3) requires that attachments must 

not impose loads to the manufactured home or its foundations unless the attachments are 

included in the manufacturer’s approved designs and installation instructions, or the attachments are designed by a registered engineer or architect consistent with the manufacturer’s 

design. Similarly, the International Residential Code® (IRC®) requires that accessory buildings 

(e.g., carports, decks, porches, and awnings) shall not be structurally supported by or attached 

to a manufactured home unless engineering calculations are submitted to substantiate any proposed construction (IRC 2006 AE 504.1).

Carports, decks, porches, and awnings should only be built as stand-alone units. Additionally, if a stand-alone deck or porch is going to be added, design criteria for vertical foundation 

members on the addition should be equivalent to those for the foundation system of the main 

structure to prevent damage to the main structure from adjacent structures. 

During the Hurricane Charley post-event assessment (2004), engineers noted the failure of 

these attached structures (in many places, occurring where wind speeds were below the design 

wind speed for the area), resulting in extensive damage to roof coverings, siding, and windows 

of the manufactured units, and generating significant amounts of debris.
3.	 Regulatory Requirements

Federal, State, and local regulatory requirements must be identified and assessed as an initial 

step in the installation of manufactured housing. The following discussion provides a summary 

of the applicable NFIP minimum requirements for the installation of manufactured homes in 

floodplains. In addition, HUD MHCSS and model building code requirements are summarized. 

Appendix A provides a list of publications for further guidance for the installation and placement of manufactured homes in hazard-prone areas. 

3.1	 Introduction to the NFIP

The NFIP has three main elements:

Flood insurance that provides financial protection for property owners to cover flood-related damage to buildings and contents. 

Floodplain management requirements within the mapped flood hazard areas that are 

designed to prevent new development from increasing the flood threat and to protect 

new and existing buildings from anticipated flood events. 

Hazard identification and mapping under which engineering studies are conducted and 

flood maps are prepared to delineate areas that are predicted to be subject to flooding 

under certain conditions. 

Federal flood insurance is designed to provide an alternative to disaster assistance and disaster 

loans for home and business owners. Disaster assistance rarely comes close to covering all of 

the costs to repair and cleanup. While available to qualified victims, disaster loans do not significantly ease the financial burden due to repayment terms. It is important to remember that 

disaster assistance is available only after floods have been declared major disasters by the President of the United States. In contrast, insurance claims will be paid any time damage from a 

qualifying flood event occurs. 

The NFIP is intended to encourage States and local governments to recognize and incorporate 

flood hazards in land use and development decisions. In some communities, this is achieved 

by guiding development to areas with lower risk. When decisions result in development within 

floodplains, application of the criteria set forth in 44 CFR 59.1 and 60.3 is intended to minimize 

exposure and flood-related damage. These are the minimum requirements that a community must adopt in order to participate in the NFIP. In general, the NFIP minimum floodplain 

management regulations require that new residential structures or substantially improved or 

substantially damaged existing residential buildings in A zones must have their low­est floor 

(including basement) elevated to or above the BFE. Non-residen­tial structures in A zones can 

be either elevated or dry-floodproofed to the BFE. In V zones, the building must be elevated 

on piles and columns such that the bottom of the lowest horizontal structural member of the 

1.

2.

3.

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lowest floor of all new construction or substantially improved existing buildings must be elevated to or above the BFE. 

Any community may exceed the minimum NFIP requirements by adopting more restrictive 

regulations. Communities often do this when they have access to information or knowledge 

of conditions that require, particularly for human safety, higher standards than the minimum 

NIFP requirements. Therefore, any floodplain management regulations that are more restrictive than the NFIP requirements shall take precedence. 

By encouraging communities to guide development to lower risk areas, and by requiring the 

elevation of new construction and existing buildings that have been substantially improved or 

substantially damaged, one of the long-term objectives of the NFIP can be achieved: reducing 

flood damage and losses. 

In addition to providing flood insurance and reducing flood damages through floodplain 

manage­ment regulations, the NFIP identifies and maps the nation’s floodplains. Mapping flood 

hazards creates a broad-based awareness of the flood hazards and provides the data needed for 

floodplain management programs and to actuarially rate new construction for flood insurance. 

3.2	 Identifying and Mapping Flood Hazards

FEMA is required by statute to identify and map the nation’s flood-prone areas and to establish 

flood-risk zones in these areas. Since the inception of the NFIP, FEMA has complied with this 

statutory requirement by performing engineering studies of flood-prone communities and producing flood maps. The flood maps are used by mortgage lenders, insurers, real estate agents, 

map determination companies, community officials, land developers, engineers and surveyors, 

State and local emergency response officials, and property owners. The flood maps are also 

used by States and communities for emergency manage­ment, land use planning, and water resources planning and by Federal agencies implement­ing Executive Order 11988, Floodplain 

Management for Federal actions proposed in or affecting floodplains. 

The NFIP would not be able to offer insurance at affordable rates without the existence of risk 

management (floodplain management) to reduce flood losses. To assess and manage the flood 

risk, a national standard was needed. The 1-percent annual chance flood was chosen on the basis that it provides a higher level of pro­tection while not imposing overly stringent requirements 

or the burden of excessive costs on property owners. The 1-percent annual chance flood (or 

100-year flood) represents a magnitude and frequency that has a statistical probability of being 

equaled or exceeded in any given year or, stated alternatively, the 100-year flood has a 26 percent (or 1 in 4) chance of occurring over the life of a 30-year mortgage.

FEMA performs a Flood Insurance Study (FIS) and creates FIRMs delineating the extent of the 

flood that has a 1-percent chance of being equaled or exceeded in any given year. The delineated area is designated an SFHA, which is the area in which development must be regulated by 

NFIP floodplain management regulations. Under the NFIP, a floodplain is defined as any land 

area susceptible to being inundated by floodwaters from any source.

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When producing a new FIS and FIRMs or updating existing ones, FEMA typically uses a combination of the two study approaches (approximate and detailed) in identifying a community’s 

flood hazards. Detailed study methods typically employ the use of engineering models and, at 

a minimum, result in the determination of BFEs or flood depths and floodways that will be displayed on the flood map. Approximate study methods are used to identify flood hazards, but 

they use more simplified engineering methods to determine the floodplain boundaries; however, BFEs and floodways are not identified or mapped. 

In general, the decision on whether to use approximate or detailed study methods is based on 

existing and anticipated development in and near the floodplain in the community. Flood hazard information for flooding sources that affect developed or developing areas are based on 

detailed studies whenever possible; approximate study methods generally are used for undeveloped or sparsely developed areas. 

When a new or updated detailed FIS is performed, the following information is usually generated: 

n BFEs are presented as either water-surface elevations or average depths of flow above the 

ground surface. These elevations and depths were historically referenced to the National 

Geodetic Vertical Datum of 1929 (NGVD 29). However, as a result of advances in technology, an updated vertical datum – the North American Vertical Datum of 1988 (NAVD 

88) – was created and has been officially adopted by the Federal Government as a new 

basis for measuring heights. Newly updated maps and BFEs are now generally referenced 

to NAVD 88.

n Water-surface elevations for the 10-year (10-percent annual chance), 50-year (2-percent 

annual chance), 100-year (1-percent annual chance), and 500-year (0.2-percent annual 

chance) floods. 

n Boundaries of the regulatory floodway. The regulatory floodway is defined as the channel 

of a stream plus any adjacent floodplain areas that must be kept free of encroachment so 

that the entire base flood (1-percent annual chance flood) discharge can be conveyed with 

no greater than a 1.0-foot increase in the BFE. 

n Boundaries of the 1- and 0.2-percent annual chance floodplains. The 1-percent annual 

chance floodplain is referred to as the SFHA. The 0.2-percent annual chance flood is the 

flood that has a 0.2 percent chance of being equaled or exceeded in any given year and is 

also known as the 500-year flood. 

The results of the FIS are presented on the FIRM. An example of a FIRM showing the map 

and title box is shown in Figure 5-1. FIRMs are now being prepared as a digital product, known 

as a Digital Flood Insurance Rate Map (DFIRM), which may involve converting an existing 

manually produced FIRM to digital format, or creating a product from new digital data sources 

using a Geographic Information System (GIS) environ­ment. A sample DFIRM is shown in Figure 3-1. 

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The results of the FIS are also presented in a report that accompanies the FIRMs or DFIRMs. 

The FIS report contains a narrative that describes the flooding conditions in the community, 

and the types of studies that were performed. The report also contains tables and Flood Profiles. 

For determining a BFE at a building site, communities should use the Flood Profiles rather than 

the BFE on the FIRM. A sample FIS summary of discharges is shown in Figure 3-2(a), a sample 

FIS floodway data table is shown in Figure 3-2(b), and a sample FIS flood profile is shown in Figure 3-2(c). Refer to Section 5.1 for additional details on obtaining FIRM and FIS data. The NFIP 

FISs and FIRMs can be viewed and ordered online at http://store.msc.fema.gov.

Flood risk zones are shown on the FIRM or DFIRM for the 1-percent annual chance flood as 

Zone A, Zone A1-30 (on older FIRMs), Zone AE, Zone AO, Zone AH, Zone AR, Zone A99, Zone 

AR/A, Zone AR/AE, Zone AR/A1-30 (on older FIRMs), Zone AR/AH, Zone AR/AO, Zone V, 

Zone V1-30 (on older FIRMs), or Zone VE. Table 3-1 provides a general description of the most 

common flood zone designations for riverine flood areas and inland flood zones in coastal areas. Table 3-2 provides a general description of the most common flood zone designations for 

coastal flood areas. The zone designations presented on the FIRM or DFIRM depend on the 

type of flood hazard and type of study performed. The 0.2-percent annual chance floodplain 

is labeled as Zone B (on older FIRMs) or Zone X (shaded). Areas outside of the SFHA are 

labeled as Zone C or Zone X (unshaded). Definitions and descriptions of the flood insurance 

risk zones can be found at: http://www.fema.gov/plan/prevent/floodplain/nfipkeywords/

flood_zones.shtm.

Figure 3-1. Sample

DFIRM.

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Figure 3-2(a). Sample FIS

Summary of Discharges

table.

Figure 3-2(b). Sample FIS

Floodway Data.

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Figure 3-2(c). Sample FIS

Flood Profile.

Table 3-1. Common SFHA Designations for Riverine or Inland Flood Zones

Zone Description

Flood Risk Zones

A Areas subject to inundation by the 1-percent annual chance flood event. Because detailed 

hydraulic analyses are not used, no BFE or depths are shown. 

AE and

A1-301

Areas subject to inundation by the 1-percent annual chance flood event determined by 

detailed methods. BFEs are shown. 

AH and AO Areas subject to inundation from the 1-percent annual chance flood that have flood 

depths of between 1 and 3 feet. AO zones can be either ponding areas or sheet flow 

and are assigned a base flood depth measured in feet above the ground. AH zones are 

generally ponding areas and are assigned a BFE.

1

 Zone designations found on older FIRMS have been replaced by the designation listed in the same box above them.

Table 3-2. Common SFHA Designations for Coastal Flood Areas

Zone Description

Flood Risk Zones

V Areas subject to inundation from the 1-percent annual chance flood in coastal areas 

where there is a velocity hazard due to wave action and, by definition, the entire 

Primary Frontal Dune. No BFEs have been determined.

VE

V1-301

Areas subject to inundation from the 1-percent annual chance flood in coastal areas 

where there is a velocity hazard due to wave action. BFEs have been determined and 

are shown on the FIRM.

1

 Zone designations found on older FIRMs have been replaced by the designation listed in the same box above them.

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In addition to the SFHAs labeled as V zones, there are some A zones, generally located landward 

of and contiguous to V zones, where flooding characteristics may include wave action, high velocity flows, and erosion or scour. These are referred to as Coastal A zones, and they are further 

described below and in Section 3.7.

Along rivers, streams, and lakes within the U.S., FEMA computes BFEs using computer models, 

statistical techniques, or both. These elevations are a function of the amount of water expected to enter a particular system by means of precipitation and runoff. The SFHAs along rivers, 

streams, and lakes are labeled as A zones. 

Along the coast, FEMA determines SFHAs by an analysis of storm surge, wind direction, wind 

speed, wave heights, and other factors. FEMA designates these areas along the coast as both V 

and A zones on the FIRM or DFIRM. 

FEMA applies the VE zone designation to those areas along the coast where water depth and 

other conditions would support at least a 3-foot wave height. FEMA also considers other factors 

in identifying VE zones, such as wave runup and the existence of a Primary Frontal Dune. FEMA 

usually designates AE zones in coastal areas landward of the VE zone. Coastal flood hazard areas mapped as AE zones can be subject to storm surge and damaging waves; however, the waves 

are less than 3 feet in height.

Currently, NFIP regulations make no distinction between the design and construction requirements for coastal AE zones and riverine AE zones. However, evidence sug­gests that design and 

construction requirements in some portions of coastal AE zones should be more like VE zone 

requirements. One of the hazard identification criteria for VE zone designation is where wave 

heights are estimated to be equal to or greater than 3 feet. Unfortunately, post-storm investigations have shown that typical AE zone construction techniques (e.g., wood frame, light gauge 

steel, or masonry walls on shallow footings or slabs, etc.) are subject to damage when exposed 

to waves less than 3 feet in height. Laboratory tests and post-disaster field investigations confirm that wave heights as small as 1.5 feet can cause failure of the above-listed wall types. Other 

flood hazards associated with coastal waves (e.g., floating debris, high velocity flow, erosion, and 

scour) also damage AE zone-type construction in these coastal areas.

As a result, for all new detailed coastal studies starting in 2009, FEMA will delineate the landward limit of waves 1.5 feet in height on the FIRMs and include this information in the DFIRM 

database as an informational layer. The landward limit of the area affected by waves greater than 

1.5 feet is called the Limit of Moderate Wave Action (LiMWA). The LiMWA will be included on 

the preliminary FIRM; however, if a community does not want to delineate the limit on its final 

FIRMs, the community may provide a written request to FEMA with justification for such a request. There are no NFIP floodplain management requirements that communities must adopt 

or special insurance ratings associated with the designation of the LiMWA. However, communities are encouraged to adopt higher standards than the minimum NFIP requirements in these 

areas to address the increased risks associated with waves and velocity action. 

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3.3	 The NFIP's Community Rating System (CRS)

The NFIP's Community Rating System (CRS) is a voluntary incentive program that recognizes 

and encourages community floodplain management activities that exceed the minimum NFIP 

requirements.

As a result, flood insurance premium rates are discounted to reflect the reduced flood risk resulting from the community actions meeting the three goals of the CRS:

Reduce flood losses

Facilitate accurate insurance rating

Promote the awareness of flood insurance

For CRS participating communities, flood insurance premium rates are discounted in increments of 5 percent; a Class 1 community would receive a 45 percent premium discount, while 

a Class 9 community would receive a 5 percent discount (a Class 10 is not participating in the 

CRS and receives no discount). The CRS classes for local communities are based on 18 creditable activities, organized under four categories:

Public Information

Mapping and Regulations

Flood Damage Reduction

Flood Preparedness

3.4	 NFIP Definitions Related to Manufactured Homes

In addressing the NFIP floodplain management requirements for manufactured homes, it is 

useful to define key terms. The following definitions are found in the NFIP floodplain management regulations at 44 CFR 59.1 unless noted otherwise. Several NFIP definitions can also be 

found in Appendix D. The definition for “manufactured home” does not include a recreational 

vehicle. 

n "Manufactured home” means a structure, 

transportable in one or more sections built on 

a chassis and designed for use with/without a 

permanent foundation when attached to utilities.

n “Manufactured home park or subdivision” means a 

parcel (or contiguous parcels) of land divided into 

two or more manufactured home lots for rent or sale. 

n “Existing manufactured home park or subdivision” 

means a manufactured home park or subdivision 

for which the construction of facilities for servicing 

the lots on which the manufactured homes are to be 

1.

2.

3.

1.

2.

3.

4.

Permanent Foundations

A permanent foundation is a foundation constructed of durable materials 

that may include masonry, reinforced 

concrete, corrosion resistant steel, 

or pressure treated timber. The foundation designs provided in Appendix 

H are considered permanent foundations in accordance with this definition.

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affixed (including, at a minimum, the installation of utilities, the construction of streets, 

and either final site grading or the pouring of concrete pads) is completed before the 

effective date of the floodplain management regulations adopted by the community.

n “New manufactured home park or subdivision” means a manufactured home park 

or subdivision for which the construction of facilities for servicing the lots on which 

the manufactured homes are to be affixed (including, at a minimum, the installation 

of utilities, the construction of streets, and either final site grading or the pouring of 

concrete pads) is completed on or after the effective date of the floodplain management 

regulations adopted by a community.

n “Expansion to an existing manufactured home park or subdivision” means the preparation 

of additional sites by the construction of facilities for servicing the lots on which the 

manufactured homes are to be affixed (including the installation of utilities, the 

construction of streets, and either final site grading or the pouring of concrete pads).

n “Substantial damage” means damage of any origin sustained by a structure whereby the 

cost of restoring the structure to the condition before damage would equal or exceed 50 

percent of the market value of the structure before the damage occurred.

n "Substantial improvement" means any reconstruction, rehabilitation, addition, or other 

improvement of a structure, the cost of which equals or exceeds 50 percent of the market 

value of the structure before the “start of construction” of the improvement. This term 

includes structures that have incurred “substantial damage,” regardless of the actual 

repair work performed. The term does not, however, include either (1) any project for 

improvement of a structure to correct existing violations of State or local health, sanitary, 

or safety code specifications that have been identified by the local code enforcement 

official and that are the minimum necessary to ensure safe living conditions, or (2) any 

alteration of a “historic structure,” provided that the alteration will not preclude the 

structure’s continued designation as a “historic structure.”

3.5	 General NFIP Floodplain Management Requirements for

Manufactured Homes 

Under the NFIP, participating communities are required to regulate all development in SFHAs. 

“Development” is defined in the NFIP regulations as:

“Any man-made change to improved or unimproved real estate, including but not 

limited to buildings or other structures, mining, dredging, filling, grading, paving, excavation or drilling operations or storage of equipment or materials.” [44 

CFR 59.1]

Before a property owner can undertake any development in the SFHA, a permit must be obtained from the community. The community is responsible for reviewing all permit applications 

to determine whether development complies with the community’s floodplain management 

regulations. Communities are also required to review proposed development in SFHAs to 

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ensure that all necessary permits have been received from those government agencies from 

which approval is required by Federal or State law, such as 404 wetland permits from the U.S. 

Army Corps of Engineers (USACE) or permits under the Endangered Species Act.

The NFIP requires manufactured homes to meet the same basic regulatory requirements that 

are required for residential structures in the floodplain. The following general NFIP minimum 

standard applies to all residential structures in the floodplain in which communities are to: 

“Review all permit applications to determine whether proposed building sites will 

be reasonably safe from flooding. If a proposed building site is in a flood-prone 

area, all new construction and substantial improvements shall (i) be designed 

(or modified) and adequately anchored to prevent flotation, collapse, or lateral 

movement of the structure resulting from hydrodynamic and hydrostatic loads, 

including the effects of buoyancy, (ii) be constructed with materials resistant to 

flood damage, (iii) be constructed by methods and practices that minimize flood 

damages, and (iv) be constructed with electrical, heating, ventilation, plumbing, 

and air conditioning equipment and other service facilities that are designed 

and/or located so as to prevent water from entering or accumulating within the 

components during conditions of flooding.” [44 CFR 60.3(a)]

The NFIP also requires that new manufactured home parks and subdivisions meet the same basic requirements as all other subdivision proposals in which communities are to:

"Review subdivision proposals and other proposed new development, including 

manufactured home parks or subdivisions, to determine whether such proposal 

will be reasonably safe from flooding. If a subdivision proposal or other proposed 

development is in a flood-prone area, any such proposals shall be reviewed to assure that:

(i) all such proposals are consistent with the need to minimize flood damages within 

the flood-prone area,

(ii) all public utilities and facilities, such as sewer, gas, electrical, and water systems 

are located and constructed to minimize or eliminate flood damage, and

(iii) adequate drainage is provided to reduce exposure to flood hazards." [44 CFR 

60.3(a)(4)]

3.6	 NFIP Requirements for Manufactured Homes in Riverine and

Inland Flood Zones 

Generally, manufactured homes must meet the same flood protection requirements as a “stick 

built” or typical housing. The general requirement for manufactured homes like typically built 

residential structures is that they must be elevated so that the lowest floor is above the BFE. This 

applies to new or substantially improved or substantially damaged manufactured homes. 

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3.6.1	 Approximate A Zones

Approximate A zones are not studied using detailed methods; therefore, BFEs have not been 

developed. When a manufactured home is proposed on an approximate A zone site, the community must make every effort to obtain any BFE data and floodway data in order to achieve a 

reasonable measure of flood protection. 

“Obtain, review, and reasonably utilize any base flood elevation and floodway data 

available from a Federal, State, or other source....”[44 CFF 60.3(b)(4)]

BFE data may be available from a variety of sources such as local hydrologic and hydraulic 

(H&H) studies, State Departments of Natural Resources, or from Federal agencies such as the 

USACE. Several other sources of BFE data are identified in FEMA 265, Managing Floodplain Development in Approximate Zone A Areas, A Guide for Obtaining and Developing Base (100-year) Flood 

Elevations. 

If BFE or floodway data cannot be obtained from these sources, the community should consider 

conducting, or requiring the applicant to conduct, a site-specific engineering analysis to determine a BFE. Guidance for developing a BFE is available in FEMA 265. 

If no BFE data are available, communities must ensure that building sites will be reasonably safe 

from flooding for proposed developments. A simplified method for estimating a BFE as well as 

other methods for ensuring a building site is reasonably safe from flooding can also be found 

in FEMA 265. 

BFE and DFE

Base flood elevation (BFE) – The water surface elevation resulting from the base, or 100-year (1 percent annual chance) flood. 

Design flood elevation (DFE) – The elevation to which development in the regulatory floodplain is 

built. At a minimum, the DFE is equivalent to the BFE. However, in some areas, the DFE includes an 

additional freeboard height above the BFE as shown in the formula below.

DFE = BFE + Freeboard

Freeboard – An additional elevation requirement some height (1, 2, or more feet) above the BFE that 

provides a margin of safety above the estimated BFE. 

As indicated above, the minimum DFE requirement for NFIP communities is the BFE. However, in communities where a higher degree of protection is promoted or required, the DFE includes a freeboard 

height above the BFE. This freeboard provides a margin of safety above the estimated BFE and against 

extraordinary or unknown risks. As in all natural hazard events, the design event can only be predicted 

in probabilistic terms and some uncertainties remain in any analysis. Freeboard is intended to allow for 

those uncertainties. The NFIP encourages participating communities to adopt and enforce freeboard 

requirements as part of their local floodplain management ordinance. This is shown in 44 CFR 60.1(d); 

which states, “Any floodplain management regulations adopted by a State or a community which are 

more restrictive than the criteria set forth in this part are encouraged and shall take precedence.”

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When a new manufactured home park or subdivision is being planned on a site that is greater 

than 50 lots or 5 acres (whichever is less), BFE data must be developed for approximate A zones 

through the use of detailed study methodologies similar to those used in creating the FIS. Under this requirement, the community must: 

“Require that all new subdivision proposals and other proposed development (including proposals for manufactured home parks and subdivisions) greater than 

50 lots or 5 acres, whichever is the lesser, include within such proposals base 

flood elevation data.” [44 CFR 60.3(b)(3)]

The community can perform a study to determine these elevations or require the developer 

to fund or complete the study. When the 50-lot, 5-acre threshold is surpassed and the development includes one or more lots that lie at least partially within the floodplain, BFE data must 

be developed through detailed study methodologies. If the developer completes the study, it is 

subject to approval by the community.

For manufactured home sites that are below the 50-lot/5-acre threshold and BFE data for those 

sites are not available from other sources, communities should consider conducting or requiring the permit applicant to develop a BFE in an approximate A zone. In addition to reducing 

flood damages, there are other advantages of developing a BFE. For example, flood insurance 

policies obtained in approximate A zones that are rated using a BFE developed using a detailed 

study methodology will often qualify for significantly lower insurance rates than policies that 

are rated without a BFE. The difference in flood insurance premiums could be substantial. 

If a manufactured home is proposed in an approximate A zone where BFE data are not available from a Federal, State, or other source, the community must require that:

“...all manufactured homes to be placed within Zone A on a community FHBM or 

FIRM shall be installed using methods and practices which minimize flood damage. For the purpose of this requirement, manufactured homes must be elevated 

and anchored to resist flotation, collapse, or lateral movement. Methods of anchoring may include, but are not limited to, the use of over-the-top frame ties to 

ground anchors. This requirement is in addition to applicable State and local anchoring requirements for resisting wind forces.” [44 CFR 60.3(b)(8)]

This minimum standard requires that manufactured homes be elevated and anchored to resist 

flood and wind forces. If elevation data are not available from a Federal, State, or other source, 

FEMA advises communities to require manufactured homes under this provision to be elevated 

at a minimum of 3 feet or higher from the highest adjacent grade to minimize flood damages. 

Guidance for anchoring is further addressed in Section 3.6.5 and in Chapter 7. The areas below the 

3-foot elevation must be constructed with flood-resistant materials and utilities, and mechanical 

equipment must be elevated to the 3-foot elevation. Any utility and mechanical components 

that must be below the BFE must be made watertight to the 3-foot elevation as described in 

3.6.7. 

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When BFEs or floodway data are obtained for manufactured home sites, the manufactured 

home must meet the elevation and floodway standards described in Sections 3.6.2 and 3.6.9.

3.6.2	 Elevation in A, A1-30, AE, and AH Zones

Most manufactured home placements are subject to the same performance standards as typically 

built residential structures. The performance standard for the placement of these manufactured homes is addressed in this section. An exception has been made for the placement of 

manufactured homes in existing manufactured home parks or subdivisions which is addressed 

in Section 3.6.3.

The NFIP requires that manufactured homes placed or substantially improved in Zones A1-30, 

AH, or AE on the community’s FIRM on sites: 

(i) “Outside of a manufactured home park or subdivision,

(ii) In a new manufactured home park or subdivision,

(iii) In an expansion to an existing manufactured home park or subdivision, or

(iv) In an existing manufactured home park or subdivision on which a manufactured 

home has incurred substantial damage as the result of a flood, 

be elevated on a permanent foundation such that the lowest floor of the manufactured home is elevated to or above the base flood elevation and be securely 

anchored to an adequately anchored foundation system to resist flotation, collapse and lateral movement.” [44 CFR 60.3(c)(6)] 

This requirement establishes the basic elevation and anchoring standards that apply to most 

manufactured home placements, including those outside of manufactured home parks and subdivisions and in new manufactured home parks and subdivisions. These manufactured homes 

must have their lowest floors elevated to or above the BFE. These requirements also apply to 

manufactured homes placed in expansions to existing manufactured home parks and on lots 

where manufactured homes are substantially damaged by flood. Once a manufactured home in 

an existing manufactured home park or subdivision is substantially damaged by flood, from that 

point on all manufactured homes placed on that lot must be elevated to or above the BFE. 

The best way to meet the elevation requirement described 

above is to elevate the bottom of the manufactured home 

steel frame to the BFE. The manufactured home must be 

securely anchored to an adequately anchored foundation 

system to resist flotation, collapse, or lateral movement. 

Section 3.6.5 and Chapter 7 address anchoring in more 

detail. 

As with stick built housing, all parts of the manufactured home below the BFE must be constructed with 

This guide recommends for A zones 

the best practice of placing the bottom of the manufactured home's 

steel frame at the BFE and not the 

lowest floor of the manufactured 

home in order to prevent flood damage to the floor and other interior 

finishes and contents (Figure 3-3).

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flood damage-resistant materials and utility systems must either be elevated or made watertight to the BFE. 

3.6.3	 3-Foot Pier Foundation

The NFIP allows for a limited exemption to elevating to the BFE by allowing elevating to no less 

than 36 inches in height above grade for lots in existing manufactured home parks in Zones 

A1-30, AE, or AH on the community’s FIRM. A manufactured home placed in an existing manufactured home park must meet either of the following requirements: 

"(1) The lowest floor of the manufactured home is at or above the base flood elevation; 

or

(2) The manufactured home chassis is supported by reinforced piers or other foundation elements of at least equivalent strength that are no less than 36 inches in height 

above grade and be securely anchored to an adequately anchored foundation system to resist flotation, collapse, and lateral movement." 44 CFR 60.3(c)(12).

The elevation requirements above apply in existing manufactured home parks or subdivisions established before the date of the community’s initial floodplain management regulations when: 

Figure 3-3.

Manufactured home with

lowest floor elevated to

the BFE.

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1) A manufactured home is being placed or replaced with a new model in an existing 

community or subdivision, or

2) A manufactured home is being replaced in an existing manufactured home park 

or subdivision when the previous home had sustained substantial damage due to 

reasons other than a flood. 

The selection of the 36-inch pier foundation does not apply to the repair or replacement of a manufactured home 

on a site in an existing manufactured home park where 

a manufactured home has been substantially damaged by 

flood. 

The option of allowing elevating 36 inches in height 

above grade is a compromise to balance the flood hazard against the economic impacts on some manufactured 

park owners that would result if elevation to the BFE were 

required. There are often practical difficulties in elevating manufactured homes to the BFE in many older parks 

due to small lot sizes and the split ownership of the manufactured home and the lot itself. 

It is important to note that, when the 36-inch pier foundation is selected, the manufactured 

home may not be protected to the level of the BFE. For this reason, care must be exercised in 

using the 36-inch pier foundation option. This option may not be appropriate in communities 

where the flood hazard may be so severe, such as deep floodplains and high velocity areas, that 

it would place lives and property at too great a risk. Some communities may find placement of 

a manufactured home in an existing manufactured home park can accommodate elevation to 

the BFE. 

If the option of elevating the manufactured home to 36 inches above grade is chosen, the 

strength of the anchoring system must be increased in order to resist the additional flood forces 

that will be exerted upon the foundation since it is possible that water will reach or exceed the 

level of the home’s floor. Therefore, under this scenario, the use of typical manufactured home 

installation methods such as foundations using dry-stack blocks and ground anchors would preclude the use of the other foundation designs in this guide. 

No matter which option is chosen for elevating a manufactured home in an existing manufactured home park, the manufactured home must be securely anchored to an adequately 

anchored foundation system to resist flotation, collapse, or lateral movement.

As with stick built housing, all parts of the manufactured home below the BFE must be constructed with flood-resistant materials and utility systems must either be elevated or made watertight 

to the BFE. The best way to meet the requirement for flood resistance is to elevate the bottom 

of the manufactured home steel frame to the BFE.

3-Foot Pier Foundation

The 3-foot pier foundation is one 

option for manufactured homes 

installation in existing manufactured home parks or subdivisions. 

However, the NFIP strongly encourages elevating to the level of 

the BFE or higher for increased 

safety and property protection 

and reduced flood insurance premiums.

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Figure 3-4 shows a typical example of a foundation used on manufactured housing per the 36-

inch pier foundation alternative. The pier, footing, and connection between the pier and the 

home must be designed to resist flotation, collapse, or lateral movement.

3.6.4	 Elevation in Zone AO

A Zone AO is an area of shallow flooding with flood depths between 1 and 3 feet with no clearly 

defined channel. FIRMs indicate a flood height above grade that is based on an average depth 

to which structures must be raised for Zone AO. 

Manufactured homes placed or substantially improved in Zone AO must meet the same standards as other residential structures and have their lowest floor elevated above the highest 

adjacent grade at least as high as the depth number specified on the FIRM. 

HUD manufactured home standard 24 CFR 3285.305 requires a minimum clearance of 12 

inches between the lowest member of the main frame and the ground surface, and the manufactured home main frame is typically 0.5 feet to 1 feet deep; therefore, the lowest floor level is 

typically 1.5 to 2 feet above grade. Under this scenario, meeting the NFIP elevation requirement 

for Zone AO may already be met or require a relatively small additional elevation of up to 1.5 

feet. This additional elevation can be accomplished using installation techniques found in this 

guide, such as placing the home on piers and securing it with ground anchors.

Figure 3-4.

Manufactured home

on reinforced pier

foundation 36 inches

high.

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Standard installation techniques provide a foundation that is typically elevated at least 3 

feet above grade, and flood depths in Zone AO are typically 3 feet or less. Therefore, the placement or substantial improvement of manufactured homes in Zone AO are typically elevated to 

or above the base flood level, even if they are being placed in an existing manufactured home 

park or subdivision. However, the recommended best practice is to elevate the bottom of the 

steel frame to the BFE in Zone AO.

As in the other A zones, manufactured homes in Zone AO must be anchored to resist flotation, 

collapse, or lateral movement. The areas below the Zone AO flood depth must be made with 

flood damage-resistant materials, and utilities and mechanical equipment must be elevated to 

the flood depth specified on the FIRM. Any utility and mechanical components that must be below the BFE must be made watertight to the Zone AO flood depth. Refer to Sections 3.6.6 and 

3.6.7 and Chapter 7 for additional details on these requirements. 

3.6.5	 Anchoring 

In addition to the elevation requirement, manufactured homes are required to be securely 

attached to an adequately anchored foundation system to resist flotation, collapse, or lateral 

movement: 

"[Manufactured homes must] ...and be securely anchored to an adequately anchored foundation to resist flotation, collapse and lateral movement." [44 CFR 

60.3(c)(6) and (c)(12)]

“Adequately anchored” is considered to be a system of ties, anchors, and anchoring equipment 

that will withstand flood and wind forces. 

Additional guidance on anchoring manufactured homes can be found in Chapter 7.

Anchor and Tie-down Systems

Anchor and tie-down systems must meet load requirements stipulated in locally adopted installation 

ordinances. Many communities across the country have adopted or are in the process of adopting the 

International Code Council’s (ICC's) model building codes, which include the International Residential 

Code® of 2009 (IRC® 2009). IRC 2009 stipulates working load requirements of 3,150 pounds with a 50 

percent overload or 4,725 pounds for manufactured home anchoring systems.

Florida’s manufactured home installation requirements, Rules of Department of Highway Safety and 

Motor Vehicles Division of Motor Vehicles, Chapter 15C-1, for homes manufactured before July 14, 

1994, have the same loading standards for anchor systems as those found in the IRC 2009, a working 

load of 3,150 pounds with an ultimate load of 4,725 pounds. However, for homes manufactured after 

July 14, 1994, the system must be able to resist a working load of 4,000 pounds with an ultimate load 

of 6,000 pounds.

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3.6.6	 Flood Damage-Resistant Materials

When a manufactured home is elevated, it is important that all parts exposed to floodwaters be 

made of flood damage-resistant materials. This includes all portions of the manufactured home 

such as foundation elements and any enclosures below the BFE. 

“[Community must] Review all permit applications to determine whether proposed building sites will be rea­sonably safe from flooding. If a proposed building 

site is in a floodprone area, all new construction and substantial improvements 

shall...(ii) be constructed with materials resis­tant to flood damage...” [44 CFR 

60.3(a)(3)]

“Flood [damage]-resistant material” is described in FEMA Technical Bulletin 2 as “any building 

product [material, component or system] capable of withstanding direct and prolonged contact 

with floodwaters without sustaining significant damage.” The term “prolonged contact” means 

at least 72 hours, and the term “significant damage” means any damage requiring more than 

cosmetic repair. “Cosmetic repair” includes cleaning, sanitizing, and resurfacing (e.g., sanding, 

repair of joints, re­painting) of the material.

For further information on flood-resistant materials refer to FEMA Technical Bulletin 2, Flood 

Damage-Resistant Material Requirements for Buildings Located in Special Flood Hazard Areas in accordance with the National Flood Insurance Program. 

3.6.7	 Utilities and Mechanical Equipment

The manufactured home’s utilities and mechanical equipment need to be protected. Their 

location below the BFE can make them susceptible to being inundated by floodwaters and damaged by floating debris. Utilities and mechanical equipment must:

“...be constructed with electrical, heating, ventilation, plumbing, and air conditioning equipment and other service facilities that are designed and/or located 

so as to prevent water from entering or accumulating within the components 

during conditions of flooding." [44 CFR 60.3(a)(3)(iii)] 

Essentially, utility systems and mechanical equipment must be elevated to or above the BFE. Elevating utilities and mechanical equipment such as furnaces, hot water heaters, and heating, 

ventilation, and air conditioning equipment (HVAC) is the best protection. With most outdoor 

HVAC compressors, the main issues presented by floodwaters are inundation, velocity flow, and 

debris impact. The control and power circuits and mechanical parts in HVAC equipment, even 

when they are designed for outdoor installations, are not designed to withstand inundation by 

floodwaters. The most effective flood-resistant design for outdoor HVAC compressors is to strap 

or bolt the equipment onto a platform above the BFE. 

Utility and mechanical components that must be below the BFE must be made watertight to the 

BFE. Examples of this include sealing on-site water well casings to prevent water intrusion or enclosing HVAC equipment in a flood-resistant enclosure so that it is protected to the BFE. 

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The NFIP does not recommend locating ductwork below the BFE for manufactured homes located in SFHAs. There is no known or cost-effective technique for designing air ducts to keep 

floodwaters from entering or accumulating within the systems components during inundation 

by floodwaters. If ductwork must be installed below the BFE, it should be minimized as much 

as possible. Placing the bottom of the steel frame at the BFE as recommended enhances protection of utility and mechanical equipment. The exception would be installation of cross-over 

duct work for manufactured housing that needs to pass under chassis frame members. If crossover ducts are used, elevating the manufactured home 1 to 2 feet above the BFE will prevent 

them from being damaged during flood events. 

Additional guidance for protecting utility and mechanical systems can be found in FEMA 348, 

Protecting Building Utilities From Flood Damage, Principles and Practices for the Design and Construction 

of Flood Resistant Building Utility Systems. 

3.6.8	 Enclosed Areas

Generally, the space beneath the lowest floor of manufactured home is covered by some type of 

skirting or screening. Manufactured homes are also often installed on an enclosed foundation 

(see Chapter 8) when the BFE is several feet above grade. 

When an enclosed area below the lowest floor of a manufactured home is created, special attention is needed for two reasons:

n The walls of enclosed areas are subject to flood damage from hydrostatic and 

hydrodynamic forces.

n People are tempted to convert enclosures that are intended to flood into areas that can 

sustain damage in a flood.

To protect enclosed areas below the lowest floor, the community must:

"Require, for all new construction and substantial improvements, that fully enclosed areas below the lowest floor that are usable solely for parking of vehicles, 

building access or storage in an area other than a basement and which are subject 

to flooding shall be designed to automatically equalize hydrostatic flood forces 

on exterior walls by allowing for the entry and exit of floodwaters. Designs for 

meeting this requirement must either be certified by a registered professional 

engineer or architect or meet or exceed the following minimum criteria: A minimum of two openings having a total net area of not less than one square inch 

for every square foot of enclosed area subject to flooding shall be provided. The 

bottom of all openings shall be no higher than one foot above grade. Openings 

may be equipped with screens, louvers, valves, or other coverings or devices provided that they permit the automatic entry and exit of floodwaters." [44 CFR 

60.3(c)(5)]

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The NFIP allows only certain uses of the enclosed area below the lowest floor of an elevated 

manufactured home. These spaces can only be used for parking of vehicles, building access, or 

storage.

Additionally, the enclosure must be designed to automatically equalize hydrostatic flood forces 

on exterior walls by allowing for the entry and exit of floodwaters. To protect the enclosure, the 

NFIP minimum requirements are:

n A minimum of two openings in walls of the enclosure having a total net area of not less 

than 1 square inch for every square foot of enclosed area subject to flooding. 

n The bottom of all openings must not be higher than 1.0 foot above the higher of the 

exterior or interior grade or floor immediately below the opening.

n Openings must permit the automatic entry and exit of floodwaters. The openings may be 

equipped with screens, louvers, valves, or other coverings or devices provided that they 

meet this requirement.

n Alternative designs are allowed but must be certified by a registered professional engineer 

or architect as meeting the requirement to automatically equalize forces on exterior walls 

by allowing for the entry and exit of floodwaters. 

To further minimize flood damages, mechanical, electrical, plumbing equipment, and other 

service facilities must be designed and/or located so as to prevent damage during conditions 

of flooding (see Section 3.6.7). Enclosures below the BFE must also be constructed with flood 

damage-resistant materials. 

If the area below the manufactured home is enclosed with rigid skirting that is less likely to be 

dislodged by wind or flood loads, openings are required to equalize hydrostatic pressure in accordance with NFIP regulations.

For further information on enclosed areas refer to FEMA Technical Bulletin 1, Openings in 

Foundation Walls and Walls of Enclosures Below Elevated Buildings in Special Flood Hazard Areas in Accordance with the National Flood Insurance Program. 

3.6.9	 Floodways

If a community receives a proposal for the development in the SFHA, the community must 

ensure that the proposal will not impose flood problems on other properties. To prevent this, 

communities must adopt floodways to designate those areas where flood flows are most sensitive 

to changes brought on by development. The NFIP defines the floodway as:

"....the channel of a river or other watercourse and adjacent land area that must 

be reserved in order to discharge the base flood without cumulatively increasing 

the water surface elevation more than a designated height." [44 CFR 59.1]

A floodway is the central portion of a riverine floodplain needed to carry the deeper, faster moving water. A floodway is included with most riverine Flood Insurance Studies and will generally 

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be shown on the FIRM or DFIRM as indicated in Figure 3-5. A floodway schematic is provided 

in Section 4.4.1.1 (Figure 4-2). The area chosen for the regulatory floodway must be designed 

to carry the waters of the 1-percent annual chance flood without increasing the water surface 

elevation of that flood more than 1 foot at any point. 

Once a community has adopted a floodway, it must prohibit development in the floodway unless it has been demonstrated through H&H analyses performed using standard engineering 

practice that the development will not result in any increase in flood levels during the base 

flood. This “encroachment analysis” is required under the following NFIP provision: 

"Prohibit encroachments in the floodway, including fill, new construction, substantial improvements, and other development within the adopted regulatory 

floodway unless it has been demonstrated through hydrologic and hydraulic 

analyses performed in accordance with standard engineering practice that the 

proposed encroachment would not result in any increase in flood levels within the community during occurrence of the base flood discharge." [44 CFR 

60.3(d)(3)]

FEMA defines “any increase” as meaning a zero increase (greater than 0.00 feet). This analysis is usually called a “no-rise” or “zero-rise” analysis and results in a “no-rise” or “zero-rise” 

certification by a qualified registered professional engineer. Remember that considerable encroachment into the floodplain was already allowed when the floodway was designated by the 

community. Although some communities or States perform the H&H analyses themselves, most 

Figure 3-5. DFIRM

showing the floodway

(cross-hatched area in

Zone AE).

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require the permit applicant to obtain the services of a qualified registered professional engineer to perform the analysis and provide the certification. Generally, very little development or 

disturbance is permitted in the floodway because even minor encroachments in the floodway 

lead to increases in the BFE. Unless the engineering analysis demonstrates that there will not be 

an increase in the BFE as a result of the development, the permit must be denied.

For streams and other watercourses where FEMA has provided BFEs, but no floodway has been 

designated, the community must review developments on a case-by-case basis to ensure that 

these increases do not occur, as stated in 44 CFR 60.3(c)(10), in which communities must:

"Require until a regulatory floodway is designated, that no new construction, substantial improvements, or other development (including fill) shall be permitted 

within Zones A1–30 and AE on the community’s FIRM, unless it is demonstrated 

that the cumulative effect of the proposed development, when combined with all 

other existing and anticipated development, will not increase the water surface 

elevation of the base flood more than one foot at any point within the community." [44 CFR 60.3(c)(10)]

3.7	 NFIP Requirements for Manufactured Homes in Coastal Flood

Areas

Flood hazard areas along coastlines that are subject to flooding from storm surge and wave impacts during coastal storms and hurricanes are designated on the FIRMs as Zones V1-30, VE, 

and V. In general, different standards than those in riverine floodplains apply in V zones to help 

manufactured homes withstand wave impacts. Many V zones are also subject to erosion and 

scour, which can undercut manufactured home foundations. 

In addition, some A zones near the coast, generally located landward of and contiguous to V zones, experience V 

zone flooding characteristics, including wave action, high 

velocity flows, and erosion or scour. These are referred to 

as Coastal A zones. While the forces in Coastal A zones 

are not as severe as those in V zones, they can still cause 

damage to manufactured housing and foundations.

Although there are no distinct NFIP floodplain management regulations for Coastal A zones, this guide 

recommends that V zone regulations be followed within 

Coastal A zones where flood forces are generally stronger 

and more destructive than those in non-Coastal A zones.

44 CFR 60.3(e) outlines the NFIP requirements for manufactured homes in coastal flood hazard 

areas in relation to elevation and anchoring, fill, enclosed areas and breakaway walls, and setbacks. Each of these requirements will be addressed in Sections 3.7.1 through 3.7.4, respectively. 

Coastal A Zones

NFIP regulations do not differentiate between Coastal and nonCoastal A zones. However, this 

guide recommends that manufactured housing in Coastal A 

zones be installed to be more 

resistant to flood forces found 

in coastal flood areas, including 

wave effects, velocity flows, erosion, and scour.

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In addition to these requirements, manufactured homes in coastal flood areas are required to 

meet many of the same requirements as manufactured homes constructed in riverine and inland flood areas, such as the use of flood damage-resistant materials below the BFE (Section 

3.6.6) and protection of utilities and mechanical equipment (Section 3.6.7). 

3.7.1	 Elevation and Anchoring

Manufactured homes placed or substantially improved in V zones must be elevated so that the 

bottom of the lowest horizontal structural member of the lowest floor is elevated to or above 

the BFE in sites: 

"(i) Outside of a manufactured home park or subdivision, (ii) In a new manufactured home park or subdivision, (iii) In an expansion to an existing manufactured 

home park or subdivision, or (iv) In an existing manufactured home park or subdivision on which a manufactured home has incurred substantial damage as the 

result of a flood, meet the standards of paragraphs (e)(2) through (e)(7) of 44 

CFR 60.3." [44 CFR 60.3(e)(8)]

Elevation requirements for structures in the V zones differ from those in the A zones. In A1-30, 

AE, A, and AO zones, the finished floor elevation (i.e., top of floor) must be elevated to or above 

the BFE. Structures in V zones must be elevated so that “the bottom of the lowest horizontal structural 

member of the lowest floor is elevated to or above the BFE.” For manufactured homes, the lowest horizontal structural member would generally be the chassis main steel beams. Having the bottom 

of the lowest horizontal structural member to or above the BFE minimizes flood damages due 

to wave action. Figure 3-6 illustrates the reference level requirement.

44 CFR 60.3 (e)(4) requires the community to: 

“Provide that all new construction and substantial improvements in Zones V1-30, 

VE, and also Zone V if base flood elevation data is available, on the community’s 

FIRM, are elevated on pilings and columns so that (i) the bottom of the lowest 

horizontal structural member of the lowest floor (excluding the pilings or columns) is elevated to or above the base flood level; and (ii) the pile or column 

foundation and structure attached thereto is anchored to resist flotation, collapse and lateral movement due to the effects of wind and water loads acting 

simultaneously on all building components. Water loading values used shall be 

those associated with the base flood. Wind loading values used shall be those required by applicable State or local building standards. A registered professional 

engineer or architect shall develop or review the structural design, specifications 

and plans for the construction, and shall certify that the design and methods of 

construction to be used are in accordance with accepted standards of practice for 

meeting the provisions of paragraphs (e)(4)(i) and (ii) of this section.”

Satisfying 44 CFR 60.3(e)(4)(ii) means that flood and wind loads are to be considered acting simultaneously in the design of elevated portions of the building, its foundation, and 

its attachment to the structure, so that the building will not be subject to flotation, collapse, 

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displacement, or other structural damage. Design wind loads are to comply with State or local 

building standards.

Manufactured homes to be placed or substantially improved on sites in an existing manufactured home park or subdivision in V zones must be elevated so that the bottom of the lowest 

horizontal structural member of the lowest floor is to or above the BFE or meet the 3-foot pier 

foundation addressed in Section 3.6.3. In the case of the 3-foot foundation, it is important to 

note that structures in V zones are subject to wave action, intense hydrodynamic pressures, and 

generally deeper flood depths than riverine areas. Foundations in V zones need to be designed 

to withstand flood forces from breaking waves and greater flood depths from storm surge. The 

3-foot foundation should not be expected to withstand these forces. For these reasons, the NFIP 

recommends that all manufactured homes in V zones be elevated to or above the BFE. FEMA’s 

Coastal Construction Manual (FEMA 55) recommends that the lowest horizontal structural members in coastal areas should be perpendicular to the expected wave crest.

As discussed in Section 3.2, post-storm investigations have indicated that typical AE Zone constructed buildings along the coast are subject to considerable damage when exposed to waves 

less than 3 feet and as small as 1.5 feet in height. Evidence from these post-storm investigations sug­gests that design and construction requirements in some portions of Coastal AE zones 

should be more like the VE zone requirements to minimize damages. 

Figure 3-6.

Manufactured home with

the bottom of the lowest

horizontal structural

member elevated to the

BFE.

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To help communities identify those Coastal AE zone areas subject to damaging wave heights, 

FEMA will delineate the landward limit of the area affected by waves greater than 1.5 feet in 

height on the FIRMs and include this information in the DFIRM database as an informational 

layer. As indicated in Section 3.2, FEMA refers to this limit as the LiMWA. Communities are encouraged but not required to adopt the coastal AE Zone area defined by the LiMWA and adopt 

and enforce higher building standards than the minimum NFIP requirements in these areas.

Furthermore, the 2009 International Building Code® (IBC®) references the American Society of 

Civil Engineers (ASCE) 24-05 Flood Resistant Design and Construction standard, which has specific 

design requirements, similar to Zone VE construction, that apply to areas that may be affected 

by waves greater than 1.5 feet (which ASCE 24 refers to as Coastal A zones). In addition, every FEMA coastal construction publication since the issuance of FEMA 55, Coastal Construction 

Manual, dated June 2000, has recommended the use of Zone VE construction practices in areas 

subject to wave heights greater than 1.5 feet. The NFIP's CRS provides credits for communities 

that adopt and enforce more stringent floodplain management requirements in these areas.

3.7.2	 Fill

Due to the wave action and more intense hydrodynamic forces experienced in coastal flood areas that lead to erosion and scour, elevating on structural fill is not permitted in V zones. Under 

the NFIP, the use of fill is prohibited for structural support.

“[The community must ] Prohibit the use of fill for structural support of buildings 

within Zones V1-30, VE, and V on the community’s FIRM.”[44 CFR 60.3(e)(6)]

While fill is not allowed for structural support, a limited amount of fill is allowed for landscaping, local drainage needs, and to smooth out a site for an unreinforced concrete pad. Fill must 

not divert floodwaters or deflect floodwaters and waves beneath elevated buildings. Fill must 

also not divert floodwaters or deflect waves such that increased flood damages is sustained by 

adjacent and nearby buildings.

For further information on fill in coastal flood areas refer to FEMA Technical Bulletin 5, Freeof-Obstruction Requirements for Buildings Located in Coastal High Hazard Areas in accordance with the 

National Flood Insurance Program. 

3.7.3	 Enclosed Areas and Breakaway Walls

The preferred method of constructing a V zone building is to leave the area below the elevated 

floor free of obstruction or to enclose the area only with latticework or insect screening. That 

way waves can freely flow under the building without placing additional loads on the foundation. The only solid foundation walls allowed below the lowest floor of a manufactured home in 

a V zone area are breakaway walls that will give way under wind and water loads without causing 

collapse, displacement, or other damage to the elevated portion of the manufactured home or 

supporting to the piles. The NFIP requirements for enclosures below an elevated building are: 

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“[The community must] Provide that all new construction and substantial improvements within Zones V1-30, VE, and V on the community’s FIRM have the 

space below the lowest floor either free of obstruction or constructed with nonsupporting breakaway walls, open wood lattice-work, or insect screening intended 

to collapse under wind and water loads without causing collapse, displacement, 

or other structural damage to the elevated portion of the building or supporting 

foundation system. For the purpose of this section, a breakaway wall shall have a 

design safe loading resistance of not less than 10 and no more than 20 pounds 

per square foot. Use of breakaway walls which exceed a design safe loading resistance of 20 pounds per square foot (either by design or when so required by 

local or State codes may be permitted only if a registered professional engineer 

or architect certifies that the designs proposed meet the following conditions.....” 

[44 CFR 60.3(e)(5)]

Just as in A zones, the space enclosed by walls is to be used solely for parking of vehicles, building 

access, or storage, and must be constructed with flood-resistant materials. Solid breakaway walls 

are allowed, as are garage doors that meet the same breakaway requirements. Solid breakaway 

walls are intended to collapse under the force of wave impacts without damaging the building's 

foundation or the elevated portion of the building. All solid breakaway walls should have their 

designs certified by a registered professional engineer or architect. The area enclosed by solid 

breakaway walls should be limited to less than 300 square feet because flood insurance rates increase dramatically for enclosures larger than 300 square feet.

For further information on enclosed areas and breakaway walls in coastal flood areas, refer to 

FEMA Technical Bulletin 5, Free-of-Obstruction Requirements for Buildings Located in Coastal High 

Hazard Areas in accordance with the National Flood Insurance Program and Technical Bulletin 9, Design and Construction Guidance for Breakaway Walls Below Elevated Buildings Located in Coastal High 

Hazard Areas in accordance with the National Flood Insurance Program. 

3.7.4	 Setbacks

44 CFR 60.3 (e)(3) requires the community to “provide that all new construction within Zones 

V1-30, VE, and V on the community’s FIRM is located landward of the reach of mean high 

tide.”

In coastal areas, there is a mean high tide line used for regulatory purposes. This means that the 

high tide line is physically shifting over time as beach and coastal areas are dynamic. However, 

the community, region, or State will have a regulatory line defined. The NFIP does not allow any 

development or construction to occur seaward of this line. Figure 3-7 illustrates how the mean 

high tide line defines the area of no development in coastal regions. Figure 3-8 shows coastal 

development in an area where setback requirements are enforced.

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3.8	 Existing Manufactured Homes in Flood Hazard Areas

3.8.1	 Relocation

The best way to prevent damage to manufactured homes and to prevent loss of life and injury 

during a flood is to install manufactured homes outside of flood-prone areas. For existing manufactured homes, relocation of the manufactured home out of the floodplain may be a viable 

option to minimize damages.

Manufactured homes are relatively easy to transport and can be moved from areas prone to 

flooding to sites located outside of the flood hazard area. This not only allows the homeowner 

to prevent damage to the manufactured home, but also provides peace of mind concerning 

safety risks and property damages due to flooding.

Figure 3-7. Mean high

tide line development

restriction.

Figure 3-8. Coastal

development well-suited

to the land: generous

setbacks, in combination

with deep lots and

avoidance of dune areas,

should afford protection

from erosion and flooding

for years to come.

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Some issues to consider when evaluating relocation as a mitigation option include:

n Will it be cost-effective (cost of incurring damages versus cost of preventing damages)? 

n Is the home in good enough condition for transporting?

n Is the removal of existing utility connections easily done prior to transportation?

n Is there a suitable new site for relocation?

n Are utilities with similar connections available at the new site?

n Is there a route that will provide adequate clearance for transporting the manufactured 

home?

The relocation process consists of nine steps that are fairly clear, but include many design considerations:

Step 1. Select a manufactured home moving contractor

Step 2. Analyze the existing site and manufactured home conditions

Step 3. Select, analyze, and design foundation for the relocated home at the new site

Step 4. Prepare the existing site

Step 5. Evaluate and plan the moving route

Step 6. Prepare the manufactured home for relocation

Step 7. Move the manufactured home

Step 8. Prepare the new site

Step 9. Restore the old site

More information on relocation as a retrofitting technique can be found in Chapter 7 of FEMA 

312, Homeowner’s Guide to Retrofitting (2nd Edition, 2009), and FEMA 259, Engineering Principles 

and Practices of Retrofitting Flood-Prone Residential Structures (2nd Edition, 2001).

3.8.2	 Evacuation

Evacuation of a manufactured home from a site when there is a flood threat can result in extensive flood damages being avoided to the home. Communities may have procedures in place for 

evacuating homes when there is a flood threat and may provide assistance to the occupant of 

the manufactured home to remove the manufactured home to a protected site. The NFIP will 

pay up to $1,000 for the reasonable expenses incurred to move an insured manufactured home 

to a place other than the described location that contains the manufactured home in order to 

protect it from flood or the imminent danger of flood. Any insured manufactured home moved 

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to a place other than the described location must be placed outside of the Special Flood Hazard 

Area or above ground level. 

Communities may allow the evacuation of manufactured homes in Zones A1-30, AH, and AE 

only in the following cases: 

n Evacuation of a manufactured home from an existing manufacture home park or 

subdivision; or 

n Evacuation of a manufactured outside of a manufactured home park or subdivision.

After the threat of flooding from a specific event has passed, these manufactured homes can be 

returned to their original site without being elevated in accordance with the requirements at 

either 44 CFR 60.3(c)(6) or (c)(12) provided the manufactured home was not subject to the 

elevation requirements in these two paragraphs when first installed on the site. 

Communities that allow existing manufactured homes to be removed to avoid flooding should 

have some type of permitting and tracking system in place to ensure that the same home is 

being returned to the original permanent site or pad. Communities may require a permit to 

ensure that utility reconnections meet local codes. If a community has developed a plan that 

pre-identifies manufactured homes that would be removed in the event of a flood, the plan 

should include procedures for determining which manufactured homes are to be removed and 

which ones are to be returned to their original site or pad. If a community has not developed 

a plan or individual manufactured homeowners initiate removal of their home on their own, 

the community is still responsible for determining whether the same home is being returned to 

the original site or pad. Since much can go wrong in trying to evacuate a manufactured home, 

evacuation is not a substitute for permanently protecting the manufactured home by elevating 

it to or above the BFE.

The community should issue some type of re-occupancy permit for owners who want to return 

the home to the original site or pad as a record to distinguish these from new or substantially 

improved manufactured homes that must meet the community’s floodplain management regulations. A variance under the NFIP floodplain management regulations at 44 CFR 60.6(a) is not 

required for returning a manufactured home that was evacuated to the original site or pad because it is not a new placement, a replacement of, or substantial improvement. 

3.8.2.1	 Manufactured Home Substantially Improved or Returned to a Different Site or Pad in an

Existing Manufactured Home Park

If the manufactured home that was evacuated due to the threat of flooding is placed on a 

different site or pad in the existing manufactured home park or subdivision or is substantially improved on the original site or pad, it would have to meet the requirements at 44 CFR 

60.3(c)(12) in all A zones (the manufactured home would have to be elevated to the BFE or be 

elevated at 36 inches in height above grade). 

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3.8.2.2	 Manufactured Home Placed in a New Manufactured Home Park or Subdivision

If the manufactured home that was evacuated due to the threat of flooding is placed in a new 

manufactured home park or subdivision or in the expanded part of an existing manufactured 

home park or subdivision, or is placed on a different site outside a manufactured home park or 

subdivision, it would have to meet the elevation requirements of 44 CFR 60.3(c)(6) in Zones 

A1-30, AE, and AH and elevate to the BFE or 44 CFR 60.3(e)(8) in Zones V1-30, VE, and V and 

elevate to the BFE.

3.9	 HUD Manufactured Home Construction and Safety Standards

Prior to 1975, there were no Federal mandatory regulations or standards governing the design and construction of manufactured homes. Since 1976, HUD’s Manufactured Home 

Construction and Safety Standards (MCHSS), or “HUD codes,” have regulated the design and 

construction of factory-built manufactured housing. 

After Hurricane Andrew in 1992, the HUD codes underwent many improvements regarding 

design and construction of manufactured homes. The lateral (wind) load provisions were updated to a modified American National Standards Institute/American Society of Civil Engineers 

(ANSI/ASCE) 7-88 design standard.

The HUD codes were revised to include three wind zones (Figure 1-1). Zone II homes are designed to resist a 100-mph (fastest mile) wind event; Zone III homes are designed for 110 mph. 

The design wind speed for Zone I homes is not specified. Rather, HUD I homes are designed to 

resist specified lateral and uplift wind pressures. Also, as point of interest, the HUD codes' energy design requirements were updated around this time frame. HUD codes program standards 

are found in 24 CFR 3280. 

HUD’s MHCSS include provisions for support and anchoring systems based on HUD Wind 

Zones I, II, and III. HUD’s standards specify design requirements for the body and frame of the 

manufactured home. However, the HUD standards do not take seismic loading into account for 

the design of foundations and attachments.

The program has been in effect since 1976, and all manufactured homes built on or after June 

15, 1976, are subject to HUD requirements. Although building codes are usually regulated at 

a local level, an exception is made with manufactured housing because it is most often built 

outside of the locality where it is eventually sited for use. Prior to the MHCSS, the quality of 

manufactured housing was unreliable, which resulted in the severe restriction or banning of the 

housing within several localities.

Under the program, manufacturers who build manufactured homes for sale in the U.S. are 

required to follow HUD-mandated design and construction standards. The standards are enforced either through HUD or approved Primary Inspection Agencies (either private firms or 

State administrative agencies) that perform reviews of manufacturers’ designs and inspect the 

homes during construction to ensure compliance with the standards. 

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According to 24 CFR 3280, the purpose of the MHCSS is to establish standards for “all equipment 

and installations in the design, construction, transportation, fire safety, plumbing, heat-producing and electrical systems of manufactured homes which are designed to be used as dwelling 

units.” 

A manufactured home designed and constructed in accordance with the MHCSS standards will 

have a data plate and a certification label attached to it. The data plate will be affixed in a permanent manner near the main electrical panel or other readily accessible and visible location. 

The data plate contains the following information:

n The name and address of the home’s manufacturing plant

n The serial number and model designation of the unit

n The date it was manufactured

n A list of the certification label numbers that are affixed to each transportable section

n A list of factory-installed equipment, including the manufacturer’s name and the model 

number for each appliance

n Reference to the roof and wind load zones for which the home is designed

The certification label will be a 2-inch by 4-inch aluminum plate. The plate will be permanently 

attached to the home on the tail light end of the home, approximately 1 foot from the floor 

and 1 foot in from the road side. Figure 3-9 is a certification plate for homes built after June 15, 

1976. 

Siting and installation requirements for manufactured homes are generally a State or local 

regulatory responsibility, where such regulatory authority exists. Some State and local regulations require licensed or registered installers, and/or require permits to be obtained prior to 

installation. Inspections are often required in conjunction with the permits, to ensure correct 

Figure 3-9. Certification

plate for manufactured

homes built after June

15, 1976.

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installation. The new installation standards in Part 3285 dictate that manufacturers’ installation 

instructions be approved by a Design Approval Primary Inspection Agency (DAPIA) and the 

protections contained therein meet or exceed all requirements of HUD’s model installation 

standards. This requirement is to ensure that the purchaser has the correct procedures available for installing the manufactured home. 

As part of the 1994 changes, the provisions for “Windstorm protection” in 24 CFR 3280.306 

that address anchoring systems were also modified to be consistent with the revisions made for 

wind design requirements for manufactured housing in high-wind areas. This section of the 

standard requires that the manufacturer provide instructions, including drawings and specifications, detailing at least one acceptable system of anchoring the manufactured home that 

has been approved by a registered professional engineer or architect (24 CFR 3280.306(b)). 

Requirements are detailed in 24 CFR 3280.306 for the instructions and stipulate wind loading 

requirements for design. At this time, the standards do not instruct the designer to consider the 

forces of floodwaters or seismic loads when designing the foundation system. 

3.10	HUD Model Manufactured Home Installation Standards

Siting and installation requirements for manufactured homes are generally a State or local 

regulatory responsibility, where such regulatory authority exists. Some State and local regulations require licensed or registered installers, and/or require permits to be obtained prior to 

installation. Inspections are often required in conjunction with the permits, to ensure correct 

installation. The new installation standards in Part 3285 dictate that manufacturers’ installation 

instructions be approved by a Design Approval Primary Inspection Agency (DAPIA) and the 

protections contained therein meet or exceed all requirements of HUD’s model installation 

standards. This requirement is to ensure that the purchaser has the correct procedures available for installing the manufactured home.

The Manufactured Housing Improvement Act of 2000 required that HUD establish minimum 

National Model Installation Standards (which were published in 24 CFR 3285 in October 2007). 

The Act also set out requirements that States must meet in order to continue to regulate manufactured home installation in their State. To be eligible, States must meet certain requirements 

as indicated in HUD’s Installation Program published in June 2008, including a requirement 

that State Standards meet or exceed HUD’s Model Installation Standards. 

State Manufactured Home Installation Programs

As of January 2009, according to HUD’s Office of Manufactured Housing Programs, the following 

States have Approved or Conditionally Approved Manufactured Home Installation Programs: Alabama, 

Arizona, Arkansas, California, Kentucky, Ohio, Pennsylvania, South Carolina, Tennessee, Washington, 

and West Virginia.

The following States have submitted applications for Manufactured Home Installation Programs: 

Georgia, Idaho, Indiana, Kansas, Maine, Michigan, Minnesota, Mississippi, Nevada, New Jersey, New 

York, North Carolina, North Dakota, Oklahoma, Texas, and Virginia.

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The installation standards in 24 CFR 3285 include requirements for pre-installation and installation considerations, site preparation, foundations, and home installation procedures. 

Additionally, standards on installation, preparation, and/or testing of optional features, appliances, utility system connections, and life safety features have been incorporated. The standard 

is designed for adoption and use by authorities who are responsible for the safety and health of 

manufactured home users. The standards address natural hazards (wind, flooding, and freezing) by incorporating the following requirements:

n A section on Installation of Manufactured Homes in Flood Hazard Areas states that FIRMs 

must be consulted prior to installation to determine a property’s flood risk and identify 

whether floodplain development permits may be necessary.

n In areas prone to flooding, elevation and anchoring techniques are required, as well 

as consultation with a registered professional engineer and local authorities to ensure 

installation conforms to all applicable codes and regulations.

n Piers, anchoring, and support systems of the foundation in flood hazard areas must be 

capable of resisting loads associated with design flood and wind events.

n Oil storage tanks in flood hazards areas must be protected from damage by anchoring and 

elevation or other design.

n Appliances located on the outside of the manufactured home and air inlets must be 

elevated to or above the design flood elevation (DFE), which is defined in Appendix D.

n Special requirements for footings and foundations for manufactured home placement in 

freezing climates apply.

n The installer is required to secure the manufactured home against the wind according to 

the HUD codes.

n Anchor type requirements (longitudinal) for manufactured homes subject to higher winds 

(Wind Zones II and III) apply.

n Provisions for maximum diagonal tie-down strap spacing, dependent on the wind zone in 

which the property is located, apply.

The standard can be viewed online at http://www.nfpa.

org/assets/files/PDF/CodesStandards/MHCCHUD/

MHCCInstallationStandardsFinalRule.pdf.

In October 2007, HUD issued 24 CFR 3285 Model Manufactured Home Installation Standards, which regulates 

manufactured home installations.

The new standard creates a baseline standard for manufactured housing installation; it presents the minimum 

requirements for installation that must be equaled or exceeded by States and are also a minimum requirement for 

manufacturers’ installation instructions. 

Manufactured Home 

Installation Standards for 

Flood Hazard Areas

24 CFR 3285.302 also requires 

manufacturers to indicate in the 

installation instructions if the foundation specifications have been 

designed for flood-resistant considerations and, if so, the conditions of applicability for velocities, depths, or wave action; or to 

indicate if the foundations are not 

designed to address flood loads.

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Model Manufactured Home Installation Standards 24 CFR 3285.302 – Flood Hazard Areas requires:

“...foundations, anchoring and support systems must be capable of resisting loads 

associated with design flood and wind events, or combined wind and flood wind 

events, and homes must be installed on foundation supports that are designed 

and anchored to prevent flotation, collapse, or lateral movement of the structure” 

3.11	Model Building Code Requirements

Model building codes can be adopted and enforced by localities and States to specifically address manufactured housing installation. Two major building codes within the U.S. are the ICC’s 

International Code Series, and NFPA 5000, Building Construction and Safety Code (2006). The 

International Code Series includes the IRC for 2006 (IRC 2006), which applies to one- and twofamily dwellings. In addition, the IRC for 2009 (IRC 2009) is now available.

3.11.1	 IRC 2006

The IRC 2006 is a comprehensive residential building code that includes provisions for development in the SFHAs that meet the minimum requirements of the NFIP. Appendix E of the IRC 

2006 contains a section dedicated to “Manufactured Housing Used as Dwellings.” Section 502 

of this appendix describes requirements for foundation systems, which, in part, refer to the general provisions of the code. Under the IRC Appendix E, all footings for manufactured housing 

shall be as follows:

n Extend below the frost line

n Be constructed of materials specified by the code, including masonry and concrete

n Be made of solid material (concrete and masonry footings)

n Be designed in accordance with seismic specifications, including minimum reinforcements 

near the bottom of the footing, and specifications on joints between footings and stem 

walls if located in a Seismic Design Category D1 or D2 area

n Be elevated to the DFE for new and replacement manufactured homes

Appendix E, Section AE604.1 of the IRC 2006 also details requirements for anchorage installation. The code ground anchor requirements are:

n The load carrying portion of the ground anchors shall be installed to the full design depth 

and shall extend below the established frost line into undisturbed soil.

n The manufacturer’s installation directions should provide preload requirements and load 

capacities for various soil types. 

n Approved ground anchors must be capable of resisting an allowable working load 3,150 

pounds in the direction of the tie plus a 50 percent overload or 4,725 pounds without 

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failure. Failure is considered to have occurred when the anchor moves more than 2 inches 

at a load of 4,725 pounds.

n Ground anchors designed to be installed so that loads on the anchors are other than 

direct withdrawal must be designed and installed to resist an applied design load of 3,150 

pounds at 40 to 50 degrees from vertical without displacing the end of the anchor more 

than 4 inches horizontally.

Appendix E, Section AE605 of the IRC 2006 establishes performance requirements for ties, materials, and installation. The requirements include:

n Ties must be attached to ground anchors with turnbuckles or other adjustable tensioning 

devices, and

n Ties must be capable of resisting a working load of 3,150 pounds with less than a 2 percent 

elongation, and a 50 percent overload of 4,725 pounds.

FEMA has evaluated the IRC 2006 and has determined that it is consistent with NFIP regulations. The IRC 2009 has also been determined to be consistent with NFIP regulations.

3.11.2	 NFPA 5000

NFPA 5000’s Chapter 39 (Flood-Resistant Design and Construction) includes many of the minimum NFIP requirements. According to the code, manufactured housing located in the flood 

hazard area must adhere to the following requirements:

n Foundations must be designed and constructed as required by Structural Engineering 

Institute (SEI)/ASCE 24, Flood Resistant Design and Construction.

n In flood areas subject to high velocity wave action, structural fill, slabs-on-ground, and 

foundation walls are prohibited.

n The lowest floor of the structure must be elevated to or above the DFE (see Appendix D 

for DFE description).

n Manufactured homes must be installed using methods that minimize flood damage and 

shall be securely anchored to a foundation (meeting the requirements of SEI/ASCE 24). 

These requirements are in addition to the manufacturer’s specifications and State and 

local anchoring requirements for resistance to wind. 

n Where temporary structures are proposed, the authority having jurisdiction must consider 

flood warning time as well as the location of the structure in relation to any floodways, 

alluvial fan areas, or high velocity wave action areas before granting a permit. 

NFPA 5000 also dictates that ASCE 7 is to be used to determine the design loads associated with 

environmental loads, including wind, seismic, snow, and ice events. The code states that all 

buildings and other structures shall be designed to resist these design loads.

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3.11.3	 NFPA 501

NFPA 501, Standard on Manufactured Housing, addresses the design, manufacture, and transportation of manufactured homes and adopts, by reference, ASCE 7-2002. NFPA 501 also added 

some seismic provisions by requiring all homes placed in seismic areas to have their water heaters braced to prevent falling or horizontal movement. 

3.11.4	 NFPA 225

NFPA 225, Model Manufactured Home Installation Standard, is the first multi-hazard installation 

standard for manufactured homes. NFPA specifies wind provisions (ASCE 7-02), includes provisions for homes sited in SFHAs, and contains the latest National Earthquake Hazards Reduction 

Program (NEHRP) criteria for homes susceptible to earthquake damages. 

FEMA has evaluated the 2005 edition of NFPA 225 and has determined it is consistent with NFIP 

regulations. In addition, the 2009 edition of NFPA 225 is now available and has also been determined to be consistent with NFIP regulations.

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4	 Site and Development

Options

Site evaluation is a critical step in the decision-making process of installing a manufactured 

home. Decisions are sometimes made without accurately assessing or considering a property’s vulnerability to flooding and other hazards. By conducting a thorough site assessment, 

a property owner or developer can mitigate the following types of losses, damage, and associated problems:

n Injuries and loss of life

n Damage or loss to buildings and contents

n Damage to attendant infrastructure

n The need for emergency evacuation

A complete and detailed understanding of the advantages and disadvantages of potential sites 

for manufactured home installation helps lead to informed decisions; not all sites are suitable 

for development. 

A thorough evaluation of property for development purposes is completed through a five-step 

process:

Compile lot/parcel information for one or more candidate properties.

Determine if local, regional, State, and Federal regulatory requirements allow the development and installation of a manufactured home and, if so, determine the requirements.

Identify the hazards present at the site (e.g., wind, flooding, seismic, etc.).

Determine whether potential effects from hazards could be avoided through siting, design, 

and construction and whether the residual risks to the site and the building are acceptable.

Proceed either with the purchase or development of a property, or reject the candidate site, 

and find and evaluate alternative sites. 

Basic information about the property (e.g., lot size, zoning and land use requirements, and utility and infrastructure availability) enables the potential property owner or developer to assess if 

the site is suitable for development as a manufactured home site. If development is feasible, the 

potential property owner should conduct Steps 3 and 4 of the process to assess the property’s 

hazard vulnerability. Based on the assessment findings, the property is determined to be either 

suitable or unsuitable for the installation of a manufactured home. The following sections provide additional information for the recommended assessment process.

.

.

3.

4.

.
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4.1	 Step 1: Compiling Site Information

After a site for possible purchase and/or development has been identified, basic information 

about the site is collected for a development analysis. It is vital that all pertinent information is 

collected in order to complete a thorough analysis.

Community building permitting or planning and zoning offices are helpful in steering potential 

property buyers toward information they need to evaluate a property. This is true of individuals 

looking at isolated lots or lots within existing manufactured housing subdivisions. A checklist of 

information to be collected is provided in Table 4-1. Not all of this information will be available 

for all sites and properties.

Although much of the information listed in Table 4-1 will need to be obtained locally, State and 

Federal government resources are also useful. Table 4-2 presents a partial listing of the most 

likely resources (i.e., local, regional, or State government) for obtaining specific information. 

Table 4-1. Information Checklist

Property Location

Municipal, township, county, or other local jurisdiction

Street address

Parcel designation (e.g., tax map ID)

Subdivision information

Special zoning or land use districts

•

•

•

•

•

Flood-prone area designation

Seismic hazard area designation 

High-wind area designation

Other hazards areas

Natural resource protection area designation

•

•

•

•

•

Property Dimensions

Total acreage

Property shape

Property elevations and topography

Location relative to adjacent properties; configuration of 

adjacent properties

•

•

•

•

Acreage outside of designated floodplain area 

or other hazard-prone area

Acreage landward/outside of natural, physical, 

or regulatory construction or development limits 

(i.e., usable acreage)

•

•
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Legal and Regulatory Information

Land use designation at property and adjacent 

properties

Zoning classification and resulting restrictions on use

Building codes and local amendments

Floodplain management requirements 

Erosion hazard area: construction setback and 

regulations

Natural resource protection area: siting, construction, or 

use restrictions

Easements and rights-of-way on property (including 

beach access locations for nearby properties or the 

general public)

Local/State siting and construction regulations

Regulatory front, back, and side setbacks

•

•

•

•

•

•

•

•

•

Local/State permitting procedures and 

requirements 

Local/State regulations regarding use, 

construction, and repair of erosion control 

measures

Riparian rights

Local/State restrictions on cumulative repairs or 

improvements

Conditions or other requirements attached to 

building or zoning permits

Subdivision plat covenants and other 

restrictions imposed by developers and 

homeowners’ associations

Hazard disclosure requirements for property 

transfer, including geologic hazard reports

•

•

•

•

•

•

•

Physical and Natural Characteristics

Soils, geology, and vegetation – site and region

Site drainage – surface water and groundwater

Storm, erosion, and hazard history of property

Development/potential flood debris upstream and 

adjacent to property

Erosion control structure on site: type, age, condition, 

and history

•

•

•

•

•

Proximity to inlets and navigation structures

Previous or planned community/regional beach/

dune restoration projects (coastal)

Relative sea-level/water-level changes – land 

subsidence or uplift (coastal)

Wind missile hazards – utility poles, trees

•

•

•

•

Infrastructure and Supporting Development

Access road(s)

Emergency evacuation route(s)

Electric, gas, water, telephone, and other utilities – 

on-site or off-site lines and hookup

•

•

•

Sewer or septic

Limitations imposed by utility, infrastructure 

locations on property use

•

•

Financial Considerations

Intended use: owner-occupied or rental property

Real estate taxes

Development impact fees

Permit fees

Hazard insurance: availability, premiums, deductibles, 

and exclusions

Property management fees

•

•

•

•

•

•

Special assessments for community/association 

projects (e.g., private roads and facilities)

Maintenance and repair of private erosion 

control structures

Increased building maintenance and repairs in 

areas subject to high winds or wind-driven rain

Building damage costs (insured and uninsured) 

from previous storms, floods, and other hazard 

events

•

•

•

•

Table 4-1. Information Checklist (continued)
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Local Regional State

Deeds and Property Records Deeds and Property Records Building Codes and Standards

Emergency Management Emergency Management Emergency Management

Environmental Protection Environmental Protection Coastal Zone Management 

Building Codes and Permitting Geographic Information Systems Geologic Survey 

Floodplain Management Health Floodplain Management 

Planning and Zoning Planning and Zoning Natural Resource Management

Municipal Engineering Post-storm Damage Assessments Post-storm Damage Assessments

Soil and Geology Soil and Geology

Subdivision Review Subdivision Review

Tax Assessor Tax Assessor

Utilities Utilities

4.2	 Step 2: Reviewing Basic Siting Information

After collecting pertinent site information, a thorough review of the information is necessary 

to ensure that development of the site is feasible. Particularly, building codes and planning and 

zoning codes requirements should be reviewed early in the evaluation process to verify that 

development of the property with the installation of a manufactured home is allowable. If installation is allowable, a review of requirements will assist in location of the manufactured home 

on the site.

4.3	 Step 3: Hazard Analysis and Risk Assessment

A hazard analysis and risk assessment should be performed to determine what hazards exist for 

a particular area or site, and then the risk level for that site should be evaluated. It is important 

to consider flooding in this analysis as well as other hazards such as dam failure, land subsidence, seismic hazards, and severe wind.

For community-wide assessments, there are several FEMA tools available for use to conduct a 

Hazards Analysis and Risk Assessment, including the FEMA software Hazards U.S.–Multi-Hazard 

(HAZUS-MH) and Mitigation Planning How-to Guides (FEMA 386). Under the Disaster Mitigation 

Table 4-2. Partial Listing of Local, Regional, and State Responsibilities
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SITE AND DEVELOPMENT OPTIONS 4

Act of 2000, communities are required to complete hazard mitigation plans, which include a 

hazard identification and risk assessment section, should they wish to remain eligible for postdisaster mitigation funding. As a result, many regional and local governments have already used 

tools like HAZUS-MH and the How-to Guides to conduct their own risk assessments, which may 

be available for public review.

4.3.1	 Flooding

Table 4-3 presents a list of the types of hazards that can result from flooding and the flood sources or types of flood-prone areas where they are generally of concern. 

Table 4-3. Flood Hazards

Flood Hazard Associated Flood Hazard Areas or Property Characteristics

Long duration flooding

Large water bodies (rivers, bays)

Water bodies with slow drainage (lakes, ponds)

•

•

Inadequate storm 

drainage

Flat or poorly graded land

Land located directly adjacent to a flood source

•

•

Impact forces

Coastal areas subject to wave action flooding

Riverine flooding areas subject to high velocity flooding

•

•

High velocity flows

Coastal areas subject to wave action

Steeply-sloped riverine flooding areas or areas otherwise subject to high 

velocity flood flows

•

•

Erosion

Coastal areas subject to wave action

Steeply-sloped riverine areas with high velocity flood flows or areas otherwise 

subject to high velocity flood flows

•

•

Sediment deposition • Coastal overwash areas

Movable stream beds • Dynamic river systems

Flood depth

Areas adjacent to the flood source

Areas with poor capacity for drainage

•

•

Two of the best sources to research a property’s susceptibility to flooding are the community’s 

FIRM and FIS. The FIRM shows the extent of the 1-percent annual chance flood, and might also 

include flooding corresponding to the 0.2-percent annual chance flood (also known as the 500-

year flood). For areas where detailed studies have been performed, FIRMs will also show BFEs 

along streams and may provide delineations of floodways. 
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An FIS summarizes the methods used to develop the hydrologic and hydraulic studies, which together determine the extent of the 1-percent annual chance floodplain. The FIS also presents a 

summary of results of the study, including information about floodwater surface elevations and 

velocities for detailed study areas. Figure 4-1 shows portions of an FIS and a FIRM.

The FIRM is used in conjunction with the FIS to determine the flood risk for a given area or 

property. For detailed study areas, while the FIRM shows the extent of the base flood and the 

BFEs, the FIS includes flood profiles that show flood elevations for various frequency events as 

well as the channel bottom elevation (for riverine flooding) over the length of a given waterway. 

If the property is located in the base floodplain where a detailed study has been performed, the 

FIRM and FIS will provide the following valuable information:

n The elevation of a base flood at a site as well as elevations for the 10-, 50- and 500-year 

floods, which can be found on the flood profiles within the FIS

n The average velocity of floodwaters within the floodway for a base flood (where a floodway 

study was performed)

n Areas of coastal wave action flooding

n Riverine flooding areas

n Areas susceptible to ponding

n Areas susceptible to sheet flow or alluvial fan flooding

n Areas protected by levees

Information about an area’s susceptibility to flooding can also be gathered from community officials and past storm event records and reports. Such reports usually provide information about 

what types of damage were inflicted on structures. For coastal areas, there may be information 

about beach and shoreline erosion.

A field assessment for a particular site is a way to collect information for a hazard analysis. Information gathered during a field visit (ideally conducted by a geologist or an engineer) can be 

used to assess what type of impact such forces will have on a manufactured home located on a 

particular site. This is determined by assessing what type of structures and potential debris are 

located upstream and adjacent to the property that could impact the manufactured home during a flood event. A field assessment can also be conducted to estimate the erosion potential for 

a site by investigating existing land slopes on the site as well as vegetation cover. Finally, the site’s 

grading/topography should be evaluated. As part of the assessment, it should be determined 

whether or not the site will drain in a reasonable amount of time in order to prevent long duration flooding. This issue is generally only relevant for smaller storm events or more localized 

flooding for which floodwaters have somewhere to drain.

4.3.1.1	 Accessibility

The flood risk to both the manufactured home and access routes should be assessed. Inhabitants of manufactured homes in flood-prone areas should prepare to evacuate an area should a 

flood watch be issued by the local or State government and should not wait for flood conditions 
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Figure 4-1. Portions of an FIS and a FIRM.
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to begin before evacuating the area. In fact, drowning in vehicles is the number one cause of 

flood-related deaths and often happens when victims mistakenly think that a washed-out roadway or bridge is still open. Therefore, access to the manufactured home during flood events 

needs to be considered during the siting process.

Inhabitants of manufactured homes in flood-prone areas should never remain within the home 

during a flood, regardless of the home’s elevation level above the BFE. They should always 

evacuate when a flood warning is issued by the local or State government and not wait for flood 

conditions to begin before evacuating the area. 

Actual damage to roadways during flood events, as well as high-wind, seismic, and land subsidence events, can leave roads impassable for several days or for weeks after a major disaster. Even 

roadways that are located outside of, but adjacent to, flood-prone areas can be undermined 

by severe erosion along stream banks. Therefore, even if a home is adequately protected from 

incurring major damage due to a flood event, its location within (or surrounded by) a floodprone area could either force occupants to repeatedly evacuate or become stranded for several 

days if access and egress routes are blocked. 

A clear access and egress path during a flood event ensures that building occupants can evacuate and that emergency and critical services can continue to be provided. Both access and egress 

should be addressed while evaluating a potential site to ensure that feasible alternatives exist. 

Necessary considerations may also include bridges, walkways, and areas of refuge for safe evacuation. 

Similarly, accessibility to potential sites after major wind and seismic events should be considered. After wind and seismic events, roadways can be blocked by debris and remain unusable for 

several days while cleanup efforts are underway. Potential property buyers or property owners 

should consider possible evacuation routes from the property in all cases.

It is important to note that accessibility is more than ingress and egress in an emergency. Frequent and long duration flooding can cause major problems for occupants of flood-prone 

structures; a site is not a good location for a manufactured home if its occupants are forced to 

evacuate several times a year due to a flood threat, or must stay out of their homes for days or 

weeks at a time when they do evacuate. Frequent and/or long-term displacement often results 

in significant financial hardships and emotional distress for flooding victims.

4.3.2	 Other Hazards

In addition to flooding, there are a variety of other hazards that should be considered during 

the manufactured home siting process. Table 4-4 presents some of these hazards, their associated hazard-prone areas, and sources to use to determine an area’s susceptibility. Historical storm 

or hazard event records are additional resources for this hazard analysis; they provide details on 

how hazards have previously affected a given area. 
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Table 4-4. Hazard-Prone Areas

Hazards Associated Hazard Areas Source for Determination

Dam failure Dam failure inundation areas located 

downstream of dam structures

• Dam failure inundation maps usually 

created at the State, regional, or local 

level

•

Land 

subsidence

• Steeply sloped land United States Geological Survey 

(USGS) Landslide Overview Map of the 

Conterminous United States (national 

level)

State Geologist

Local Public Works Director

•

•

•

Seismic • Area within 6 miles of a known fault USGS Seismic Maps (as found in FEMA 

450 and ASCE 7)

Building Codes

•

•

Wind/Debris Coastal areas

Tornado-prone areas

Alluvial fans

•

•

•

Wind Zone Maps (as found in FEMA 320 

and ASCE 7)

Building Codes

•

•

Dam failure inundation maps provide the best tool for determining the risk from a dam failure 

event and are usually available at the State or local level. Every State has a Dam Safety Officer responsible for coordinating its Dam Safety Program. State-specific information about State dam 

safety regulations is available at the Association of State Dam Safety Officials’ web site: http://

www.damsafety.org.

To avoid damage that can be caused in areas subject to land subsidence, manufactured homes 

should be sited away from steep slopes and areas where land subsidence has been known to occur in the past. While the USGS Landslide National Map indicates relative risk on a national 

basis, State geologists and local public work directors can provide better community-specific 

information about landslide risk. A listing of State geologists is available at the Association of 

American State Geologists web site: http://www.stategeologists.org.

USGS seismic maps as well as local officials are the best sources for determining the seismic risk 

for a site. The types of soils present on the site should also be taken into consideration. Poor 

soils, often found in flood-prone areas, can greatly amplify ground motions and seismic forces. 

During wind events, large structures or items such as trees and telephone poles can be damaged 

and either collapse or become separated from their anchoring to the ground to become sizable 

pieces of debris. Manufactured homes should be sited away from or in a manner that provides 

protection from such items since airborne debris can act as missiles during a wind event.
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Generally, coastal areas are susceptible to high winds from storm and/or hurricane events. A 

good reference to determine the susceptibility of a property to wind and seismic hazards is 

ASCE 7, which provides maps that show the risk of wind and seismic activity for the entire United States and detail design considerations for areas based on the risk posed by these hazards. 

4.4	 Step 4: Protecting Properties In and Near Hazard-Prone Areas

The best way to avoid damage to a manufactured home is to site the home away from areas 

susceptible to hazards. If the home is to be sited in or near a hazard-prone area, the developer 

must establish that the risk for the site is acceptable by ensuring that the home can be installed 

on the proposed site in accordance with applicable hazard-resistant codes and standards. In addition, mitigation techniques can be used during the installation process to further reduce the 

hazard risk. 

NFIP floodplain requirements and HUD installation standards exist to lessen manufactured 

home vulnerability to flood hazards. Two aspects to consider during this process include the 

following: 

n Placement options

n Design and construction techniques 

4.4.1	 Placement Options

One way to reduce the risk of damage to manufactured housing and attendant infrastructure is 

to select a site outside the limits of the hazard areas. This option is most applicable to areas subject to flooding and landslides where hazard areas can be reasonably well defined. Flood and 

landslide hazards are often localized so that all or a portion of the proposed site development 

can be located outside of the hazard-prone area. Note that this method is best suited for land 

that is initially subdivided or in the overall design of a manufactured home park.

4.4.1.1	 Flood-Prone Areas

Within floodplain areas, portions of an individual site may be outside the defined floodplain. 

Portions of the site outside the identified floodplain typically have higher ground elevations 

and are less susceptible to flooding. Locating the manufactured home outside the identified 

flood hazard area reduces the home’s exposure to the hazard. For individual sites located entirely within the flood-prone area, this method is not an option. Another option for siting in 

flood-prone areas is to determine if there are portions of the property that, while susceptible to 

flooding, are not exposed to some of the more destructive characteristics of flooding such as 

long duration flooding, high flood levels, and high velocity flows. 

Generally, the farther a manufactured home is sited from the flood source, the less likely it is 

to be exposed to high velocity flows. Within riverine flooding areas, the highest velocity flows 

are usually recorded within and near the floodway. In areas vulnerable to coastal flooding, wave 

action and/or storm surge can cause floodwaters to be particularly destructive. These types of 
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forces are the strongest in FEMA high hazard coastal zones labeled as V zones on FIRMs and the 

portions of A zones adjacent to V zones.

Similarly, flood levels usually decrease as the distance from the flood source increases. Setbacks 

from the flood source help to keep structures out of areas most susceptible to high velocity and 

high depth flooding. Damage to structures is directly related to the depth of floodwaters to 

which they are exposed.

Additionally, floodwaters can make access to and from manufactured homes difficult both during and after a flood event and greatly increase the safety hazards of flooding. For this reason, 

manufactured home installation in the floodplain is strongly discouraged within areas of increasing flood depths (greater than 2 to 3 feet) and high velocity flood flow (greater than 5

fps).

The duration of flooding is generally dependent on the flood source (size of the watershed), 

the slope of the stream, and the type of flood event (e.g., hurricane, snow melt event). 

Figure 4-2 shows a floodplain/floodway schematic. The floodway is generally the area where 

floodwaters are deepest and have the fastest flow rate for most waterways. Development in the 

floodway is restricted through floodplain management regulations and, in some communities, 

it is prohibited. Under minimum NFIP regulations, in order to place a new structure within the 

floodway, a floodplain development permit applicant must show that the development will not 

result in any increase to the 1-percent annual chance flood elevation. In most situations, development in the floodway is not permitted by communities because it almost always leads to a 

water surface elevation increase.

Figure 4-2. Floodplain/floodway schematic.
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4.4.1.2	 Areas Subject to Landslides

In areas subject to landslides, placing manufactured housing away from steeply sloped portions 

of a site, including relatively flat areas at the bottom of steep slopes, reduces exposure of the 

structure to damage. This is an option where landslides occur as the result of increased stresses 

from natural or manmade conditions. Some of the events or activities that can cause increased 

stresses include heavy rainfall, rapid snowmelts, melting of permafrost, groundwater withdrawal, 

soil liquefaction, and land development.

4.4.2	 Design and Construction Techniques

A thorough review of design and construction techniques (as they relate to manufactured housing) and their applicability and feasibility is another consideration in the evaluation of site 

alternatives. A site located in a hazard-prone area cannot be deemed suitable for manufactured 

home installation if mitigation methods cannot be implemented at the site. 

4.4.2.1	 Flood-Prone Areas

Ideally, all manufactured homes should be located outside of the SFHA; however, if that is not 

possible, manufactured homes located in the SFHA must be installed to resist damage from 

flooding. Some questions for consideration in determining if design and construction techniques that prevent flood damage are feasible for a particular site follow:

n What are the flood risks for manufactured home installation at the site?

n Can structural fill be placed to elevate the site? If fill is placed, will it withstand forces from 

velocity flows? 

n If long-duration flooding is an issue at the site, can the foundation be designed and 

constructed to resist long-duration floodwaters?

n Can the foundation be designed and constructed to withstand the expected design velocity 

flows?

n Can adequate storm drainage be provided? 

Chapters 9 and 10 of this guide present methods for the design and construction of manufactured home foundations installed in flood-prone areas.

4.4.2.2	 Dam Failure Inundation Areas

In dam failure inundation areas, the decision to install a manufactured home at the site will depend on assessment of the inundation risk. In higher risk areas where dam failures can trigger 

high flood depths and/or velocities, manufactured homes should be located elsewhere. High 

velocity flood surges can overwhelm typical residential foundation systems, including those 

used to support manufactured homes. However, in lower risk areas where relatively small flood 

depths and velocities are anticipated, the developer should determine if a foundation can be 

constructed that will provide adequate protection from expected inundation forces. 

As with all hazards, safety issues such as warning times are also of vital importance. 
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4.4.2.3	 Areas Subject to Landslides

Although it is preferable to site housing outside of areas vulnerable to damage from landslides, 

it might be feasible to undertake a grading project to reduce the risk of landslides. A developer 

must determine if this is feasible and consider whether it would be cost-effective. Licensed professionals specializing in earth movement (geotechnical, structural, and/or civil engineering) 

should be consulted. 

4.4.2.4	 Areas Subject to Seismic Events

For manufactured housing being placed in areas subject to seismic events, it is important to 

consider whether or not the foundation can be economically designed and constructed to withstand the expected forces for a design seismic event. Additionally, the property owner must 

consider the cost and effort of securing non-structural items in the manufactured home to 

prevent them from being damaged or dislodged. Relatively heavy items like domestic water 

heaters and HVAC units are particularly vulnerable to seismic events. Sewer, water, and gas lines 

should be sufficiently flexible to allow the home to move without damaging them. Inspections 

of manufactured homes after seismic events reveal that homes can be destroyed by broken utility connections (particularly natural gas connections) even where only minor damages occur to 

the home’s structure. 

4.4.2.5	 Areas Subject to Wind/Debris Hazards

Manufactured housing being placed in areas subject to wind/debris events need to have a 

foundation that is designed and constructed to withstand the expected forces associated with 

a design wind event. In areas subject to seismic activity and wind, the design standard for the 

hazard requiring the most resistance should be used. Even if the design of a foundation to meet 

this requirement is cost- effective, the developer still must consider whether or not the manufactured home can withstand the forces expected from a design wind event and debris impact.

4.5	 Step 5: Deciding on Property Development: Proceed or Reject

After reviewing the results of the data/information in Steps 1 through 4, deciding whether 

property development continues is the last step. Is the property suitable for development and 

for the installation of a manufactured home? If the siting evaluation shows that development 

of the property is feasible, and a hazard analysis and risk assessment also indicates development 

may be feasible, the proposed installation must be designed and constructed to adequately resist the hazard threat. If the proposed site satisfactorily addresses all of the conditions listed 

above, the site can be considered suitable for manufactured home installation.

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5	 Natural Hazards – Design

Considerations

Many regions in the U.S., such as coastal areas, are subjected to severe flooding and wind events 

at the same time. Other areas are simultaneously subjected to snow and seismic loads. Manufactured homes can be subjected to more than one hazard at the same time and should be 

designed to appropriate concurrent loading (or load combinations) based on ASCE 7 design 

standards. However, Manufactured Home Construction and Safety Standards (24 CFR 3280) do not 

require that designs include simultaneous flood and earthquake loads. The MHCSS specifies 

that snow loads shall not be considered as simultaneously acting with the wind loads. HUD 

standards also require that foundations for manufactured homes be designed to resist loads for 

combined wind and flood events. 

The first part of this chapter describes what kind of information is provided in FIRMs and FIS 

reports. The remainder of this chapter describes the characteristics of several natural hazards 

(flood, wind, and earthquake) that must be considered when making decisions about siting and 

selecting a foundation system for a manufactured home. 

5.1	 Flood Data

FIRMs and FIS reports are two sources that can provide vital information about flooding characteristics. Both of these tools are generally available for viewing at community permitting offices 

such as Zoning and Planning Departments or Building Permit Offices. The local permit official 

can go over these tools and help to explain floodplain development requirements. These tools 

are also available from FEMA’s Map Service Center and can be accessed online at http://store.

msc.fema.gov. 

FIRMs provide key information about a property’s flood vulnerability by showing the extent of 

the floodplain, the flood zone, the floodway (when floodway studies are performed), and the 

BFE (for detailed studies). Figure 5-1 shows an example of a FIRM. For example, Point 1 (white 

box) is located in the AE zone and has a BFE of 9 feet. For rivers and streams, FIS reports contain stream profiles that provide more accurate BFE data and should be used when determining 

BFEs for manufactured home installations or other types of development.
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There are two principal digital options for using flood maps. The simplest option is FIRM Scan 

data. These are simply digital images of a map and are available everywhere FEMA has a published FIRM. FEMA provides tools to view these maps and create a FIRMette, which is a printout 

of the area of interest, along with FIRM title block, scale bar, and north arrow. There is an online FIRMette tool that allows you to find, view, and print any map when you need it. There is 

also a Desktop FIRMette tool that can be faster and easier to use if you frequently use the maps 

for one area or need to use them without an internet connection. You can download or order 

the FIRMette Desktop and FIRM Scans you need. 

The second digital option is flood data in GIS format. GIS technology provides powerful abilities to create custom maps and perform sophisticated analyses. It also requires the user to have 

specialized skills and software to use. FEMA now produces a GIS product called the DFIRM Database for use with all map updates. Beginning on or after October 1, 2009, FEMA will provide 

a single paper flood map and FIS to each mapped community. FEMA will convert all other distribution of maps and FIS reports for digital delivery. FEMA will continue to provide free digital 

map products and data to Federal, State, Tribal, and local NFIP stakeholders. 

Base Flood Elevation

ZONE AE

(EL 9)

Flood Insurance Zone

Portion of 

FIRM Panel

FIRM Title Box

1

Figure 5-1. Example of a FIRM.
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FIS Reports

Several key components of an FIS report can help users identify specific flooding characteristics. For example, summary of discharge tables report the flow rate of water within streams 

for a given frequency storm (Figure 5-2). These values can be used with updated topography 

and cross-section information to conduct hydraulic modeling and to estimate updated flood 

elevations.

 FISs generally generate water surface elevations for the 10-year (10-percent annual 

chance), 50-year (2-percent annual chance), 100-year (1-percent annual chance), and 500-year 

(0.2-percent annual chance) floods (Figure 5-3). Floodway data tables provide cross-section specific information, including the width of the floodway, mean floodway velocity, and the BFE 

where there is no encroachment in the flood fringe (or the regulatory BFE) and where there 

is full encroachment in the flood fringe (Figure 5-4). However, these tables are only available 

when a floodway study has been performed. See Appendix C for additional information.

 Hydraulic modeling can be performed to estimate updated flood elevations where new or more accurate topography 

data are available. However, if the modeling shows lower flood elevations, they cannot be used for regulatory purposes 

unless a map revision (e.g., Letter of Map Amendment (LOMA) or Letter of Map Revision [LOMR]) is granted.

Figure 5-2. FIS

Summary of Discharges

table.
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Figure 5-3. FIS Stream

Flood Profile.

Figure 5-4. FIS

Floodway Data table.
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5.2	 Flood Characteristics

A variety of factors contribute to the type and strength of flood forces. For instance, structures 

and their foundations are subject to different forces if they are flooded by standing or slowly 

moving floodwater versus high velocity or wave action flooding. Similarly, factors such as the 

duration of flooding and flood depth help to define how structures and foundations will be affected by flood events. 

5.2.1	 Frequency, Duration, and Rate of Rise

Frequency of flooding is the rate of flood occurrence at a particular location. This frequency is 

the probability, expressed as a percentage, that a flood of a specific size will be equaled or exceeded in any year. For example, the flood that has a 1-percent probability (1 in 100) of being 

equaled or exceeded in any given year is referred to as the “base flood” and sometimes called 

the “100-year flood;” the latter term is simply a convenient way to express probability. The base 

flood is particularly important for homeowners because it is used by the NFIP to determine 

flood insurance rates and define regulatory floodplain management requirements.

A 1-percent annual chance flood is a probability term and does not mean that a flood will happen approximately once every 100 years. Nor does it imply that once a 1-percent annual chance 

flood occurs there is little chance of another 1-percent annual chance flood occurring in the 

near future. To the contrary, changes in climatic conditions, such as those caused by El Niño or 

La Niña, often result in a cluster of floods that occur over relatively short periods of time at the 

same location. 

Manufactured homes located within the 100-year floodplain on a FIRM might also be in the 

10- or 5-year floodplain and subject to even more frequent flooding. Although more frequent 

floods such as the 10- or 20-percent annual chance flood (10- or 5-year flood) cause less damage to the manufactured home than the 1-percent annual chance flood, the repetitive damages 

can result in significant financial loss to the homeowner. Data describing the extent of more 

frequent but less intense flooding is also important in determining the foundation system most 

appropriate for resisting forces imposed by the smaller floods.

Information about the base flood can be found in the FIS and the FIRM, including the flood 

elevation and the extent of flooding. In areas where detailed studies have been performed, 

flood elevation information may also be available for the more frequent 10- or 2-percent annual 

chance flood as well as the less frequent 0.2-percent annual chance flood (500-year flood).

For historical flood events, flood frequency estimations are sometimes found in the FIS under 

the Principal Flood Problems section. Documentation of past flood events is sometimes available from other Federal sources as well as State and local agencies. Table 5-1 lists such sources 

and agencies. 
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Table 5-1. Sources for Information About Past Flood Events

Other Federal Sources State Agencies Local or Regional Agencies

USACE Floodplain Information Reports Departments of Environmental 

Conservation/Protection

Flood Control Districts

USGS Water Resources Investigations State Floodplain Management 

Office

Levee Improvement Districts

Natural Resources Conservation 

Service (NRCS) Watershed Studies

Departments of Natural 

Resources

Local Planning Commissions

Federal Highway Administration 

(FHWA) Floodplain Studies

Departments of Transportation Local Public Works Departments

Tennessee Valley Authority (TVA) 

Floodplain Studies

Departments of Water 

Resources

Municipal Utility Districts

Geologic Surveys River Basin Commissions

Water Control Boards

Flood duration is the time from the inundation to the recession of floodwaters. Flood duration provides an estimate of how long a manufactured home’s foundation (not the home itself, 

assuming the home is properly elevated) will be subjected to pressures and forces exerted by 

floodwaters, the degree of floodwater seepage, and the length of time that a structure may be 

inaccessible and/or inhabitable for the occupants. For identical flood characteristics, long duration flooding is usually much more destructive than short duration flooding.

The rate of rise is a measure of how rapidly water depths increase during flooding. A slow rise 

of floodwaters will allow seepage of water into a manufactured home (only if it is not elevated 

above the flood level) and may prevent buoyancy forces from acting on a home. When water rises rapidly, however, there may be insufficient time for seepage, exposing the home to buoyancy 

forces. The buoyancy forces could result in failure of the foundation, the floor, or the framing 

of the home. 

The rate of rise (and fall) also affects the amount of warning for an impending flood. For example, homeowners in the floodplains of large rivers like the Mississippi may know days in advance 

of upstream flooding that will eventually reach their homes. But in the floodplains of streams 

with high rates of rise, homeowners may have only a few hours’ notice (or none at all) of an approaching flood. 

The rate of rise and fall is important in planning emergency evacuations and determining the 

feasibility of emergency loss mitigation procedures. Although surrounding and upstream terrain is a good indicator of the rate, flooding duration for particular areas is best determined 

from historical data and accounts of past flood events. This information might be available locally from accounts given by homeowners and local emergency management staff, or it may be 

documented in the FIS or local, State, or Federal studies. See Table 5-1 for a list of potential 

sources of information.
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5.2.2	 Flood Elevation and Depth

The BFE is the elevation of the flooding, including wave height, if applicable (in V zones), having a 1-percent chance of being equaled or exceeded in any given year. The elevation of the 

flooding is the floodwater’s height above or below an established reference datum. Ground elevation is the height of the ground surface also above or below an established reference datum. 

The standard datum used by most Federal agencies and many State and local agencies are the 

NGVD 29, and the NAVD 88; however, other datum are used. Ground elevations are established 

by topographic surveys; flood elevations are calculated for a particular flood event. BFEs can be 

found on FIRM panels where a detailed study has been performed, and should be estimated using the flood profile in the corresponding FIS.

Some flood zones are delineated by approximate methods without BFEs determined. When a 

manufactured home is proposed on an approximate A zone site, the community must make 

every effort to obtain, review, and reasonably utilize BFE data and floodway data from a Federal, State, or other source in order to provide a reasonable measure of flood protection in 

accordance with 44 CFR 60.3(b)(4). If BFE or floodway data cannot be obtained from available 

sources, the community should consider conducting or requiring the applicant to conduct a 

site-specific engineering analysis to determine a BFE or floodway. FEMA has developed Quick2: Computation of Water Surface Elevations in Open Channels, which is useful in developing a BFE. 

Quick 2 is available from the FEMA web site http://www.fema.gov/plan/prevent/fhm/dl_qck22.

shtm. If no BFE data are available through Federal, State, or other sources, communities must 

BFE and DFE

Base flood elevation (BFE) – The water surface elevation resulting from the base, or 100-year (1 percent annual chance) flood. 

Design flood elevation (DFE) – The elevation to which development in the regulatory floodplain is 

built. At a minimum, the DFE is equivalent to the BFE. However, in some areas, the DFE includes an 

additional freeboard height above the BFE as shown in the formula below.

DFE = BFE + Freeboard

Freeboard – An additional elevation requirement some height (1, 2, or more feet) above the BFE that 

provides a margin of safety above the estimated BFE. 

As indicated above, the minimum DFE requirement for NFIP communities is the BFE. However, in communities where a higher degree of protection is promoted or required, the DFE includes a freeboard 

height above the BFE. This freeboard provides a margin of safety above the estimated BFE and against 

extraordinary or unknown risks. As in all natural hazard events, the design event can only be predicted 

in probabilistic terms and some uncertainties remain in any analysis. Freeboard is intended to allow for 

those uncertainties. The NFIP encourages participating communities to adopt and enforce freeboard 

requirements as part of their local floodplain management ordinance. This is shown in 44 CFR 60.1(d); 

which states, “Any floodplain management regulations adopted by a State or a community which are 

more restrictive than the criteria set forth in this part are encouraged and shall take precedence.”
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ensure that building sites will be reasonably safe from flooding for proposed developments. A 

simplified method for estimating BFEs as well as other methods for ensuring a building site is 

reasonably safe from flooding can be found in FEMA 265, Managing Floodplain Development in 

Approximate Zone A Areas, A Guide for Obtaining and Developing Base (100-year) Flood Elevations. Additional guidance on approximate A zones and requirements for developing BFEs in subdivision 

proposals and other proposed developments can be found in Section 3.6.1. 

Flood depth is the difference between the water surface elevation and the grade elevation of the 

flooded area (Figure 5-5). Any differences in datum between the flood elevation and the grade 

elevation must be taken into account when calculating flood depths. 

Many communities have adopted more stringent requirements for an additional elevation (e.g., 

1 foot, 1.5 feet, etc.) above the BFE. The additional height (or freeboard) is used to establish the 

DFE. Freeboard provides a margin of protection above the estimated base flood. 

5.2.3	 Hydrostatic (Buoyancy) Forces

For manufactured homes that are not elevated to above the BFE or DFE, hydrostatic forces can 

cause significant damages. Hydrostatic pressures are also a concern for enclosed areas underneath elevated manufactured homes.

When a body or a structure is immersed in water, the body is subjected to forces exerted by 

the surrounding water. These forces are called hydrostatic forces and act perpendicular to the 

surface on which they are applied. For example, hydrostatic forces exert lateral forces on vertical walls and vertical forces on horizontal floors.

The total vertical force (also called buoyancy force) on a submerged structure is equal to the 

weight of the displaced volume of water. For each foot of fresh water submersion, a buoyancy 

force of 62.4 psf is created. Salt water, with a greater density than fresh water, creates 64 psf for 

each foot of submersion. That is

Fbuoy = γ x VOL

Where: γ = specific weight of water (62.4 pcf for fresh water; 64 pcf for salt water)

VOL = volume of water displaced by the submergence object (ft3)

The lateral force of water acting against a surface is related to the water’s depth (d) and specific 

weight (γ). Hydrostatic forces create a triangular loading on vertical surfaces with zero psf on 

the water surface to a force equal to γ (pcf) multiplied by d (feet). 

FL = 1⁄2 γ d2
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Where: FL = lateral force

γ = specific weight of water (pcf)

d = depth (ft)

 FL is applied at a distance 2/3 d from the 

water’s surface.

Hydrostatic forces act perpendicular to submerged surfaces. If the water depth is the same on both sides of a 

wall, the lateral component of hydrostatic forces are 

equal such that the resultant force is zero. 

If floodwater rises above the lowest floor, hydrostatic forces can lift an inadequately anchored 

manufactured home off its foundation. If buoyancy forces exceed the weight of a manufactured home, the home will float off its foundation if it is not securely fastened to the foundation. 

Floods do not need to be deep to displace a home. Flood depths of only 4 to 5 inches above the 

lowest floor can be capable of floating unsecured manufactured homes off their foundations. 

The walls and floors of manufactured homes are not typically designed to resist hydrostatic forces, which can cause extensive structural damage. Floodwaters in contact with a home generally 

seep into it through openings around doors, windows, vents, and utility entrances. Floodwaters 

inside the structure add weight to the structure, thus reducing the net buoyancy force of the 

structure. Figure 5-5 shows hydrostatic forces acting on the walls and lowest floors below the 

flood level. As illustrated in Figure 5-5, an effective method of avoiding damage from hydrostatic forces is to elevate the home above the flood levels.

5.2.4	 Hydrodynamic Forces

Forces due to moving floodwaters are called hydrodynamic forces. The magnitude of these 

forces depends on the floodwater depth, floodwater velocity, and the drag coefficient. Higher 

d

 FL = 

γ ⋅ d

γ ⋅ 

Figure 5-5. Buoyancy

forces acting on a

structure.
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depths and velocities produce greater hydrodynamic forces. The drag coefficient is a function 

of the shape of the body subjected to the hydrodynamic forces. 

Moving floodwaters can cause foundation failure by pushing (sliding failure) (Figure 5-6) or 

by overturning. Either can topple foundation elements and destroy a home. In addition, moving floodwaters can cause erosion and scour that can undermine foundation elements (Figure 

5-7).

Flood velocity depends on the slope and roughness of the terrain. For example, water moves 

faster along streams in steep mountains than streams in flatter areas, and water moves faster 

over a parking lot with a paved surface as opposed to an area with dense vegetation or other obstacles. Flood velocities are not shown on the FIRMs, but can be found in the FIS for floodways 

Figure 5-7. A

manufactured home

destroyed by the

hydrodynamic forces of

flooding.

Figure 5-6. Failure due

to sliding.
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NATURAL HAZARDS – DESIGN CONSIDERATIONS 5

where floodways have been studied. Mean floodway velocities can be obtained from the FIS by 

matching the cross-section on the FIRM with the cross-section in the floodway data table. The 

floodway’s mean velocity usually overestimates the flood velocity within the flood fringe; floodwaters generally move slower as they extend outward from the floodway. However, the floodway 

velocities can be used as a general measure to determine cross-section locations within the 

floodplain where floodwaters will move relatively faster or slower, and provide an upper limit 

for velocities in the flood fringe. One of the best sources for flood velocities is records of past 

flood events. In addition to the FIS, a list of potential sources for documentation of past flooding is included in Table 5-1.

Coastal areas are particularly hazardous because of flooding accompanied by wave impacts that 

are associated with storm surges from coastal storms. Flow velocity can be further increased by 

manmade or natural obstructions that restrict floodwaters and channel the flow.

The hydrodynamic force exerted by a fluid in the direction of the flow stream is defined as the drag force Fd. 

Hydrodynamic forces are calculated as follows:

Where: Fd = hydrostatic force (pounds)

Cd= dynamic coefficient (1.25 for round piers; 

2.0 for square or rectangular piers; 1.4 for 

continuous foundations) 

γ = specific weight of water 

g = acceleration due to gravity (32.2 ft/s2)

V = floodwater velocity (fps)

A = projected vertical area submerged (sq. ft.)

 A = d x w

 w = width of the submerged object 

An alternative approach to consider the dynamic effects of moving water is to use an equivalent 

hydrostatic load approach. When water velocities are less than 10 fps, ASCE 7 permits adding an 

equivalent surcharge water depth to the design flood elevation to simulate the hydrodynamic 

load.

d

FL = 1⁄2 γ (d+dh)(d+dh)" and "γ (d+dh)"

NOTE: Formulas should be identical. Symbols should be "gamma" fo

Cd x A x γ x V 2

2g

Fd =

FL = 1⁄2 γ 

NOTE: Fo
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The equivalent surcharge water depth (dh) is determined by: 

Where: Cd = dynamic coefficient (1.25 for round piers; 2.0 for square or rectangular piers; 

 1.4 for continuous foundations)

V = average velocity of water (fps) 

g = acceleration due to gravity (32.2 ft/s2)

Hydrodynamic forces apply to any portion of the home or its foundation exposed to moving 

floodwaters. Elevating the home so that its lowest floor is at the BFE reduces the effects of hydrodynamic forces on the building envelope. However the floor framing, chassis frame, and 

the foundation elements remain exposed to the hydrodynamic forces. For open foundations, 

elevating the home so the lowest chassis frame member is at the BFE reduces the effects of hydrodynamic forces by reducing the number of components exposed to those forces during a 

base flood event.

5.2.5	 Erosion and Scour

Erosion is the result of moving waters removing soil from the ground surface. Scour is the result 

of those moving waters removing soil beneath and around objects located in the path of flow. 

Scour under and around building foundations and erosion of fill embankments can result in 

foundation failure. For example, when Hurricane Ike struck Galveston, Texas, in 2008, several 

homes were destroyed due to a loss of lateral support of pile foundations caused by scouring. 

Moving floodwaters will erode soil around any obstruction placed in the flow until a maximum 

scour depth is reached. The maximum scour is a function of flow velocity, soil particle size, and 

obstruction geometry. Determining maximum potential scour is critical in designing a foundation system that prevents failure during and after flooding. In coastal areas, scour depth can be 

significant due to both storm-induced erosion and localized scour resulting from storm surge. 

Because storm-induced erosion is difficult to predict, local regulatory officials should be contacted for further information and historical perspectives. 

Scour susceptibility is a function of foundation geometry, flow depth, flow velocity, and the soil 

particle size. Fine grain soils (i.e., silts and clays) scour at a lower flow velocity than coarse grain 

soils. Coarse grain, non-cohesive soils can erode and/or scour quickly, possibly reaching the ultimate scour depth in a single flood event. Fine grain, cohesive soils can scour just as deep but 

take longer to reach the ultimate depth. It is important to note that foundations frequently are 

well above the ultimate scour depth and that foundation undermining can occur in a relatively 

short time period. Undermining of foundations can result in the collapse of the home.

Because flood velocities are not shown on FIRMs, the FIS must be consulted to determine flood 

velocities, which are only provided for floodways. In some SFHAs, floodways and mean floodway 

velocities have not been determined as part of the FIS.

Cd x V 2

2g

dh

=
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NATURAL HAZARDS – DESIGN CONSIDERATIONS 5

The preferred scour mitigation technique is to place the manufactured home’s foundation below the predicted ultimate scour depth. In areas where placing foundations below maximum 

scour depth is not economically feasible, alternative scour protection methods include reinforcing foundation soils with large diameter materials such as coarse aggregate or riprap. 

5.2.6	 Debris Impact Forces

Floodwaters can transport objects of all types (e.g., trees, portions of flood-damaged buildings, 

automobiles, boats, storage tanks, manufactured homes, etc.). In cold climates, winter floods 

can also carry large pieces of ice. Substances such as dirt, oil, gasoline, sewage, and various types 

of debris add to the dangers of flooding. Even when flow velocity is relatively low, large objects 

carried by floodwaters can easily damage windows, doors, walls, and, more importantly, critical 

structural components. As the flood velocity increases, the danger of damage from floodborne 

debris also increases. 

Waterborne debris impacts caused significant damage to buildings and enclosures, slabs, decks, 

utilities, and other ancillary features when Hurricane Ivan struck the Florida panhandle and 

coastal Baldwin County, Alabama in 2004. Manufactured homes typically are not designed to 

withstand loads to walls or floor systems that may be exerted by attached carports, decks, porches, or awnings, although some manufacturers have designed homes that are “awning ready” to 

provide a load path through the structure to handle the increased stresses. The alternative is 

to build the attachments with adequately anchored foundations to resist these debris impact 

forces.

Damages caused by floodborne debris impacting a manufactured home depend not only on 

the weight and velocity of the debris, but also on the size and material. Historically, the forces 

developed when floating debris hits a home have been estimated using the principles of conservation of momentum with impact duration of 1 second. Recent research, however, indicates 

that the impact from floodborne debris occurs over intervals of only 0.01 to 0.05 second. Since 

impact force is inversely proportional to duration (i.e., the shorter the duration, the larger the 

impact force), historical methods may underestimate impact forces. Typical impact duration 

ranges from 0.1 second for stiff foundations (such as concrete) to 1 second for flexible foundations (such as wood).

Fi = (W/g) x (Vb/t)

Where: Fi = impact force (pounds)

W = weight of floodborne object (pounds)

Vb = velocity of object (fps)

t = time for object to decelerate from Vb to zero (seconds)

g = acceleration due to gravity (32.2 ft/s2)
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ASCE 7-05 updates the methods of calculating impact debris forces. Recent research indicates 

that the impact durations are much shorter and impact loads are significantly higher than 

previously recommended. Foundations for manufactured homes located in SFHAs should be 

designed to resist floodborne debris impact forces calculated based on the most recent edition 

of ASCE 7 protocols.

The relatively light and frequently unreinforced foundation systems used to support and anchor manufactured homes are susceptible to impact damage from floating debris. The cost to 

strengthen foundations, ground anchors, and anchor straps to resist debris impact may be economically impractical. An alternative method is to install additional foundation elements to 

provide redundancy in the system. For example, reducing the frame span length by reducing 

pier spacing allows the frame to span across a damaged pier without excessive deflections or failure. This method requires post-flood inspection and maintenance to repair or replace damaged 

foundation elements to allow the home’s foundation to survive any subsequent events. Also, if 

more foundation elements than what have been assumed are damaged, a finite risk of system 

failure remains.

5.3	 Wind

Design and installation of foundations 

of manufactured homes is a controlling 

factor on the homes’ ability to withstand 

forces from wind events. Connections 

used to secure the manufactured homes 

to the foundations have historically lacked 

the attention given to the manufacturing 

of the homes themselves. Lack of proper 

connections has caused numerous homes 

to fail during high-wind events. Figures 5-

8 and 5-9 are examples of wind damage to 

modular and manufactured homes.

Several factors influence the direction and 

magnitude of wind forces on a structure, 

which should be considered in manufactured home foundation design. 

When a structure is exposed to these 

wind forces in combination with flood 

forces, the damage potential is greatly 

increased.

Figure 5-8. Failure of a modular home due to

high winds. The home lifted off of its foundation

(concrete slab) when the connections failed.
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5.3.1 	 Wind Forces on Structures

The texture and roughness of the terrain, as well as its surface contours and topography, have a 

profound effect on the wind loads. A structure in an open flat area with few trees and obstructions is likely to see a substantially higher wind load than one situated in a developed area with 

numerous buildings or other obstructions. Similarly, a structure situated on a hilltop may be 

subjected to significantly greater loads than one on flat terrain. All structures should be built to 

withstand the forces caused by the strongest wind speeds likely to happen at the site.

The basic design wind speeds for the United States can be obtained from the map given in Figure 6-1 of ASCE 7. The wind speeds correspond to 3-second gust speeds in mph at 33 feet (10 

meters) above the ground for open terrain with scattered obstructions having heights generally 

less than 30 feet (9.1 meters), Exposure C category. This includes flat open country and grasslands. 

When a building is exposed to wind, the wind flows over and around the building. This imposes 

positive pressure on the windward side of the building and negative pressure (suction) on the 

leeward side and roof of the building. 

5.3.2	 Wind Forces in Combination with Flood Forces

Damage potential is increased when wind forces are exerted on a structure in combination with 

flood forces, as is often the case in coastal and mountainous areas. For example, tensile forces 

can develop at the windward piers and the home can be lifted. If the windward piers are exposed to moving floodwaters, they can topple when no longer in contact with the home’s steel 

frame. 

Elevating a manufactured home to minimize the effects of flood forces does not significantly 

increase the wind loads on the home.

Figure 5-9. A

manufactured home that

failed during a high-wind

event.
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The HUD Model Manufactured Home Installation Standards require foundations to consider flood 

and wind load combinations. Sections 2.3.3 and 2.4.2 in Chapter 2 of ASCE 7, Minimum Design 

Loads for Buildings and Other Structures contain load combinations that should be used.

5.4	 Earthquakes

Designing resistance of structures and foundations for earthquakes requires a different approach than that used for flooding and wind. Because earthquakes can affect internal building 

elements without requiring penetration of the building like flooding or wind, there are design 

considerations specific to earthquakes. 

Often designing for wind load standards will allow a manufactured home installation to be adequate for protecting the home from earthquake forces. However, there are instances when 

forces resulting from earthquakes can result in greater loads than those for wind. 

5.4.1	 Design Philosophy

Earthquake-caused ground accelerations cause forces on building elements attached to or into 

the ground. Foundation movements are transmitted to structural and non-structural components. Consequently, exterior and interior building components should be designed for seismic 

loads to be transferred to the remaining elements. 

In the event of an earthquake, a structure will be simultaneously subjected to vertical and horizontal accelerations. The weight of a manufactured home on its foundation is not sufficient to 

resist a moderate or severe earthquake. Therefore, the manufactured home and its foundation 

should be properly connected to avoid damage during an earthquake event. Soil liquefaction 

resulting from an earthquake is also an important consideration for earthquake design because 

soil liquefaction is a major cause of damage in a seismic event. Refer to Chapter 6 for a discussion on liquefaction.

5.4.2	 Design Standard

Regulations governing the construction and design of manufactured homes currently do not 

specifically address seismic loads but rather specify loading requirements primarily to provide 

basic wind resistance. However, seismic requirements exist in most model codes and standards. 

Seismic requirements have been included in the Model Manufactured Home Installation Guide

(NFPA 225). 

Historically, communities with low seismicity have considered wind load provisions as sufficient 

for resisting seismic events. Although this may be true in the case of transverse loading (i.e., 

wind pressures acting on the long walls) where wind loads are high, it is often not the case with 

longitudinal loading where wind loads are low. Also, the nature of seismic loading is greatly different than wind loading. These differences make comparisons between wind resistance and 

seismic resistance tenuous at best. 
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NATURAL HAZARDS – DESIGN CONSIDERATIONS 5

Studies list the following typical damages to manufactured homes during design earthquakes:

n Homes falling off support systems

n Damage to floors from piers puncturing them 

n Disruption of gas, water, and electrical lines from seismic motions of the home

n Fire resulting from damaged water heaters, and gas and electric lines. (A simple and costeffective seismic mitigation measure for manufactured homes is to strap the water heater to 

a wall to prevent it from falling over.) 

For a manufactured home installed on a foundation system, providing adequate resistance to lateral movement, uplift, and rotation is very important. It is also necessary to provide tensile connections between the main frame and the piers supporting the home to resist all seismic forces. 

Earthquake-resistant bracing systems (ERBSs) can be installed to minimize damage to the home, 

but they do not provide the protection of a seismic-resistant foundation system. ERBSs are secondary supports that do not resist seismic forces, but rather allow the home to fall from its 

primary supports and “catch” it before it hits the ground. 

The State of California has had an ERBS certification program since 1987. To be certified, the 

ERBS must be able to limit seismic movement and limit vertical drop of the manufactured 

home to 2 inches. A list of certified ERBSs is available from the State of California, Department 

of Housing and Community Development, Division of Codes and Standards. The price range of 

ERBSs is from approximately $2,000 up to $5,000; the average cost of an ERBS is about $2,500. 

The State of California also permits the use of other systems that do not require the home to fall 

off supports, as is the case with ERBSs.

5.5	 Evaluation of Multi-Hazards

Manufactured homes, like all buildings, can be simultaneously subjected to many natural hazards, and the combined effects of the forces generated from these hazards must be considered. 

The different loads that must be considered in the design of a manufactured home foundation 

are shown in Table 5-2.

Table 5-2. Load Combination Nomenclature (ASCE 7)

Nomenclature Load Description

D Dead load

L Live load

F Load due to fluid with well-defined pressures and maximum heights

Fa Flood loads

H Load due to lateral earth pressure, groundwater pressure, or pressure of bulk 

materials

T Self-straining force
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Nomenclature Load Description

Lr Roof live load

S Snow load

R Rain load

W Wind load

E Earthquake load

Load combinations are used in the design process to take into account the simultaneous occurrence of different hazards. Manufactured homes and their foundations should be designed to 

the load combinations indicated by the applicable building code for the area. In the absence of 

a building code, the manufactured home should be designed in accordance with the load combinations given in ASCE 7. 

5.5.1	 Load Combinations (ASCE 7)

5.5.1.1 	Strength Design (Load and Resistance Factor Design)

Strength design is defined as a method of proportioning structural members such that the computed forces produced in the members by the factored loads do not exceed the member design 

strength.

According to ASCE 7, the basic load combinations for strength design (using the nomenclature 

in Table 5-2) are as follows:

1. 1.4(D+F)

2. 1.2(D+F+T) + 1.6(L+H) + 0.5(Lr or S or R)

3. 1.2D + 1.6(Lr or S or R) + (L or 0.8W)

4. 1.2D + 1.6W + L + 0.5(Lr or S or R)

5. 1.2D + 1.0E + L + 0.2S

6. 0.9D + 1.6W + 1.6H

7. 0.9D + 1.0E + 1.6H

When a structure is located in a flood zone, the following load combinations must also be considered:

1. In V zones or Coastal A zones, 1.6W in combinations (4) and (6) shall be replaced 

with 1.6W + 2.0Fa.

Table 5-2. Load Combination Nomenclature (ASCE 7) (continued)
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2. In non-Coastal A zones, 1.6W in combinations (4) and (6) shall be replaced by 0.8W 

+ 1.0Fa.

5.5.1.2	 Allowable Stress Design (also known as Working Stress Design)

Allowable stress design (ASD) is defined as a method of proportioning structural members such 

that computed stresses produced in the members by nominal loads do not exceed specified allowable stresses.

According to ASCE 7, the basic load combinations for ASD are as follows:

1. D + F

2. D + H + F + L + T

3. D + H + F + (Lr or S or R)

4. D + H + F + 0.75(L + T) + 0.75(Lr or S or R)

5. D + H + F + (W or 0.75E)

6. D + H + F + 0.75(W or 0.7E) + 0.75L + 0.75(Lr or S or R)

7. 0.6D + W + H

8. 0.6D + 0.7E + H

When a structure is located in a flood zone, the following load combinations must be considered:

1. In V zones or Coastal A zones, 1.5 Fa shall be added to other loads in combinations 5, 

6, and 7 and E shall be set equal to zero in 5 and 6.

2. In non-Coastal A zones, 0.75Fa shall be added to combinations 5, 6, and 7 and E shall 

be set equal to zero in 5 and 6.

Either ASD or strength-based design can be used for manufactured housing. In both design 

methods, wind and seismic loads need to be evaluated. In addition, increases in allowable 

stress shall be used with these load combinations where allowed by the locally adopted building 

code. 

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6	 Soils

Soil properties can significantly impact manufactured homes exposed to flooding. Properly 

determining soil type and allowable bearing capacity is important for the selection and performance of the foundation. It is recommended that a geotechnical engineer make these 

determinations. For example, highly erodible soils are not desirable as fill for elevating a structure in a high velocity area because it can be washed away by moving floodwaters. Other soils 

lose strength as moisture content increases, reducing a foundation’s load capacity. This chapter 

discusses the effect of flooding on soil properties.

6.1	 Bearing Capacity

Bearing capacity is a soil’s capability to support load without catastrophic failure. Bearing capacity is derived from the soil shear strength, which is expressed as:

s = c + ρ (tan φ)

Where s = soil shear strength

 c = soil cohesive strength

 ρ = lateral confining pressure 

 φ = soil angle of internal friction

The general bearing capacity equation is:

q = (γb/2)Nγ + cNc + ρN

ρ

Where q = soil bearing capacity

 γ = unit weight of soil

 b = footing width

 N

γ = bearing capacity factor for soil density and footing size

 Nc = bearing capacity factor for cohesion 

 N

ρ = bearing capacity factor for surcharge

As previously discussed, soil strength frequently changes as a function of moisture content. In 

granular soils, as moisture content approaches saturation (i.e., intergranular voids are 100 

percent filled with water), the effective unit weight is reduced, thus reducing the confining pressure (ρ).
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For preliminary design purposes, allowable bearing capacities can be established based on 

national model codes, local building codes, and soil surveys. Local officials may also have information on soil types and soil bearing capacities.

Soil classifications of a general nature and a typical range of their maximum allowable bearing capacities can be found in publications such as the IBC ( 2009 Edition), NFPA 5000 (2009 

Edition), and NFPA 225 (2009 Edition). These documents can provide the allowable bearing 

pressures considered sufficient to support foundations at well drained sites. Excessive foundation movement or settlement may occur where unusual soil or moisture conditions are 

encountered. A geotechnical engineer should be consulted when unusual soil or moisture conditions are present. 

6.2	 Effects of Flood Duration and Frequency on Soil

For granular soils located below the water surface, the effective confining pressure (ρ’) is used 

to determine the effective shear strength’. The effective confining pressure is calculated using 

the effective unit weight of the soil where:

	 γ’ = γs

 – γw

	 γ’ = effective unit weight of soil

	 γs = total unit weight of soil and

	 γw = unit weight of water

The submerged weight can be estimated as about half of the total weight of many granular soils. 

The angle of internal friction (φ) is not appreciably changed by submergence. Therefore, if the 

water table rises from a depth greater than the width of the footing to the ground surface, as 

can be the case of flooding, the bearing capacity of the soil is reduced by approximately 50 percent.

Cohesive soils (silt and clay) are bound together by electrochemical bonds between individual 

particles. Increasing the moisture content can change the distance between particles, decreasing the strength of the inter-particle bonds. The decrease in bond strength results in a decrease 

in cohesion and a loss of shear strength. 

6.3	 Soil Liquefaction

Seismic soil liquefaction is an important consideration for earthquake design because soil liquefaction can be a major cause of damage in a seismic event. Liquefaction is defined as the 

significant loss of strength resulting from an increase in soil pore pressure. Seismic vibrations 

act to realign soil particles into a denser configuration. The realignment decreases the inter-particle void space, thus increasing the water pressure in the voids. The increase in water pressure 
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SOILS 6

has the effect of decreasing the effective strength of the soil. A sufficient increase in pore pressure can reduce the effective strength to zero, at which time the soil mass behaves as a liquid. 

The loss of shear strength in soils supporting buildings can be catastrophic. Building foundations can slide or unevenly settle, bridges can collapse, and empty fuel tanks buried under 

ground can rise to the surface. 

Liquefaction-induced soil movements can push foundations out of place to the point where the 

manufactured home will lose support. 

Loose clean sands with relatively few fines, and silty soils with little or no clay, are vulnerable to 

seismic induced liquefaction. It is necessary to identify the presence of liquefaction susceptible 

soils expected to support building foundations, and design the manufactured home foundation 

accordingly. 

To reduce liquefaction hazards, the best approach is to avoid construction on soils susceptible 

to liquefaction. Other options such as making the structure liquefaction resistant or improving 

the strength, density, and/or drainage characteristics of the soil are probably not economical 

for most manufactured home sites. 

6.4	 Recommended Soil Testing and Criteria for Manufactured

Home Installations

Shallow subsurface investigations should be conducted to evaluate the soil bearing conditions 

at the proposed manufactured home site. An experienced geotechnical engineer can conduct 

an appropriate investigation using hand augers, field strength tests, and visual classification 

methods. Field strength testing includes dynamic cone penetration tests, pocket penetrometer 

tests for cohesive soils, and torvane tests for granular soils. The investigation should evaluate 

soils to a depth of twice the width of the bearing surface or 5 feet, whichever is greater. The investigation will provide information on soil type and bearing capacity. 

When footings are eccentrically loaded, the effective bearing area is reduced so footing sizes 

must be increased accordingly. Eccentric loading occurs when piers are not centered over footings and when a manufactured home frame is not centered on its pier. Eccentrically loaded 

foundation conditions should be avoided.

Ground anchors consist of a specific anchoring device designed to transfer home anchoring 

loads to the ground (24 CFR 3285.5). Axially loaded ground anchors support loads by mobilizing soil shear strength along a subsurface shear plane. Ground anchors loaded non-axially 

resist loads through both the axial shear strength and passive soil resistance against the horizontal component of the non-axial load. Development of passive soil resistance requires some soil 

movement. The required movement will generally result in larger total movement in non-axially loaded anchors than axially loaded anchors in the same soil conditions.

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7	 Ground Anchors

Both the NFIP regulations (44 CFR 60.3) and the HUD Model Manufactured Home Installation 

Standards (24 CFR 3285.305) require that manufactured homes installed in flood-prone or 

SFHAs be anchored to resist flotation, collapse, or lateral movement.

Ground anchors consist of a specific anchoring assembly designed to transfer home anchoring 

loads to the ground (24 CFR 3285.5). They are used extensively in manufactured home installations. Ground anchors are economical, readily available, and can be installed with relatively 

lightweight tools and equipment. 

Ground anchors have potential limitations. One significant limitation arises from multiple soilanchor response mechanisms as a function of soil type, anchor depth, and load configuration. 

Ground anchors have historically been allowed to move up to 3 inches horizontally or 2 inches 

vertically as the basis for determining acceptable performance. (Appendix E of the IRC established a ground anchor moving 2 inches in the direction of pull of the load as being acceptable.) 

When a manufactured home is secured with ground anchors, it too can move up to 3 inches 

when exposed to flood, wind, or seismic events. 

Three inches of horizontal movement can produce eccentrically applied loads on piers under 

a home, which can cause the piers to topple and the home to collapse. Two inches of vertical 

movement can allow the home to lift and lose contact with the supporting piers. Unless the 

home is securely fastened to the piers and the piers constructed as monolithic units, the loss of 

contact can make the piers vulnerable to being displaced by moving floodwaters.

Ground anchor movements of several inches can have significant negative impacts on long-term 

performance. In cohesive soils, such anchor movements in a vertical direction can approach or 

exceed the soil’s shear strength. In such cases, the ground anchor is supported by the soil’s residual shear strength, resulting in a decrease in anchor capacity. In granular soils, large lateral 

movements may produce failure planes that can reduce the strength on the vertical direction.

Anchor assemblies, including ground anchors, should be inspected periodically, especially after 

hazard events. Loose anchor straps need to be retightened, and loose or failed anchors need 

to be reset or replaced. If movement has made the manufactured home structurally unstable, 

it should be reset to center it on its supporting piers. If this maintenance is not performed, the 

anchor assembly may fail during subsequent hazard events, resulting in significant damage to or 

destruction of the home. Tightening ground anchor straps typically costs a few hundred dollars 

or less. Replacing anchors or resetting a home is estimated to cost at least $1,000. 

 44 CFR 60.3(a), 44 CFR 60.3(b), and 24 CFR 3285.305 use the phrase “flotation, collapse or lateral movement.” 

44C FR 60.3(c) uses the phrase “flotation, collapse and lateral movement." To account for both usages, this publication uses “or.”
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7.1	 Types of Anchors and Installed Configurations

7.1.1	 Types of Anchors

Several styles of anchor assemblies are available that can adequately secure a manufactured 

home to resist flood, wind, and seismic forces. Helical earth anchors, cast-in-place concrete footings, drilled concrete anchors, and cross drive anchors are just a few of the types available. 

7.1.1.1 	Helical Earth Anchors

Helical earth anchors are designed to be augured (screwed) into the ground and are often referred to as ground anchors. Helical earth anchors typically consist of a shaft, head, and one or 

more helixes. The head is used for installing and fastening the anchor to the home, and tensioning the anchor. Toward the bottom of the shaft, there is one (single) or more helical disks 

for the anchor to be screwed into the soil. The helix provides much of the anchor’s load capacity. The typical lengths of helical ground anchors are 30, 36, 48, and 60 inches.

Helical anchors also may be installed with stabilizer plates to increase the lateral capacity of the 

anchor by enlarging the surface area used to develop passive soil resistance. 

7.1.1.2 	Concrete Anchors

Concrete anchors use dead weight of a concrete footing or a combination of concrete weight 

and soil uplift resistance. Uplift resistance can be increased by the use of drilled concrete piers. 

In both cases, the home must be securely attached to the concrete elements using anchor bolts, 

tie-rods, or other structural connection. Anchor attachments placed in concrete must be installed with adequate depth to develop the required strength. The load capacity of the anchors 

must be sufficient to resist applicable design loads.

7.1.1.3 	Cross Drive Anchors

Like helical ground anchors, cross drive anchors 

are constructed with a head secured to a metal 

shaft. Cross drive anchors are shafts driven into 

competent soils to develop their resistance. As 

their name implies, cross drive anchors are driven in pairs that form an “X” or cross (Figure 

7-1). The heads of the anchors are secured to the 

home with metal straps.

Figure 7-1. Cross drive anchor. (Courtesy of Tie Down

Engineering)
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7.1.2	 Anchor Construction and Capacity

Steel ground anchors are the most common anchor assembly application for manufactured 

home installations. Ground anchors are typically constructed with a circular shaft of one or 

more helixes; a head connects at the opposite end of the anchor, which then connects to the 

home’s frame and/or sidewalls with steel straps or cables. Anchor shafts are typically 5/8 inch 

to 3/4 inch in diameter, and helixes range from 3 inches to 8 inches in diameter. Most anchors 

used for manufactured home applications have one helix, although anchors with two to four 

helixes are available (Figure 7-2). 

Most anchor heads are “U” shaped and contain predrilled bolt holes. The bolts connect the 

anchor to the home’s frame or sidewalls with 11⁄4-inch anchor straps. Some anchors have heads 

with closed eyes for cable connections. The bolts in “U” shaped anchor heads can be used to 

pre-tension the anchor. Pre-tensioning an anchor with closed-eye heads requires using other 

devices like turnbuckles. 

HUD requires anchoring equipment and anchoring assemblies to be capable of resisting allowable minimum working loads of 3,150 pounds and ultimate loads of 4,725 pounds without failure 

of either the anchoring equipment or the attachment point on the manufactured home. 

7.1.3	 Anchor Selection

Ground anchors should be selected based on the specific soils conditions at the manufactured 

home site. Short anchors with smaller helixes may be used in firm, well compacted soils. Longer 

anchors or anchors with larger or multiple helixes are required in weaker soils.

Most anchors are selected based on standard torque probe tests conducted at the site. During 

those tests, a 5-foot long auger probe is screwed into the ground to the approximate depth of 

the anchor helix. A torque wrench measures the torque required to advance the probe. The 

resulting torque value is used to classify the soils and select an appropriate anchor based on 

Figure 7-2. Single and

double helix ground

anchors with strap

connection and single

helix anchor with a

closed-eye connection.
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recommendations of the anchor manufacturers. Table 7-1 depicts how one anchor manufacturer classifies soils for anchor selection. 

7.1.4	 Anchor Installation

Anchors can be machine-installed or handinstalled. Machine installation involves using 

portable torque equipment to rotate and advance the anchors into the supporting soils. 

Figure 7-3 shows an electric installation device, also known as a portable anchor drive 

machine.

Hand installation involves excavating holes 

for the anchors, placing the anchors into 

the holes, and then backfilling and compacting the removed soil. Hand installation 

disturbs significantly more soil than machine 

installation and reduces anchor load capacity. Manufacturers typically limit the depth of 

pre-drilling to two-thirds the anchor length. They also recommend against hand installations in 

areas with poor soils.

Anchors are typically installed vertically or inclined slightly from vertical to facilitate installation. 

Typical inclinations range from approximately 5 to 15 degrees from vertical. Where required, 

stabilizer plates are installed adjacent to the anchor shaft (Figure 7-4). This allows anchors to 

Table 7-1. Soil Classifications Using a Standard Torque Probe (STP) 

Figure 7-3. Electric anchor drive machine.

(Courtesy of Tie Down Engineering)
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be installed after the manufactured home is positioned and locates the exposed anchor heads 

behind the manufactured home’s skirting. Anchor straps are then installed to the manufactured home frames and, in HUD Wind Zones II and III, to the wall ties installed when the home 

is fabricated. When used in this configuration, stabilizer plates are installed to provide additional lateral resistance for the anchor assembly.

When fastened as vertical wall ties and loaded axially, ground anchors may be used without 

stabilizer plates. When ground anchors are used with some proprietary foundation systems, anchors can be installed 45 degrees to the horizontal and loaded axially (Figure 7-5). 

7.1.5	 Anchor Performance

The capacity of anchoring systems is a function of the soil response to loads applied to the anchor assembly. For ground anchors, important geotechnical considerations include soil type or 

classification, soil shear strength, load-deformation characteristics (i.e., modulus of subgrade 

reaction, or stiffness), and moisture condition. Research has identified additional considerations impacting the capacity and performance of ground anchors, including anchor geometry, 

anchor depth, anchor orientation, and direction of load relative to that orientation.

Soil response mechanisms are different for axially and non-axially loaded ground anchors. Soil 

response for axially loaded ground anchors is the result of shear stresses along the failure plane. 

Figure 7-4. Typical

ground anchor

installation.
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Failure occurs when the stresses exceed the soil shear strength. The geometry of the failure 

surface varies based on the critical depth of the anchor. Ground anchors with an embedment 

depth less than the critical depth to anchor base width ratio ((D/B)cr) respond as a shallow 

foundation. Ground anchors with a depth greater than (D/B)cr respond as a deep foundation. 

Non-axially loaded ground anchors resist lateral movement by mobilizing the passive resistance 

of the surrounding soil. As small diameter rods develop little passive soil pressure resistance, 

stabilizer plates are used in the contact area to increase the passive resistance and reduce movement.

Figure 7-6 plots anchor load (in pounds) as a function of anchor head displacement (in inches), 

and shows the typical response for an anchor installed vertically and loaded axially. A test load 

of 6,000 pounds resulted in a movement of less than 2 inches. The graph also indicates that little 

or no movement occurred at loads less than about 225 pounds. 

Vertical, axially loaded anchors are relatively efficient. The soil shear failure is a symmetrical, 

roughly truncated cone extended from the anchor bearing plate to the ground surface. Nonvertical, axially loaded ground anchors are somewhat less efficient due to the asymmetrical 

shape of the shear failure surface. The asymmetry is the result of the shallow depth at the top 

side of the anchor relative to the bottom side.

Movements of axially loaded anchors are proportional to the applied loads. The load-movement 

is approximately linear in granular or non-cohesive soils. Cohesive soils tend to be less linear, 

particularly as the amount of clay in the soil increases. Soil related anchor failure in granular 

soils may occur rapidly and may cause ground surface movements around the anchor. Anchors 

in cohesive soil typically fail more slowly. 

Figure 7-5. In line ground

anchor installation. 
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GROUND ANCHORS 7

Anchor failure may also result from weld failure between the anchor shaft and the anchor head, 

weld failure between the shaft and the helix, collapsing of the anchor helix, or metal tearing 

around the anchor head strap bolts.

Anchors used with stabilizer plates respond differently than axially loaded anchors. Figure 7-7 

shows the typical response for non-axially loaded anchors used with stabilizer plates. The small 

diameter shaft does not create much passive resistance in the soil; therefore, small loads produce relatively large movements. After the anchor shaft contacts the stabilizer plate, lateral 

movement as a function of load decreases significantly as a result of a larger area of passive resistance mobilized by the stabilizer plate. After the shaft contacts the stabilizer plate, stiffness, 

as indicated by the slope of the load vs. movement curve, increases significantly. However, the 

apparent stiffness, as indicated by the slope of the curve, remains less than axially loaded anchors.

For both axially and non-axially loaded anchors, the design stiffness should be selected to account for the variability of individual anchor performance during anchor tests, including an 

assessment of the upper limit of the applicability of the stiffness factor.

Shear strength in saturated soil is typically less than in non-saturated soils. Thus, anchors in 

saturated soils have a lower load capacity than non-saturated soils, and may exhibit decreased 

stiffness, especially in clays. 

Figure 7-6. Typical response for an axially loaded anchor.
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7.1.6	 Anchors and Other Foundation Elements

Ground anchors are used with masonry piers, wood posts, or steel jack stands. For satisfactory 

performance, the piers, posts, and stands must function with relatively large displacements that 

the anchors may experience when subjected to flood, high-wind, or seismic loads. Large displacements are especially likely for non-axially loaded ground anchors. If subjected to relatively 

large movements, many foundation components cannot maintain their integrity. Piers, for example, can experience failure due to sliding of unmortared blocks or overturning of reinforced 

piers supported on ground surface pads (Figure 7-8).

7.2	 FEMA Anchor Test Program

7.2.1	 Anchors in Saturated Soils

Flooding can decrease soil shear strength and change the subsurface hydrostatic pressures as 

a result of an increase in the pressure gradient. The two mechanisms affect ground anchor 

performance differently. The effects are a function of soil characteristics, anchor end plate diameter, installation depth, and change in hydrostatic pressure. Research has indicated that, for 

a fixed depth, the ultimate capacity of screw anchors in dry sand is higher than screw anchors in 

submerged conditions. The impact of the reduction in capacity is significantly greater for shallow anchors than for deep anchors.

In 2002, FEMA participated in a field test program of 120 anchors to provide performance data 

for developing pre-engineered foundation designs using ground anchors. Sixty anchors were 

tested in a saturated site, and 60 anchors were tested in an adjacent dry site as a control group. 

Figure 7-7. Typical response for a non-axially loaded anchor used with a stabilizer plate.
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The results generally support the findings of previous research and have been used to develop 

the foundation designs included in this guide. 

The tests were conducted by representatives from FEMA and its engineering consultants, 

HUD, the Manufactured Housing Institute (MHI), Florida Manufactured Housing Association 

(FMHA) and several ground anchor manufacturers (Tie Down Engineering, Oliver Technologies, Minute-Man, and Style Crest).

7.2.2	 Anchor Test Results

Tables 7-2 and 7-3 contain the test results for dry site anchors and wet site anchors, respectively. 

Figure 7-9 shows three anchor configurations described in Tables 7-2 and 7-3. The data shown 

are for anchor stiffness or the amount of load the anchors develop per inch of anchor head 

movement. The data are raw data and are not indicative of design or working load values.

The data also include the standard deviations for the data groups. The data show that the performance of individual ground anchors varies significantly from the average performance of 

the group of anchors. This variance, or “spread” in ground anchor performance has a significant effect on appropriate design values. The greater the relative spread in individual anchor 

performance, the lower the appropriate design value.

Figure 7-8. Failed

foundation system that

used masonry piers.
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Table 7-2. Dry Site Anchor Data

Anchor Style and Installation

Anchor Stiffness (pound/inch)

Min Max Average Standard

Deviation

5-foot anchor installed at 45° and loaded 

axially (Figure 7-9a) 1,505 3,283 2,426 543

5-foot anchor installed vertically and loaded 

axially (Figure 7-9c) 1,774 6,639 3,801 1,394

5-foot anchor installed 15° from vertical and 

used with an 11-inch by 17-inch stabilizer plate 

(Figure 7-9b)

1,006 2,001 1,475 294

4-foot anchor installed 15° from vertical and 

used with an 8-inch by 24-inch acrylonitrile 

butadiene styrene (ABS) stabilizer plate 

(Figure 7-9b)

1,019 2,612 1,721 497

4-foot anchor installed 15° from vertical and 

used with an 11-inch by 17-inch stabilizer plate 

(Figure 7-9b)

817 1,614 1,197 257

Table 7-3. Wet Site Anchor Data

Anchor Style and Installation*

Anchor Stiffness (pound/inch)

Min Max Average Standard

Deviation

5-foot anchor installed at 45° and loaded 

axially (Figure 7-9a) 2,874 9,019 5,506 1,829

5-foot anchor installed vertically and loaded 

axially (Figure 7-9c) 2,166 6,581 4,112 1,391

5-foot anchor installed 15° from vertical and 

used with an 11-inch by 17-inch stabilizer plate 

(Figure 7-9b) 

833 1,412 1,094 193

4-foot anchor installed 15° from vertical and 

used with an 8-inch by 24-inch ABS stabilizer 

plate (Figure 7-9b)

655 2,006 1,538 370

4-foot anchor installed 15° from vertical and 

used with an 11-inch by 17-inch stabilizer plate 

(Figure 7-9b) 

1,024 1,894 1,319 300

* See Figure 7-9.

Note that anchor stiffness values are based on the linear portion of the load deflection data. In 

most cases, the non-axially loaded anchors’ significant movements occurred prior to reaching 

the linear response range. It should also be noted that the upper and lower limits of applicability of the stiffness values have not been fully evaluated. For axially loaded anchors in dry sand, 

the linear zone appears to exceed 6,000 pounds, about the limit of the test. Vertically loaded 

vertical anchors in wet sand had similar results; however, axially loaded anchors installed at 45 
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GROUND ANCHORS 7

degrees in wet sand had an upper limit of about 4,500 to 5,000 pounds. Similar differences were 

observed for inclined anchors.

7.3	 Recommended Ground Anchor Certification, Performance, and

Design Values

7.3.1	 Recommended Ground Anchor Certification Performance

Currently, there is no consensus based standard protocol for certifying ground anchors used to 

anchor manufactured homes. Some States have developed certification procedures and a task 

group under HUD’s Manufactured Housing Consensus Committee (MHCC) has been working 

on a Ground Anchor Test Protocol (GATP) for certifying ground anchors. Although no standard protocol had been completed at the time this guide was published, the ground anchor 

design strengths included in this guide assume that the ground anchors will achieve a level of 

performance that can be relied upon to anchor the home and resist applied loads from a design event. 

7.3.2	 Recommended Ground Anchor Design Values

The following data present recommended ground anchor design values for use when ground 

anchors have not been load tested or otherwise certified for a specific capacity. The information, while limited, is based on the best available data from the FEMA ground anchor testing 

program. FEMA’s testing program gathered data on ground anchor performance and soil saturation effects, and was conducted on sandy Class 4b soils.

Figure 7-9. Three

graphics showing the

anchor configurations

described in Tables 7-

2 and 7-3. From left to

right, they are (a) ground

anchor installed at 45

degrees and loaded

axially, (b) ground

anchor installed at 15

degrees from vertical and

loaded 45 degrees from

horizontal, and (c) ground

anchor installed vertically

and loaded axially.
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The recommended design loads contained in Table 7-4 were developed by applying the statistical factors required to produce a design load that provides a 10 percent lower exclusion limit 

with a 90 percent confidence level. The factors were applied to the FEMA test data. 

Table 7-4. Recommended Design Loads – from FEMA Ground Anchor Testing Program

Anchor Style and Installation – FEMA Tests Recommended Design Load (pounds)

5-foot anchor installed at 45°and loaded axially 3,150

5-foot anchor installed vertically and loaded axially 3,000

5-foot anchor used with an 11-inch by 17-inch stabilizer plate 2,000

4-foot anchor with an 8-inch by 24-inch ABS stabilizer plate 2,000

4-foot anchor used with an 11-inch by 17-inch stabilizer plate 2,000

Recommended design stiffness values were selected that provide a 90 percent confidence level; 

this means that no more than 10 percent of individual anchors would provide resistances less 

than design values. This level of statistical performance is less conservative, but similar to statistical performance levels applied to other structural materials like steel or concrete. 

The lowest stiffness values from the dry and wet sites were selected for design. Generally, the dry 

site values controlled for axial pull anchors while the saturated site anchor values controlled for 

anchors using stabilizer plates. Table 7-5 lists the recommended design stiffness for the anchors 

tested.

Table 7-5. Recommended Design Stiffness for Tested Anchors

Anchor Style and Installation

Anchor Design

Stiffness

(pound/inch)

Controlling Soils

5-foot anchor installed at 45° and loaded axially 1,200 dry

5-foot anchor installed vertically and loaded axially 1,010 dry

5-foot anchor used with an 11-inch by 17-inch stabilizer plate 675 wet

4-foot anchor with an 8-inch by 24-inch ABS stabilizer plate 708 wet

4-foot anchor used with an 11-inch by 17-inch stabilizer plate 659 wet

With a 3-inch displacement, the axially loaded anchors provide working loads greater than 

the 3,150-pound loads required by HUD (e.g., 1,200 pounds/inch x 3 inches = 3,600 pounds). 

However, the anchors with stabilizer plates provide only about two-thirds of the HUD required 

capacity. The anchor stiffness values in Table 7-5 are based on the linear portion of the load-deflection curve. Designers need to evaluate potential deflections at loads below the linear range, 

particularly for inclined anchors loaded non-axially.
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7.4	 Ground Anchors in Seismically Active Areas

While data exist on the performance of ground anchors in static saturated soils, no definitive 

testing has been completed that predicts how ground anchors will perform in saturated soils 

during a seismic event. Seismic events can result in soil liquefaction, particularly in fine sands 

like those in the test program. Liquefaction may result in a significant and potentially total loss 

of anchor capacity during a seismic event. A registered engineer should be consulted to evaluate the liquefaction potential at the proposed manufactured home installation site.

8	 Foundation Systems

8.1	 Introduction

Properly designed and constructed manufactured home foundations can significantly reduce 

the potential for damages from flooding, high winds, and seismic events. Many styles of foundations are available for supporting manufactured homes. Continuous perimeter walls, constructed 

of concrete, masonry, or treated wood; concrete or masonry piers; wood posts and piles; and 

systems consisting of piers and ground anchors are some of the options available to a manufactured homeowner and installer. 

To be effective, manufactured home foundations must perform the following:

n Support the weight of the home, its contents, and its occupants

n Resist design loads from wind, snow, seismic events, and moving floodwaters

n Elevate the home sufficiently to prevent losses from a design flood event 

Elevated foundations are classified as enclosed or open. As the name implies, enclosed foundations enclose the space below the elevated home. Perimeter masonry, concrete, or sheathed 

wood walls are enclosed styles. Open foundations consist of a system of individual members that 

support the home only at discrete locations. Pier, post, and pile foundation systems are open 

systems. Many open foundations have a non-structural skirting to enclose the space below the 

home. The skirting is primarily for aesthetics and does not add to the strength of the foundation. Skirting can help to protect piping installed below the home from freezing and reduce 

both heat loss from the home and the potential for animal or insect entry into the home. 

Selecting an appropriate foundation system depends on a number of variables, including the 

building site conditions (elevation, slope, access restrictions, drainage); the flood zone; the 

design loads (wind, flood, snow, and, if appropriate, seismic) at the site; the availability of materials; on-site soil characteristics; local construction practices; and cost. Whatever foundation 

system is chosen, both the HUD Code and NFIP require a foundation to resist flotation, collapse, 

or lateral movement during a design event to prevent damages to the home and surrounding 

structures. Tables 8-1 through 8-3 at the conclusion of this chapter provide guidance to the selection of appropriate manufactured home foundation systems as a function of flood depth and 

flow velocity.

Proprietary systems are also an option for the manufactured homeowner. Proprietary systems 

are discussed in Section 2.2.2 and the system must meet the design and performance criteria 

described in Chapter 9.

More information on proprietary foundations is contained in the SBRA’s Guide to Foundation 

and Support Systems for Manufactured Homes available at http://www.research-alliance.org/pages/

foundations_guide.htm.

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8.2	 Enclosed Foundations

Enclosed foundations consist of perimeter foundation walls placed on continuous footings. The 

walls enclose the area below the living space of the home (Figure 8-1). Perimeter walls are commonly constructed of concrete, masonry, or wood. Enclosed foundations are occasionally used 

to raise manufactured homes, often creating the appearance of a site-built home (Figure 8-1). 

NFIP regulations permit enclosed foundations for manufactured homes in SFHA A zones with 

a requirement that the foundation walls include flood vents. 44 CFR 60.3(c)(5) requires that: 

“all new construction and substantial improvements, that fully enclosed areas below the lowest floor that are usable solely for parking of vehicles, building access 

or storage in an area other than a basement and which are subject to flooding 

shall be designed to automatically equalize hydrostatic flood forces on exterior 

walls by allowing for the entry and exit of floodwaters. Designs for meeting this 

requirement must either be certified by a registered professional engineer or 

architect or meet or exceed the following criteria: A minimum of two openings 

having a total net area of not less than one square inch for every square foot of 

enclosed area subject to flooding shall be provided. The bottom of all openings 

shall be no higher than one foot above grade. Openings may be equipped with 

screens, louvers, valves, or other covering or devices provided that they permit 

the automatic entry and exit of floodwaters.”

This elevation technique should not be used in high-velocity or highly erosive flood conditions, 

and is not permitted in V zones. Additional information on wall vents for floodwater flow is provided in FEMA Technical Bulletin 1, Openings in Foundation Walls and Walls of Enclosures (2008).

Figure 8-1. A

manufactured home

elevated on a perimeter

foundation wall. Although

it appears the openings

are too high, the bottoms

of the openings are less

than 1 foot above the top

of the interior slab.

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Perimeter walls of enclosed foundations should include adequate reinforcement to resist unbalanced hydrostatic and/or hydrodynamic loads that may occur in fast rising flood events. Such 

floods may result in water levels higher on the exterior side of the wall than the inside until flow 

through the flood vents can equalize them.

8.3	 Open Foundations and Breakaway Walls

NFIP regulations require manufactured homes in SFHAs designated V1-30, VE, or V on the 

community’s FIRM have the space below the lowest floor either free of obstruction or constructed with non-supporting breakaway walls (44 CFR 60.3(e)(5)). Elevating a manufactured home 

on an open foundation involves raising it onto piers, posts, or piles. If the home is located in 

an area of coastal flooding, an open foundation is the only way to safely elevate the home. If 

the home is subjected to high-velocity riverine floodwaters, significant water depth, or potential 

erosions, the home should also be elevated on an open foundation. Open foundations are intrinsically more resistant to moving floodwaters and breaking waves than enclosed foundations 

since the home has a smaller surface area exposed to flood forces. Selection of the proper open 

foundation for various flooding and site characteristics is critical to the success of the foundation.

Breakaway walls, including non-structural skirting around a manufactured home, are walls that 

are not part of the structural support of the home. They are designed and constructed to fail under the loads imposed by floodwaters without jeopardizing the elevated portion of the home or 

the structural support of the home. Because such enclosures are designed to fail at a lateral load 

of no greater than 20 pounds per square foot, they will transfer minimal additional loads to the 

foundation. Insect screening and latticework both allow floodwaters to pass through. Detailed 

discussions on design and construction for breakaway walls can be found in FEMA Technical 

Bulletin 9, Design and Construction Guidance for Breakaway Walls (2008).

8.3.1	 Pier Systems

Most manufactured homes are placed on pier foundations. Although there are many variations, 

pier foundations fall into two general styles. One style contains pier foundations combined with 

other components (such as anchors and frame straps) for lateral stability; the second style relies 

on the piers and their footings to resist all imposed loads. The foundation styles that use supplemental lateral-supporting devices often do not require reinforced piers. The determination of 

the need for and the amount of reinforcing required is part of the design analyses. The final 

design requirements for pier reinforcing is a function of the combination of flood, wind, and 

seismic loads acting on the manufactured home, and the resulting pier capacity required to resist flotation, collapse, or lateral movement.

When flood velocities are less than 1 fps, piers can be constructed using unbonded (dry-stacked) 

concrete blocks or steel piers. When pier foundation systems are used without separate components to resist lateral loads, the piers and footings must be much stronger. They not only have 

to transfer all imposed loads to the bearing soils, but also must do so in a manner that does not 

damage the piers and footings or overload the supporting soils.

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Piers designed to resist lateral and/or uplift loads without the use of ground anchors or straps 

typically consist of reinforced brick masonry, reinforced concrete masonry units, or reinforced 

cast-in-place concrete with steel reinforcing bars for both the piers and the below-grade footings. Because the ground around pier footings in SFHAs can be susceptible to erosion and 

scour, the footings must be embedded below the anticipated scour depth. 

In areas exposed to moving floodwaters, special consideration must also be given to controlling 

scour around the pier foundation elements. Moving floodwaters can remove soil around and 

beneath foundations, potentially reducing their load capacity to the point of foundation failure. 

Floodwaters with high flow rates and floodwaters that carry a large sediment load create more 

scour than low velocity or clear water flood flow. Because of this, scour is particularly damaging 

where floodwaters converge or abruptly change direction. Scour is generally greatest around 

discrete interior piers and the corners of perimeter wall foundations. 

Scour removes soil particles from beside and, in severe cases, from beneath foundations. The 

loss of soil around and/or beneath a foundation affects its capacity to support the design loads. 

The loss of vertical foundation capacity can result in large settlements and potential collapse. 

The loss of lateral capacity not only reduces the capacity of the foundation to resist lateral wind 

and flood loads, but also can reduce the vertical capacity. The loss of lateral support for long 

thin vertical elements (e.g., single block masonry stack piers) can result in buckling under the 

design vertical loads.

Piers designed to resist lateral and/or uplift loads must be constructed using mortared horizontal joints between courses and reinforced grout used to fill the vertical cell. The piers must be 

firmly attached to the supporting footings. 

8.3.1.1	 Reinforced Pier Systems

Reinforced piers typically have steel reinforcements placed 

inside of the piers. Reinforced pier systems are commonly 

constructed of concrete masonry units (CMUs) or cast-inplace concrete (Figure 8-2). 

In high-wind areas and in areas exposed to seismic or hydrodynamic loads, piers may require much larger footings 

to ensure applied loads do not exceed the soil’s bearing 

capacity or allow tensile forces to develop in foundation 

elements in the upstream and windward side of the home. 

Laterally bracing the piers can allow the structure to distribute imposed loads to the entire foundation system. 

Lateral bracing also provides protection against buckling 

failure of vertical elements due to loss of confining soil 

from erosion or scour.

Concrete Masonry unit 

(CMu) Reinforced Piers

The International Residential Code 

limits the height of solid masonry 

piers, including hollow concrete masonry units filled solidly with concrete or Type M or S mortar to ten 

times their least dimension. The 

height limitation for unfilled hollow concrete masonry unit piers 

is four times their least dimension 

(IRC§606.6).

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Adequate connections between the piers and the manufactured home are necessary for the 

manufactured home and its foundation to resist lateral and uplift loads from floods, winds, and 

earthquakes. Generally, multiple fastener bolted connections are needed to connect the top of 

the piers to the manufactured home frames when the piers must transfer moments. If the piers 

must resist only uplift loads, fastening requirements may be simplified. Regardless of the complexity of the connection, consult the manufactured home manufacturer to ensure the factory 

built components are not overloaded. Figure 8-3 shows a method used to fasten a home’s steel 

frames to reinforced masonry piers using nuts, steel plates, and bolts grouted into the piers. 

8.3.1.2	 Unreinforced Pier Systems

Unreinforced brick or CMU pier systems have no reinforcing steel and, therefore, have very little resistance 

to overturning, sliding, and uplift. Because of this, 

unreinforced piers should always be provided with other 

stabilizing devices like ground anchors.

When used with stabilizing devices, many styles of 

unreinforced piers are available to support manufactured 

homes; the styles provide varying degrees of strength to 

resist loads on the home. These systems can be constructed without mortar (called dry-stacked masonry); with 

dry-stacked block lightly secured with a surface bonding 

material; with mortared block; or with fully mortared, grouted block.

Figure 8-2. Reinforced masonry and concrete piers.

Dry-Stacked Piers

Dry-stacked block piers can fail 

when a home is exposed to combined wind and flooding (ASCE 7, 

Load Combination #6). Failure results when wind forces lift the 

manufactured home’s frame off its 

windward piers. Without the weight 

of the home to stabilize the piers, 

the piers can easily fail by sliding 

or overturning.

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Figure 8-3. Bolted

connection between

frame and reinforced pier.

When placed directly on concrete footings or pads, 3-foot tall piers constructed with single, 

dry-stacked blocks to create an 8-inch by 16-inch pier can only resist flood velocities of approximately 1.0 fps. Three-foot tall (16-inch by 16-inch) double-stacked piers can resist flood 

velocities of approximately 1.75 fps. Dry-stacked piers or posts supported on ABS pads fail at 

lower velocities than piers supported on concrete. The failure at lower velocities is due to a reduced frictional resistance between the pier blocks or posts and the ABS pad. The design flood 

velocity for double-stacked piers on ABS pads is 1.25 fps.

If dry-stacked piers are not fully submerged, they can resist higher flood velocities. This is because the portion of the pier above the water line adds to the pier’s stability but does not add 

to the flood load that the pier must resist. Engineers can calculate the ability of partially submerged piers to resist moving floodwaters or their resistance can be determined by testing.

Applying surface-bonding materials strengthens the piers by increasing their shear resistance. 

However, surface bonding has limited impact on their resistance to bending moments from 

lateral loads; therefore, surface bonded piers still need to be used with other foundation components. The increased shear resistance allows 3-foot tall (16-inch by 16-inch) piers to resist 

hydrodynamic loads from floodwaters moving at approximately 2.0 fps for single-stacked piers 

and 3.0 fps for double-stacked piers. This resistance to moving floodwaters can only be achieved 

when the surface bonding materials not only bond the individual pier blocks to each other, but 

also the pier to the concrete footing below. A FEMA sponsored testing program conducted at 

the Haynes Coastal Engineering Laboratory, Texas A&M University verified the design values.

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The Texas A&M testing program included piers constructed using a polyurethane based masonry adhesive as joint bonding material. The Illinois Tool Works (ITW) TACC Division's Mason 

Bond was certified by ICC Evaluation Service as meeting the IBC and IRC for Types M, N, O, 

and S Portland cement/lime mortar. Test results showed the single stack adhesive bonded piers 

to be the strongest configuration tested.

Mortared block and fully grouted mortared block are much stronger than dry-stacked and surface-bonded piers, but their strengths do not match piers with #3 reinforcing steel bars grouted 

into the vertical cells. Mortared and fully grouted piers usually require other foundation components (like shear walls) to resist lateral loads. 

Reinforced piers (constructed by introducing reinforcing steel to fully grouted piers) can be 

made to resist lateral and vertical loads when used with other foundation components like large 

concrete footings. 

Unreinforced piers cannot be used in V zones. In Coastal A zones, fully grouted piers may be 

adequate for low flood velocities, but reinforcing with steel is recommended.

8.3.2	 Pile Foundations

Pile foundations provide protection for the broadest range of flooding conditions. This foundation system consists of the pile supports, horizontal beams, longitudinal support under the 

manufactured home, and foundation bracing for additional resistance to lateral wind, floodwaters, and seismic events. A properly designed pile foundation can withstand high-wind and water 

velocities, and can resist erosion and scour around its base if embedded to an adequate depth 

(Figure 8-4). Because of this, pile foundations meet the NFIP requirements for installation in 

V zones. Pile foundations are also appropriate for Coastal A zones and for areas exposed to highvelocity riverine flooding.

Figure 8-4. Manufactured

home on a pile

foundation.

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8 FOUNDATION SYSTEMS

Saturated soils with low bearing capacities are less of a stability problem for a pile foundation 

than for a pier foundation; thus, pile foundations are preferable in coastal areas. The design of 

pile foundations requires determining the number, size, length, and location of piles appropriate to the particular manufactured home, soil conditions, and flooding situation at the site. A 

pile design methodology is provided in the Coastal Construction Manual (FEMA 55) that requires 

input parameters, including soil property and loading information.

Piles are vertical supports similar to posts, but differ in the method and depth of placement. 

Piles are embedded much deeper than posts and do not rest on footings for resistance. Instead, 

the piles are driven until they rest on a solid support layer, such as bedrock, or until they are 

embedded deep enough that the friction between the ground and the piles will enable them to 

resist the gravity, lateral, and uplift loads expected to act on them. 

The most commonly used piles in residential construction are wood. Steel and precast concrete 

piles are also used. Pile foundations are primarily used in areas where other elevation methods 

are not feasible, such as V zones. 

A major consideration in the effectiveness of pile foundations is the method of installation. Piles 

are placed into the ground by impact driving, water jetting, augering, or some combination of 

these methods (Figure 8-5). Piles are often driven by a single- or double-acting diesel hammer 

or an air/steam hammer. Pile driving is an excellent method due to the strength of the pile and 

the ability of the pile and its soil interface to resist vertical and horizontal loads. 

A less desirable, but frequently used method is jetting. Jetting inserts piles into sandy soil by 

forcing a high-pressure stream of water through a pipe along the side of the pile. The stream of 

water creates a hole in the sand while the pile is continuously pushed or dropped to the desired 

depth. Jetting results in a lower load capacity due to loose soils that create decreased friction between the piles and the surrounding soil. Jetted piles must be inserted deeper into the ground 

than driven piles in order to achieve the same load capacity.

Figure 8-5. Pile driving

methods.

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Another method is the use of an auger to pre-drill holes for piles. If the soil is composed of adequate clay or silt, using an auger to create holes for piles is sufficient. Additionally, some sands 

may contain enough clay or silt to permit the use of an auger. This method can be used by itself 

or in conjunction with pile driving. 

Pile installation methods, including driving, jetting, and, to a lesser extent, augering can make 

precise location control difficult. Also, irregularities in the piles and soil will often prevent the 

piles from being driven perfectly vertical. When using piles to support manufactured housing, 

wood beams are typically secured to the piles and the home is secured to the beams.

When soils near the top of a pile are lost due to scour, the pile loses some of its ability to resist 

vertical and lateral loads. Erosion and scour must be taken into account when determining pile 

embedment depth and lateral bracing requirements. Due to the variability associated with differing installation methods and erosion/scour potential, a geotechnical engineer should be 

involved in the design process to verify that intended pile capacities are achieved.

8.4	 Bracing

Bracing is often used to lower the point of application of lateral loads to reduce moments 

applied to the foundation system (cross bracing) or to provide lateral support to resist buckling (knee bracing). Diagonal bracing runs diagonally from one vertical supporting member to 

another, stiffening the vertical supporting members and increasing their strength and lateral 

stability (Figure 8-6). Unfortunately, with greater strength comes a larger exposure to wave and 

debris impact. Diagonal bracing is too slender to resist compressive forces and is typically only 

designed to carry tension forces. This technique of bracing is especially beneficial in higher elevated homes. 

Steel rods are often used to diagonally brace wood posts or piles. The rods are fitted through 

drilled holes filled with wood preservative and fastened with nuts and cast beveled washers. 

Rod bracing offers two important benefits. One, rod bracing can easily be fitted with turnbuckles that allow bracing to be tightened after an event that creates loads in the bracing; two, rod 

bracing has smaller cross-sections than bracing created with dimensional lumber and thus is 

exposed to lower flood forces. 

Knee bracings are short diagonal braces that run from a vertical support member to a horizontal support member (Figure 8-7). Knee braces can be effective in supporting the pile against 

the lateral forces of wind and water, Knee bracing increases the strength and stiffness of the 

extended pile foundation by retraining rotation near the top of the pile and reducing the pile 

bending length. Knee bracing is not as stiff as diagonal bracing. Knee braces have an advantage 

over diagonal braces in that they present less obstruction to waves and debris. Knee braces are 

shorter than diagonal braces and are usually designed for both tension and compression forces. 

Engineers should be consulted to determine bracing designs, particularly for knee bracing.

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8 FOUNDATION SYSTEMS

8.5	 Footings

Footings are the components of a foundation system that transfer loads applied to a home to 

the earth below it. Footings continuously support gravity loads (and are generally well designed 

for this purpose), but they also must transfer lateral and uplift loads produced by wind events, 

seismic events, snow accumulation, and moving floodwaters.

Figure 8-6. Diagonal bracing.

Figure 8-7. Knee bracing.

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The soils below the footings must support the home and resist all the loads applied to the 

home. When soils are strong, footings can be relatively small and foundation systems relatively 

compact. When soils are weak, however, the footings become large, complicated, difficult to 

construct, and quite expensive. Even when placed on firm soils, footings often need to be oversized to provide sufficient weight to resist uplift forces and overturning moments.

The design size of a pier footing is a direct function of soil bearing capacity. Soil bearing capacity can be directly determined by soils tests. In some jurisdictions, approximate bearing capacity 

can be assessed by soil classification. Model building codes (such as the IBC and NFPA 5000) 

have presumptive soil bearing capacity values that can be used in the absence of soils test data. 

The depth of the footings depends on local frost levels and expected scour depths (whichever 

is greater). Local codes may provide specific requirements for the depth of footings based on 

local soil conditions. 

8.6	 Foundation Materials Selection

As stated in 44 CFR 60.3(a)(3), all structural and non-structural materials at or below the BFE 

must be flood damage-resistant. A flood damage-resistant material is defined as any building 

material capable of withstanding direct contact with floodwaters for 72 hours without sustaining 

significant damage (i.e., damage requiring more than low-cost cosmetic repair, such as painting). In addition, materials should be durable and resistant to decay and corrosion.

Some flood damage-resistant foundation material choices include the following:

n Pressure-treated lumber 

n Naturally decay-resistant lumber (only for applications above grade)

n Concrete: a minimum 28-day compressive strength of 5,000 lb/in2 is recommended in 

coastal environments 

n Masonry: reinforced and fully grouted in coastal environments

n Steel: corrosion-resistant

n Closed-cell foam insulation

n Other flood damage-resistant materials approved by local building officials

The most commonly used foundation materials are wood, concrete, steel, and masonry. Their 

properties, advantages, and special considerations are discussed in Sections 8.6.1 through 8.6.4. 

Additional information on materials’ durability can be found in FEMA 55 and trade organization publications. FEMA’s Technical Bulletin 2, Flood Damage-Resistant Materials Requirements 

(2008) also contains information on appropriate materials used in SFHAs. 

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8 FOUNDATION SYSTEMS

8.6.1	 Wood Foundations

Wood is a very workable material and one of the most cost-effective; however, it is susceptible to 

decay, insect infestation, marine borers, and weathering. Wood must be adequately maintained 

to ensure the foundation’s integrity. All wood used in foundation piles, girders, beams, braces, 

and walls must be pressure-preservative treated or, when not in direct contact with the ground, 

naturally decay-resistant. No wood with natural resistance to decay is considered to have sufficient decay resistance for ground contact or partial water immersion. Wood exposed to the 

ground and exterior elements should be pressure-preservative treated to increase its resistance to 

infestation and decay. The degree of resistance depends on the treatment chemical and the 

amount of retention in lb/ft3

 of wood.

The preservatives used in pressure treating wood for foundation applications; e.g. piles, piers, and posts, include:

n chromated copper arsenate (CCA) 

n pentachlorophenol

n creosote

n copper azole

8.6.2	 Concrete Foundations

Concrete is an economical and workable foundation 

material that is extremely good at resisting compressive loads. Concrete can be reinforced to 

increase its ability to withstand tensile loads that often result from flood, wind, and seismic activity. Corrosion of the reinforcement and the cracking of the concrete weaken the concrete 

structural element, reducing its ability to resist loads. Providing adequate concrete cover to 

reinforcement is the best defense against corrosion. Consult the latest version of American 

Concrete Institute (ACI) publication 318, Building Code Requirements for Structural Concrete, for 

minimum concrete cover requirements. Additional protection from corrosion can be achieved 

by using epoxy-coated reinforcement.

Proper mixing, placement, and curing are essential for durable concrete. During placement, 

concrete will normally require vibration to eliminate air pockets and voids in the finished surface. The vibration must be sufficient to eliminate the air, but not to separate the concrete or 

water from the mix. Appropriate freeze protection may be needed if pouring is done in cold 

temperatures. Concrete placed in cold weather takes longer to cure, and the uncured concrete 

may freeze, which will adversely affect its final strength. Methods of preventing concrete from 

freezing during curing include the following:

n Heating adjacent soil before pouring

n Warming the mix ingredients before batching

n Placing insulating blankets over and around the forms after pouring

n Selecting a cement mix that will shorten curing time

ACQ- or ACZA-Treated Wood

Designers and builders considering 

the use of ACQ- or ACZA-treated 

wood should consider using stainless steel hardware and fasteners or 

obtain the latest information on the 

chemicals’ interaction with hardware 

due to reports of alleged problems 

with corrosion of galvanized framing 

hardware and nails in contact with 

wood treated with these chemicals.

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FOUNDATION SYSTEMS 8

Because the environmental impact of salt-laden air and moisture make the damage potential 

significant for concrete, this guide recommends that all concrete construction in and near 

coastal flood hazard areas (both V and A zones) be built with the more durable 5,000-pounds 

per square inch (psi) minimum compressive strength concrete regardless of the purpose of the 

construction and the design loads.

8.6.3	 Steel Foundations

Prefabricated steel stands are available for supporting manufactured housing. Like unreinforced 

masonry piers, steel stands have little resistance to overturning and should only be used in 

conjunction with other foundation components like ground anchors or perimeter shear walls. 

Metal stands should also be firmly secured to the homes’ frames. 

8.6.4	 Masonry Foundations

Reinforced masonry has much more strength and ductility than unreinforced masonry for resisting large flood, wind, and earthquake forces. It is recommended that permanent masonry 

construction in and near coastal flood hazard areas be reinforced and fully grouted regardless 

of the purpose of the construction and the design loads. 

Moisture can have a damaging effect on masonry construction. Moisture-borne salts in coastal environments 

entering the piers through cracks or openings in the 

masonry joints can cause cracking and spalling of the 

masonry. Moisture entering piers in cold weather environments can expand upon freezing, causing small cracks 

to become large cracks. The entry of moisture into reinforced masonry construction can lead to corrosion of the 

reinforcement and additional cracking and spalling of the masonry. Moisture resistance is highly influenced by the quality of the materials and the quality of the masonry construction at the 

site. For CMUs, choosing Type I “moisture controlled” units and keeping them dry in transit 

and on the job will minimize shrinkage and cracking. For optimum crack prevention, Type S 

mortar should be used for below-grade applications and Type M mortar may be used for abovegrade applications. 

In addition to Portland cement/lime based mortar, polyurethane based masonry adhesives are 

now available. Care must be taken in the selection and application of masonry adhesives. As a 

minimum, the selected adhesive must be certified by a nationally recognized organization as 

meeting or exceeding the requirements for Types M, O, and S cement/lime based mortar and 

approved for use in masonry construction designed in accordance with applicable provisions of 

the IRC and IBC.

8.7	 Foundation Selection and Flood Resistance

Flooding can have a dramatic effect on the suitability and stability of a manufactured 

home foundation. But not all floods are the same, and the type of flooding, along with its 

Open Masonry Foundations

Open masonry foundations in earthquake hazard areas require special 

reinforcement detailing and pier proportions to meet the requirements 

for increased ductility.

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8 FOUNDATION SYSTEMS

char-acteristics and severity at a site, will eliminate many foundation types. Sections 8.7.1 through 

8.7.5 provide guidance (for installers, owners, community officials, and designers) on selecting 

foundations suitable to different flood types and site conditions. 

8.7.1	 Flooding Types

Flooding can be divided into seven major types: coastal, riverine, flash flood, alluvial fan, mudflow, lake or pond overflow, and poor drainage. Unique hazards are associated with each of 

these flooding types. 

n Coastal flooding is usually accompanied by waves, high velocity flow, and erosion. Damages 

to structures are usually the results of erosion and scour, and direct impact from wave 

action. 

n Riverine flooding is associated with dominant hazards, including velocity, depth, and 

duration. These hazards are determined by several factors, including the slope of the 

channel and watershed, land uses within the watershed, and the extent, intensity, and 

duration of precipitation. (Both coastal and riverine flooding can transport damaging 

debris. Debris impact can weaken a structure and make it more vulnerable to damages 

from flooding.)

n Flash floods are accompanied by rapidly rising water and extreme flood velocities, with 

high debris carrying potential. Although they are generally of short duration, the forces 

exerted on structures by the high velocity floodwaters and debris can cause extensive 

damage in a very short time.

n Alluvial fan flooding is distinct in that the region of greatest flood hazard is not welldefined, and floodwaters can follow many different paths across a normally dry area. 

Alluvial fan floods usually occur in arid areas at the base of steeply sloping terrain, and can 

have extreme flood velocities and debris loads. 

n Mudflows are proximately caused by flooding and can be considered a river of liquid 

and flowing mud on the surfaces of normally dry land areas. Mudslides usually offer little 

warning and can be very destructive due to their debris load and velocity.

n Lake or pond overflow is rarely as hazardous as coastal or riverine flooding, and is usually 

limited to inundation by slowly moving or standing water.

n Poor drainage can lead to backups and generally results in ponding type flooding. Poor 

drainage is usually caused by a lack of topographic relief to allow for natural drainage from 

a site, or it might be the result of a specific hindrance such as blockage of a drainage ditch 

or an undersized culvert. In areas where drainage impediments become severe, they can 

compound risk and damages from other types of flooding such as riverine or flash floods. 

8.7.2	 Flood Characteristics

The flood source, its proximity to a site, and the flood hazard zone will provide information 

about flood characteristics. However, historical flood events at and near the site can also provide important clues about expected flood conditions at the site. All should be considered when 

home foundations are evaluated.

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FOUNDATION SYSTEMS 8

High velocities, large waves, and large floating debris can cause many home installations to fail, 

especially those on stacked masonry block, crawlspace, slab, shallow pier, and post foundations. 

Large flood depths can float or wash homes off their foundations. Long-duration floods can 

weaken soils, foundations, and anchor systems. Refer to Section 5.2 for more information on 

flood characteristics.

8.7.3	 Flood Hazard Zones

Table 8-1 is provided as general guidance for the selection of foundation systems for manufactured homes located at different flood hazard zones, Different flood hazard zones are 

represented on FIRMs by different zone designations. Detailed information pertaining to flood 

hazard zone designations and current NFIP regulations is provided in Chapter 3. 

8.7.4	 Proximity to Flood Source

Proximity to the flood source will, to a large extent, determine whether a home site is in a more 

or less hazardous location. As discussed in Chapter 4, sites in a floodway or closest to a river or 

stream will be subject to the greatest flood depths, highest velocities, and greatest debris potential. Sites outside the floodway and far from a river or stream and closer to the landward limit of 

the floodplain will be subject to reduced flood hazards (e.g., shallow flood depths, lower velocities, low erosion potential, and only small debris). 

Sites in a V or Coastal A zone will be subject to the highest waves, greatest flood velocities and 

depths, greatest erosion potential, and largest debris. Sites outside the V zone and far from the 

shoreline will be subject to reduced wave, velocity, depth, erosion, and debris conditions.

Closeness is a relative issue, however. Therefore, it may be useful to look at the location of a 

home site relative to the flood source or floodplain boundary. Two approaches may be useful, a 

distance approach and a floodplain width approach. 

The distance approach relies on a distance measurement between the home site and the 

stream or river bank or floodway (in the case of a riverine flood source), or between the 

home site and the V zone boundary or shoreline (in the case of an A zone in a coastal 

area).

The floodplain width approach considers the relative position of the home site within the 

floodplain. (See Figure 4-2, which is a schematic of a floodplain/floodway.)

For riverine areas outside the floodway:

Distance approach: If a home site lies within a few hundred feet of a river or stream or 

floodway, it should be considered “close,” and foundations should be appropriate to sites 

with greater flood hazards.

Floodplain width approach: If a home site lies within the third of the floodplain closest to 

the river or stream or floodway, it should be considered “close,” and foundations should be 

appropriate to sites with greater flood hazards. 

1.

2.

1.

2.

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8 FOUNDATION SYSTEMS

For Coastal A zones:

Distance approach: If a home site lies within a few hundred feet of the V zone boundary (or 

shoreline, if a V zone has not been mapped), it should be considered “close,” and V zone 

foundations should be used. Beyond that point, A zone foundations may be suitable.

Floodplain width approach: If a home site lies within the half of the floodplain closest to the 

V zone boundary (or shoreline, if a V zone has not been mapped), it should be considered 

“close,” and V zone foundations should be used. Beyond that point, A zone foundations may 

be suitable.

8.7.5	 Foundation Selection Guidance

The recommendations contained in Tables 8-1 through 8-3 should be evaluated in light of specific soil, terrain, and base flood conditions at a home site. The pier information contained in 

the tables is appropriate for piers used in foundation systems that contain other components 

(like ground anchors) that are properly selected, designed, and installed to resist flood and 

wind forces on the manufactured home itself. The pier construction styles listed are those required to resist flood forces on the piers themselves.

Areas exposed to flash flooding, alluvial fans, and mudslides pose unique (and often not specifically known) hazards. Foundations for homes in those areas should be developed by licensed 

engineers working closely with local floodplain managers.

Table 8-1. Recommended Manufactured Home Foundation Selection for Lake/Pond Flooding (for very low velocity

less than 1 fps)

Lake/Pond Flooding (standing water; maximum flood flow velocity 1.00 fps)

Flood Zone/Foundation Type A, AE, A 1-30, AO/AH

Steel pier 3

Dry-stacked masonry block Single block stack 3

Double block stack 3

Dry-stacked masonry block with 

surface-bonded mortar

Single block stack 3

Double block stack 3

Mortar or adhesive-bonded masonry 

block

Single block stack 3

Double block stack 3

Reinforced and grouted masonry 

block

Single block stack 3

Double block stack 3

Fill/slab 3

Posts 3

Perimeter foundation walls 3

Piles 3

3 = OK

fps = feet per second

1.

2.

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Single block stack (8 inch by 16 inch) on concrete footing or ABS pad

Double block stack (16 inch by 16 inch) on concrete footing or ABS pad

Table 8-2. Recommended Manufactured Home Foundation Selection for Riverine Flood Zones (and maximum flood

flow velocity)

Riverine Flooding

Flood Zone/Foundation Type Floodway1 A, AE, AE1-30, AO/AH

Steel pier 3

Vmax=1.00 fps

Dry-stacked masonry block

Single block stack Do Not Use 3

Vmax=1.25 fps2

Double block stack Do Not Use 3

Vmax=1.75 fps3

Dry-stacked masonry block 

with surface-bonded mortar

Single block stack Do Not Use 3

Vmax=2.00 fps

Double block stack Do Not Use 3

Vmax=3.00 fps

Mortar or adhesive-bonded 

masonry block

Single block stack Do Not Use 3

Vmax=2.50 fps

Double block stack Do Not Use 3

Vmax=3.00 fps

Reinforced and grouted 

masonry block

Single block stack Do Not Use 3

Vmax=5.00 fps

Double block stack Do Not Use 3

Vmax=5.00 fps

Fill/slab Do Not Use 3

Posts Do Not Use 34

Perimeter foundation walls Do Not Use 3

Piles5 3 3

1

 Any construction in the floodway requires certification that the construction will not cause a rise in flood levels.

2 Vmax shown for single stack block on concrete pad or footing. Vmax = 1.00 fps for single stack block on ABS pad.

3 Vmax shown for double stack block on concrete pad or footing. Vmax = 1.25 fps for double stack block on ABS pad.

4 Scour protection is recommended around shallow foundations where velocities exceed 2 fps.

5 Pile foundations are suggested for all sites exposed to flood velocities greater than 5 fps unless designed by a licensed engineer 

or architect.

3 = OK

Vmax = maximum design flood velocity (ft/sec) for foundation type

Single stack (8 inch by 16 inch)

Double stack (16 inch by 16 inch) 

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Table 8-3. Recommended Manufactured Home Foundation Selection for Coastal Flood Zones

Coastal Flooding

Flood Zone/Foundation Type V, VE, V1-30

A, AE, A1-30, AO/

AH

(LiMWA area)2

A, AE, A1-30, 

AO/AH

(Outside LiMWA 

area)2

Steel pier Do Not Use Do Not Use 3

where Vmax=1.00 fps

Dry-stacked 

masonry block

Single block stack Do Not Use Do Not Use 3

where Vmax=1.25 fps3

Double block stack Do Not Use Do Not Use 3

where Vmax=1.75 fps4

Dry-stacked 

masonry block with 

surface-bonded 

mortar

Single block stack Do Not Use Do Not Use 3

where Vmax=2.00 fps

Double block stack Do Not Use Do Not Use 3

where Vmax=3.00 fps

Mortar or adhesivebonded masonry 

block

Single block stack Do Not Use Do Not Use 3

where Vmax=2.50 fps

Double block stack Do Not Use Do Not Use 3

where Vmax=3.00 fps

Reinforced and 

grouted masonry 

block

Single block stack Do Not Use Do Not Use 3

where Vmax=5.00 fps

Double block stack Do Not Use Do Not Use 3

where Vmax=5.00 fps

Fill/slab Do Not Use Do Not Use 3

Posts Do Not Use Do Not Use 35

Perimeter foundation walls Do Not Use Do Not Use 3

Piles1 3 3 3

1

 Pile foundations are suggested for all sites exposed to flood velocities greater than 5 fps unless designed by a licensed engineer 

or architect.

2

 The Limit of Moderate Wave Action (LiMWA) is the inland limit of the area affected by waves greater than 1.5 feet.

3

 Vmax shown for single stack block on concrete pad or footing. Vmax = 1.00 fps for single stack block on ABS pad.

4

 Vmax shown for double stack block on concrete pad or footing. Vmax = 1.25 fps for double stack block on ABS pad.

5

 Scour protection is recommended around shallow foundations where velocities exceed 2 fps.

3 = OK 

Vmax = maximum design flood velocity (ft/sec) for foundation type

Single stack (8 inch by 16 inch)

Double stack (16 inch by 16 inch)

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9-

9	 Recommended Design

Process and Criteria for

Manufactured Home

Foundations in SFHAs

9.1	 Performance Criteria

Performance criteria for manufactured home foundation systems are provided in the HUD 

Manufactured Home Construction and Safety Standards (24 CFR 3280) and Model Manufactured Home 

Installation Standards (24 CFR 3285). The HUD regulations require that manufactured homes 

installed in flood hazard areas use foundations, anchoring, and support systems capable of resisting loads associated with the design flood and wind events. Foundation, anchoring, and 

support systems must be capable of resisting flotation, collapse, or lateral movement.

The International Residential Code for One and Two Family Residential Dwellings also requires that 

residential structures, including manufactured home foundations, be designed and connected 

to resist flotation, collapse, or lateral movement due to structural loads and stresses from flooding equal to the design flood event (R324.1.1). IRC Appendix E: Manufactured Housing Used as 

Dwellings requires that foundations be designed and constructed to minimize differential movements (IRC AE502.4).

The NFIP regulations also establish performance requirements for structures, including manufactured homes, installed in flood-prone SFHAs. The NFIP requires homes be:

"adequately anchored to prevent flotation, collapse, or lateral movement of the 

structure resulting from hydrodynamic and hydrostatic loads, including the effects of buoyancy." [44 CFR 60.3(a)(i)]

9.2	 Design Criteria

To meet the performance requirements outlined in Section 9.1, design criteria must be established that identify the anticipated design loads the structure is expected to experience. Design 

loads can be categorized as dead loads, live loads, or environmental loads.
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9 Recommended Design Process and Criteria for Manufactured Home Foundations in SFHAs 

Dead loads depend on the weight of the manufactured home, its foundation, and any accessory supported by the structure such as walls, piping, electrical service panels and conduits, and 

HVAC equipment.

Live loads depend on the weight of occupants, furnishings, and non-fixed equipment and the 

function of the building or specific space.

HUD 24 CFR 3280.305(g) requires floors be designed to support a minimum uniform live load 

of 40 psf. 

Environmental loads are loads for given recurrence intervals specified by the engineering community and vary with locality. Wind loads (which are proportional to the square of the design 

wind speed) can exceed 80 psf near coastal areas, while those inland can be as low as 15 psf. 

Snow loads are non-existent in much of the southern U.S., but can exceed 50 psf in northern 

and mountainous regions. 

Environmental loads that act on manufactured homes and foundations include the following:

Flood loads depend on flood depth, flood velocity, surrounding debris sources, wave 

effects, elevation of the building relative to the flood condition, and surrounding topography and exposure. Flood loads include hydrodynamic and hydrostatic (including 

buoyancy) and floating debris impact loads. 

Wind loads depend on the geometry (particularly roof shape and pitch) and building 

height, exposure, and, of course, on the design wind speed for the site. Coastal areas in 

the U.S. are generally exposed to greater wind speeds than interior areas, but special 

wind regions exist in mountainous areas. 

Seismic loads depend on the mass of the building, additional loads that the building 

may occasionally support (e.g., snow loads), the building geometry and distribution, its 

lateral force resisting system, and local soil conditions. 

Snow loads depend on roof geometry, building orientation, and the geographical location of the site.

Most model building codes contain requirements on estimating the normal and environmental loads on a structure. After the design loads are identified, how these loads interact with the 

structures can be assessed and the foundation system can be designed. 

9.3	 Design Process

After the performance criteria are established, the design process can begin. The design process involves the following steps:
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Each step of this process is described below. The entire design process is based on the fundamental premise that anticipated normal and environmental loads can and must be transferred 

through the manufactured home to the foundation in a continuous path to the supporting soils. 

Any weakness in the continuous path is a potential point of failure of the building, and any failure creates the possibility for large property losses and the loss of life.

This guide does not cover every combination of loads, materials, building shapes and functions, 

hazard risks, and elevations. Engineering judgment will need to be applied to a range of problems during the design of a manufactured home foundation located in an SFHA. Therefore, 

the intent of this guide is to provide sufficient background for designers to effectively design 

manufactured home foundations for forces and issues encountered when sited in an SFHA.

9.3.1	 Step 1: Determine Design Criteria

As discussed in Section 10.1, HUD’s Model Manufactured Home Installation Standards require manufactured home foundations be capable of resisting loads associated with the design flood and 

wind events. However, the designs do not consider flood or seismic loads (24 CFR 3285.303 footnotes to Tables 1, 2, and 3). The HUD standards do not provide guidance on determining the 

design criteria for flood and seismic events.

The IRC establishes required minimum design loads for residential structures, including manufactured homes. The IRC requires minimum design wind loads where basic wind speeds exceed 

100 miles per hour (mph) in hurricane-prone areas or 110 mph elsewhere shall be in accordance 

with ASCE 7, Minimum Design Loads for Buildings and Other Structures. ASCE 7 is a state-of-the-art 

consensus standard that establishes minimum required design loads to be resisted by structures 

and their foundations.
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9.3.2	 Step 2: Select a Design Methodology and Assess Load Combinations and Failure

Modes

9.3.2.1 	 Design Methodology

The two predominant design methods being used in engineering practices today are allowable, or 

working, stress design (ASD) and strength design. ASD (sometimes referred to as elastic design) 

is based on the calculated stress in members due to service loads. Strength design (also known as 

limit state design) is based on ultimate loads (i.e., the load that will cause a member to fail) and 

the mode associated with the failure. For designing manufactured home installation, either ASD 

or strength design methodology is acceptable. 

9.3.2.2 	 Load Combinations

After the design criteria are determined, the possibility of more than one hazard occurring at 

the same time must be considered for all modes of failure as well as the possibility of imposed 

loads counteracting gravity loads. For example, it is common to expect simultaneous flooding with high winds during hurricane events. In similar situations, uplift forces due to the high 

winds from hurricanes can counteract gravity loads. Load combinations are used to assess the 

probability of more than one hazard occurring simultaneously and the probability of imposed 

loads exceeding gravity loads. Refer to Section 5.5.1 for a discussion of ASCE 7 load combinations.

Load combinations must be resolved directionally so that all loads in a given combination are 

acting in the same direction, either vertically or horizontally. The Commentary in ASCE 7 states 

“Wind and earthquake loads need not be assumed to act simultaneously. However, the most unfavorable effect of each should be considered in design, where appropriate. In some instances, 

forces due to wind might exclude those due to earthquake, while ductility requirements might 

be determined by earthquake loads.” 

All design loads create forces in and on the building. All forces acting on the manufactured 

home must be transferred through the foundation and into the soil that supports the structure. 

The foundation designer must ensure integrity of this load path. The primary failure modes in 

load path integrity include uplift failure, overturning, and sliding (or shearing).

9.3.2.3 	 Primary Failure Modes

Uplift failure occurs when vertical forces caused by wind or buoyancy exceed the weight of both the structure and the strength of the 

soil anchorage. The structure fails by being lifted off its foundation 

or because the foundation pulls out of the soil.

Uplift failure mode is particularly likely when the 3-foot pier rule is used in areas where the BFE 

is greater than 3 feet above the ground elevation. In that situation, the home will become submerged and exposed to buoyancy forces. For relatively low levels of flooding (9 to 12 inches or 

less), buoyancy forces can exceed the weight of the home and lift the home off its foundation. 

To resist buoyancy, foundations must provide sufficient resistance to overcome buoyancy (and 
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wind) and all connections between the home’s foundation must be designed to prevent flotation, collapse, or lateral movement, of the submerged home (Figure 9-1). 

Overturning failure occurs when the combined forces of wind, 

waves, and buoyancy or earthquake exceed the resisting forces of 

the building’s weight and anchorage. The building fails by rotating off its foundation or rotating out of the soil (see Figure 7-8).

Sliding or shearing failure occurs when horizontal forces exceed 

the friction force or strength of the foundation. The building fails 

by sliding off its foundation, shear failure of components transferring loads to its foundation, or the foundation sliding. 

All three of these failure modes can result in a wide range of damage, 

from minor, such as loosening of the anchor straps, to catastrophic, such as structural collapse. It is important to emphasize that other load combinations and 

failure modes may exist, and professional judgment should be exercised in any situations not 

covered by this guide. 

9.3.3	 Step 3: Select Foundation Type and Material

After loads are determined and failure modes are assessed, the foundation type and materials 

must be selected. Both are described in Chapter 8 as well as advantages and special considerations for the selection process. 

Figure 9-1. A home that

was partially submerged

and displaced from its

foundation by hydrostatic

forces. Although it

appears the openings

are too high, the bottoms

of the openings are less

than 1 foot above the top

of the interior slab.
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9.3.4	 Step 4: Determine Forces at Connections and on Foundation Components

The loads on components and at connections through the load path must be determined to 

design an adequate foundation. Some examples of critical points of the load path include the 

following: 

n Connections between the site-built foundation and the home’s steel frames (and the 

home’s wall ties if in higher wind zones)

n Load path connections through the site-built foundation

n Load path connections from the foundation to footings

n Adequacy of the footing and surrounding soil

9.3.5	 Step 5: Specify Connections and Framing Methods Along with Component

Dimensions to Satisfy Load Conditions

The loads determined in Step 4 are used in the design and detailing of connections, foundation 

components, and anchoring. The loads at connections will be used to design the connections 

(i.e., number of bolts or nails, size of clip angles). The reactions at the support points will be 

used to design the foundations (i.e., size of piers and footings, amount of reinforcement, and 

spacing between piers and ground anchors).

Components and connections design should meet the requirements of an appropriate material 

standard, some of which are listed in Table 9-1.

Table 9-1. Design Standards and Publishers for Building Materials

Building Material Standard and Publisher

Wood National Design Specification (NDS) for Wood Construction and Commentary published 

by American Forest and Paper Association (AF&PA)/American Wood Council (AWC)

Steel Manual of Steel Construction (AISC 325-05) published by American Institute of Steel 

Construction (AISC)

Concrete Building Code Requirements for Structural Concrete (ACI 318) published by American 

Concrete Institute (ACI)

Masonry Building Code Requirements for Masonry Structures (ACI 530) published by ACI

9.3.6	 Step 6: Note All Design Assumptions and Details on Drawings

To ensure a quality design and installation, all assumptions, calculations, and details should be 

clearly documented or noted on the construction documents so that installers and floodplain 

managers clearly understand the design and design assumptions. 

Appendix F provides a detailed example of vertical and lateral load, overturning moment, and, 

foundation and ground anchor calculations. 
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10	 Recommended

Foundations

Manufactured home foundations must be designed and constructed to resist gravity, lateral, 

and uplift loads. Foundation systems provide load resistance by transferring the loads to soils 

at the site. Load transfer can be accomplished using a single element or a combination of elements. For example, driven piles are an effective foundation element for transferring gravity, 

lateral, and uplift loads to the soil. Unreinforced masonry piers can be effective at transferring 

gravity loads, but are not effective in transferring lateral and uplift loads. 

This chapter provides descriptions and design recommendations for six foundation systems 

suitable for use in many SFHAs in the U.S. The six foundation systems presented are: reinforced 

masonry, wood framed, braced masonry pier, wood H-frame, ground anchors with stabilizer 

plate, and ground anchor (in-line). These foundations can be used for sites in A zones with low 

to moderate floodwater velocities and depths. They are not suitable for V zones, Coastal A zones, 

and home sites in floodways or other high velocity areas. 

Also included in this chapter are three foundation designs suitable for all seismic regions in the 

U.S. The seismic designs can only be used for sites with maximum 90-mph design wind speeds. 

Although the foundation designs can be used in many flood areas, they are not “all inclusive” 

and do not provide a “one-size-fits-all” design. Because natural loads vary greatly from site to 

site, any single design may be overly conservative in some areas, but would provide inadequate 

protection in other areas. 

10.1	 Design Criteria for Recommended Foundations

Criteria used in the design of the recommended foundations were selected to provide a balance between cost and applicability. Homes placed in areas where anticipated events exceed 

the design criteria used will require custom designed foundations. Foundation designs provided in this guide are not appropriate for manufactured homes within V zones, Coastal A zones, 

floodways, and all sites where design event flood velocities exceed 5 fps. The recommended 

foundations were designed to the following criteria:

n Maximum flood depth of 36 inches above finished grade.

n BFE no higher than the bottom flanges of the home’s I-beams.

n Maximum flood velocities of 5 fps. 

n No breaking wave forces.
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n 3-second gust wind speeds of 90 mph, 110 mph, 130 mph, and 150 mph. The seismic 

foundations (drawings SP, SM, and SWF in Appendix H) may be used in areas with design 

3-second gust wind speeds of 90 mph or less.

n Manufactured home with width of at least 14 feet wide for single units and 28 feet wide for 

double units. Minimum home length of 48 feet for both single and double units.

n Manufactured home eave heights of 8 feet 2 inches or less.

n Chassis main beam spacing of 82 to 96 inches (for ground anchor foundations).

n Maximum snow loads of 40 psf. 

n Manufactured home weight of at least 25 psf and not greater than 40 psf.

Foundations have been designed to resist loads and load combinations calculated using ASCE 7. 

Where additional design guidance was needed, other documents have been used, including the 

Coastal Construction Manual (FEMA 55). A brief description of each design follows in Sections 

10.1.1 through 10.1.5. The drawings are presented in Appendix H.

10.1.1	 Reinforced Masonry Perimeter Foundation Walls

(Drawing Numbers: M-1 to M-4)

The reinforced masonry (RM) foundation consists of reinforced masonry walls supported on 

cast-in-place concrete strip footings. 

Reinforced masonry perimeter walls transfer gravity loads to the soil through the bearing pressure imposed on the strip footing. Lateral loads are resisted by a combination of frictional 

resistance between the strip footing and the soil, and lateral resistance of the soil against the 

below grade portion of the wall footing. Uplift loads are resisted by a combination of the weight 

of the footing, soil resistance to vertical failure, and increased bearing pressures in the leeward 

footing.

RM foundations are likely the most expensive of the recommended foundations that have been 

developed. For example, properly grouting the masonry wall cores will likely require the home 

to be crane set on the RM foundation. 

Openings or vents, required by the NFIP, are spaced throughout the foundation walls. The 

openings are designed to automatically allow floodwater to enter the crawlspace and equalize 

hydrostatic pressures on the foundation walls. Flood openings must automatically open to allow 

floodwaters to flow and must be installed within 1 foot of grade. Detailed guidance for flood 

openings is provided in FEMA Technical Bulletin 1: Openings in Foundation Walls and Walls of 

Enclosures, August 2008. A minimum of two flood vents with a total net area of 1 square inch per 

square foot of enclosed space must be provided (44 CFR 60.3(c)(5)). Vents for moisture control 

are typically installed at the top of the foundation wall. In areas where high crawlspace moisture 

levels predominate, additional vents may need to be installed in the upper portions of the foundation walls for adequate ventilation. 
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Final connections between the factory-built home and the site-built foundation are made by 

bolting the home to the foundation wall anchor bolts (if an adequately attached pressure-treated sill plate is provided with the home) or by anchoring a pressure-treated sill to the foundation 

wall and installing framing clips between the site-installed sill plate and the home’s band joist 

(if a pressure-treated sill is not provided with the home).

10.1.2	 Wood Framed Perimeter Foundation Walls

(Drawing Numbers: WF-1 to WF-4)

The wood framed (WF) foundation consists of 2-inch by 6-inch (nominal) treated framing 

sheathed with plywood that is treated to resist rot and wood destroying insect damage. The WF 

foundation walls are constructed over continuous poured concrete strip footings. Like the RM 

foundation, the WF foundation encloses the underside of the home to create a crawlspace, and 

flood vents are required to equalize hydrostatic loads and provide crawlspace moisture control. 

Because wood framed walls resist less shear per unit length than masonry, the WF foundation 

design requires interior shear walls to adequately resist lateral (wind, flood, and seismic) loads. 

In higher wind zones, plywood sheathing is needed on both sides of the shear walls.

Unlike the RM foundation wall design, the WF foundation was designed to enable “building to 

the box.” Footings can be poured prior to placing the home (for installation tolerance, they can 

be constructed slightly wider), the home can be rolled onto the sites, and the walls can be constructed between the factory-built home and the site-built foundation. 

A home destined to be placed on a WF perimeter foundation will require some fabrication 

modifications. Most notable is the need to shorten outriggers to avoid interferences with the 

foundation wall. Shear reinforcement may be necessary to adequately transfer lateral and vertical loads from the home to the foundation. Manufacturers should provide connection details 

specific to their homes.

10.1.3	 Braced Masonry Pier Designs

(Drawing Numbers: BM-1.1 to BM-2.2) 

The braced masonry pier (BMP) designs utilize materials developed for more typical foundations. The BMP designs use metal straps and masonry piers found in ground anchor foundations. 

But instead of using ground anchors to resist vertical and lateral loads, the weight of continuous concrete footings are used to resist uplift and soil pressure while friction forces resist lateral 

loads. 

Pier construction varies with flood velocity. With low flood velocities, piers can be dry stacked 

and adhered with surface bonding mortar. For increased flood velocities, piers with greater 

lateral strength are needed to ensure that they do not become dislodged under the combined 

effects of wind (or seismic) loads and flood loads. Increasing design flood flow velocities require that the masonry piers be strengthened to resist higher loads. Section 10.3 outlines flood 

flow velocity design considerations for several masonry pier configurations. 
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The metal straps are an integral component in the braced masonry foundation. Failure of any 

strap will redistribute loads to adjacent portions of the foundation. If straps are loaded to their 

maximum working load, redistributing the load can lead to progressive strap failure and home 

collapse. To reduce potential for progressive failure, redundant straps have been included in 

the designs. 

10.1.4	 Wood H-Frame Designs

(Drawing Numbers: HF-1.1 to HF-2.2)

The wood H-frame designs are similar to pile foundations used to elevate structures in coastal 

areas. The greatest difference between the two designs is the use of the weight of concrete footings to resist uplift and lateral forces instead of the friction between earth soils and piles.

The designs allow setting the manufactured home on the site and constructing the H-frames 

from below. This “building to the box” provides flexibility in site construction, but some installers 

have expressed concerns regarding the amount of work required under the home. Temporary 

piers or metal jack stands can support the home during construction, or all H-frames can be 

built and the home can be set with a crane.

A critical component to the H-frame designs is the post bases that connect the vertical posts to 

the concrete footing. The design specifies cast-in-place bases be accurately positioned during 

the concrete pour. The fasteners used to construct the frames are sized to resist uplift and lateral forces only. They are not sized to support all gravity loads. Because of this, the posts must 

be constructed tight to the bottoms of the I-beam frames or provided with shim pairs to transfer 

gravity loads.

10.1.5	 Ground Anchor Designs

(Drawing Numbers: GASP-90-1.1 to GASP-110/130/150-2.2 and GA-90-1.1 to 

GA-110/130/150-2.2) 

In many areas of the U.S., ground anchors are used as part of the support and anchorage systems for manufactured homes. Anchors are inexpensive, easily installed in many types of soils, 

and readily available in many portions of the U.S. Despite these advantages, support and anchorage systems using ground anchors do not perform as well as typical foundations. Most of 

the shortcomings result from the fact that shallow earth anchors are relatively weak and their 

performance is highly variable, and foundations using ground anchors lack the structural stiffness that typical foundation materials provide. 

Ground anchors can be effective in resisting uplift and lateral loads. Loads are resisted by 

mobilizing the soil shear strength along the potential failure surface. The surface area of the 

potential failure plane is a function of helix area and embedment depth. Mobilization of soil 

shear strength requires movement along the failure plane. Shallow anchors typically used for 

manufactured home installations require mobilization of a significant portion of the ultimate 

soil shear strength, which results in relatively large movements.
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Ground anchors used in manufactured home installations frequently use stabilizer plates to 

enhance lateral load resistance capacity. The stabilizer plates resist lateral loads by mobilizing 

passive resistance of the soil. Development of soil passive resistance requires deformations, or 

movement, in the direction of the applied load. As with uplift, the greater the amount of passive 

resistance mobilized, the larger the horizontal movement required.

Two recommended foundation designs using ground anchors have been developed. One design uses ground anchors with stabilizer plates. The ground anchor/stabilizer plate design is 

similar to a “standard set,” but includes additional aspects required to resist flood forces and 

flood damage.

The second ground anchor design has been developed using ground anchors installed for axial (in-line) loading. In-line anchors offer superior performance to anchors used with stabilizer 

plates because fewer are needed for the same level of performance. More importantly, in-line 

anchors are less prone to becoming loose and can withstand repeated loading better than anchors with stabilizer plates. One consideration of in-line anchors is that they need to be installed 

prior to home placement. This requires additional advanced planning of home placement and 

also requires using installers with experience in the use of in-line anchors.

10.2	 Summary of Recommended Foundations

A summary and a comparison of the six different recommended foundation systems proposed 

in this guide are given in Table 10-1. 

Table 10-1. Summary of Recommended Foundations

Type of Foundation Advantages Disadvantages

Reinforced masonry

(M-1 to M-4) 

• Little or no maintenance Need crane to set home

Expensive

Need custom feature built into home 

to accommodate foundation wall

Shear reinforcement needed

•

•

•

•

Wood framed

(WF-1 to WF-4)

• No crane installation necessary Need modification to home to 

accommodate foundation wall

Shear reinforcement needed

•

•

Braced masonry pier

(BM-1.1 to BM-2.2)

• Typical installation methods Adjustment of straps needed after a 

design event

•

Wood H-frame

(HF-1.1 to HF-2.2)

Uses common materials

Easily adapted to site conditions

•

•

Need crane to set home

Amount of work under home

Accurate placement of vertical posts 

at concrete footings

•

•

•
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Type of Foundation Advantages Disadvantages

Ground anchors with 

stabilizer plate

(GASP-90-1.1 to GASP110/130/150-2.2) 

Inexpensive

Readily available 

Easy to install 

Anchors installed after home is in 

place

•

•

•

•

Not as strong as typical foundation

Human intervention needed after a 

design event

Home likely to move during a design 

event

Home may need to be lifted and recentered on foundation

•

•

•

•

Ground anchor – in-line

(GA-90-1.1 to GA110/130/150-2.2)

Better performance than anchors with 

stabilizer plate – uses fewer

Anchors readily available

•

•

Adjustment of straps after a design 

event

Anchors need to be accurately 

installed prior to placement of home

•

•

10.3	 Floodwater Velocity Design Considerations for Pier

Foundations

As is noted on the design drawings for the recommended foundations (Appendix H), the design of the pier is dependent on the floodwater velocities. There are four basic pier designs, 

each of which has a different design flood flow velocity. Furthermore, whether or not a single or 

double block design is used will also affect the design floodwater velocity. Pier designs and the 

corresponding recommended limit design velocities are provided in Table 10-2.

Table 10-2. Design Flood Flow Velocity for Concrete Masonry Unit Foundations1

Pier

Construction Dry Stack Dry Stack withFace Mortar Bonded Stack2

Fully Grouted,

Reinforced, and

Anchored to Concrete

Foundation

Single Stack 1.00 fps3

1.25 fps4 2.00 fps 2.50 fps 5.00 fps

Double Stack 1.25 fps5

1.75 fps6 3.00 fps 3.00 fps 5.00 fps

1 Velocities are the maximum design flood flow for each masonry block pier shown. Design velocity applicable to piers up to 36 

inches high.

2 Bonded piers use Type M or S Portland cement and lime mortar to adhere horizontal and vertical joints between block. 

Polyurethane based masonry adhesive certified by a recognized national testing agency for use in masonry construction in 

accordance with the IRC and IBC also may be used.

3 Dry stack pier design velocity of 1.00 fps applies to single stack block piers supported on ABS pads.

4 Dry stack pier design velocity of 1.25 fps applies to single stack block piers supported on concrete pad or footing

5 Dry stack pier design velocity of 1.25 fps applies to double stack block piers supported on ABS pad

6 Dry stack pier design velocity of 1.75 fps applies to double stack piers supported on a concrete pad or footing

Table 10-1. Summary of Recommended Foundations (continued)
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Methods of determining flood velocities are presented in Appendix C.

10.4	 Recommended Foundation Designs for Seismic Areas

The recommended foundation designs proposed in this chapter are designed mainly for flood 

and wind loadings. They are not specifically designed for seismic loadings. However, some of 

the designs have some seismic resisting capabilities. In Sections 10.4.1 through 10.4.3, three recommended foundation designs suitable for seismic areas are introduced. These are designs that 

have been included in the most recent release of Model Manufactured Home Installation Standards

(NFPA 225) and are presented in Appendix H. 

10.4.1	 Concrete Masonry Pier Foundation Designs

(Drawing Numbers: SP-1/2.1 to SP-1/2.2)

This design applies to concrete masonry piers located along and attached to the chassis beams, 

providing both vertical and lateral force support to a manufactured home. The pier cannot 

be less than 16 inches square and must be supported on a concrete footing. Each concrete 

masonry pier must be provided with no less than two No. 4 vertical reinforcing bars located 

in diagonally opposite corners. Anchorage shall be provided between the support system and 

manufactured home that is capable of resisting the greater of the load and forces specified in 

Standard on Manufactured Housing (NFPA 501), Table SP-1.2 (Drawing SP-1/2.2), and the reactions specified by the home manufacturer.

The grouting required for pier construction will likely require homes to be crane set on this 

style of foundation.

10.4.2	 Masonry Wall Foundation Designs

(Drawing Numbers: SM-1/2.1 to SM-1/2.2)

This design applies to foundation walls located at the manufactured home perimeter and to 

provide lateral force support. Foundation stem wall construction must consist of concrete or 

concrete masonry units supported on a continuous concrete footing. The stem cannot be less 

than 6 inches thick.

Each concrete or concrete masonry foundation wall must use no less than No. 4 vertical reinforcing bars at a space not exceeding 4 feet with a standard hook of not less than 8 inches in the 

footing. Anchorage shall be provided between the support system and manufactured home that 

is capable of resisting the greater of the loads and forces specified in NFPA 501, Table SM-1.1 

(Drawing SM-1/2.2), and the reaction specified by the home manufacturer. 
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10.4.3	 Wood Framed Foundation Designs

(Drawing Numbers: SWF-1/2.1 to SWF-1/2.2)

This design applies to continuous wood framed cripple walls located at the manufactured home 

perimeter to provide lateral force support. The continuous concrete perimeter footing cannot 

have a width less than 12 inches. Also, the concrete shall extend less than 8 inches above the 

highest adjacent grade.

When the home length exceeds 60 feet, two interior transverse shear walls with continuous footings must be added with spacing between walls not exceeding 30 feet. Access openings shall be 

provided at the center transverse interior footings. The height of the cripple walls shall exceed 

4 feet above the top of the footing.

The concrete footing shall be provided with not less than two No. 4 horizontal reinforcing 

bars. Lap splice No. 4 bars no less than 24 inches straight and provide no less than 12 inches 

bend around corners. Anchorage shall be provided between the cripple wall and manufactured 

home that is capable of resisting the greater of the reaction forces specified in NFPA 225, Table 

13.4.7, the reactions specified in SWF-1.1 (Drawing SWF-1/2.2), and anchorage requirements 

specified by the home manufacturer. If anchorage requirements from the home manufacturer 

are not available or if the manufactured home’s weight and dimensions are not within the criteria established in these designs, anchorage requirements shall be determined by engineering 

analysis based on loads specified by ASCE 7.

Anchorage between the cripple wall sill plate and the concrete footing shall be provided not 

less than 1/2-inch diameter anchor bolts, starting with no more than 8 inches from the end of 

each foundation wall. Anchor bolts cannot be less than 8 inches embedded into the concrete. 

Also, anchor bolts shall exceed 16 inches on center for an end wall of single-wide homes, and 32

inches on center for the other walls. Each anchor bolt shall be provided with a steel plate washer 

dimensions of no less than 1/4 inch by 3 inch by 3 inch that has a hole diameter of 11/16 of an 

inch that is installed with a standard cut washer.

The wood framed perimeter design should allow the foundation to be built to the dimensions 

of the manufactured home; therefore, crane installation should not be required. 

10.5	 Design Drawings

The design drawings for this guide are listed in Table 10-3 and are contained in Appendix H.

Table 10-3. Foundation Drawings

Drawing No. Title

GN-1.1 Recommended Foundation – General Notes

M-1 Single Unit Masonry Foundation Plan 
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RECOMMENDED FOUNDATIONS 10

Drawing No. Title

M-2 Double Unit Masonry Foundation Plan

M-3 Masonry Wall Foundation Detail

M-4 End Wall Foundation Detail

WF-1 Single Unit Wood Framed Foundation Plan 

WF-2 Double Unit Wood Framed Foundation Plan

WF-3 Wood Framed Foundation Detail

WF-4 Wood Framed Shear Wall Detail

BM-1.1 Single Unit Braced Masonry Pier Foundation Plan 

BM-1.2 Braced Masonry Pier Detail

BM-2.1 Double Unit Braced Masonry Pier Foundation Plan 

BM-2.2 Braced Masonry Pier Detail

HF-1.1 Single Unit Braced Wood H-Frame Foundation Plan 

HF-1.2 Single Unit Wood H-Frame Detail

HF-2.1 Double Unit Braced Wood H-Frame Foundation Plan 

HF-2.2 Double Unit Wood H-Frame Detail

GASP90-1.1 Single Unit Ground Anchor Foundation Plan 

GASP90-1.2 Ground Anchor and Pier Detail (Single Unit)

GASP90-2.1 Double Unit Ground Anchor Foundation Plan 

GASP90-2.2 Ground Anchor and Pier Detail (Double Unit)

GASP110/130/150-1.1 Single Unit Ground Anchor Foundation Plan 

GASP110/130/150-1.2 Ground Anchor and Pier Detail (Single Unit)

GASP110/130/150-2.1 Double Unit Ground Anchor Foundation Plan 

GASP110/130/150-2.2 Ground Anchor and Pier Detail (Double Unit)

GA90-1.1 Single Unit Ground Anchor Foundation Plan 

GA90-1.2 Ground Anchor and Pier Detail (Single Unit)

GA90-2.1 Double Unit Ground Anchor Foundation Plan

GA90-2.2 Ground Anchor and Pier Detail (Double Unit)

GA110/130/150-1.1 Single Unit Ground Anchor Foundation Plan 

GA110/130/150-1.2 Ground Anchor and Pier Detail (Single Unit)

GA110/130/150-2.1 Double Unit Ground Anchor Foundation Plan

GA110/130/150-2.2 Ground Anchor and Pier Detail (Double Unit)

APD-1.1 Alternative Pier Details for Flood Velocities Not Shown on Plans

AS-1.1 Lateral Ground Anchor Spacing

AS-1.2 Longitudinal Ground Anchor Spacing and Anchor Notes

Table 10-3. Foundation Drawings (continued)
10-10 PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards 

A Multi-Hazard Foundation and Installation Guide

10 RECOMMENDED FOUNDATIONS

Drawing No. Title

SP-1/2.1 Single Unit Concrete Masonry Pier Foundation Plan

SP-1/2.2 Concrete Masonry Pier Details

SM-1/2.1 Single Unit Concrete Masonry Wall Foundation Plan

SM-1/2.2 Concrete Masonry Wall Foundation Detail

SWF-1/2.1 Single Unit Wood Framed Foundation Plan

SWF-1/2.2 Wood Framed Foundation Detail

Table 10-3. Foundation Drawings (continued)
PROTECTING Manufactured Homes in Flood Hazard Areas 

A Multi-Hazard Foundation and Installation Guide

A-

A.	 References

American Concrete Institute, Building Code Requirements for Masonry Structures (ACI 530), 2008.

American Concrete Institute, Building Code Requirements for Structural Concrete (ACI 318) and 

Commentary (318R), 2005.

American Forest and Paper Association and American Wood Council, National Design Specification (NDS) for Wood Construction and Commentary, 2006.

American Institute of Steel Construction, Manual of Steel Construction (AISC 325-05, 13th Edition), 2005.

American National Standards Institute/American Institute of Steel Construction, Specification 

for Structural Steel Buildings (ANSI/AISC 360-05), 2005. 

American Society of Civil Engineers, Minimum Design Loads for Buildings and Other Structures, 

(ASCE 7-98, 7-02, and 7-05).

American Society of Civil Engineers, Flood Resistant Design and Construction, (ASCE 24-05).

Bowles, J.E., Foundation Analysis and Design, McGraw-Hill Book Company, 1977.

Code of Federal Regulations (CFR), General Provisions (44 CFR 59), 2004.

Code of Federal Regulations (CFR), Criteria for Land Management and Use (44 CFR 60), 2004.

Das, B. M., Seeley, G. R., “Breakout Resistance of Shallow Horizontal Anchors,” Journal of the Geotechnical Engineering Division, ASCE, Vol. 101(T9), 999-103, 1975.

Das, B. M., Seeley, G. R., “Inclined Load Resistance in Sand,” Journal of the Geotechnical Engineering Division, ASCE, Vol. 101(9), 995-998, 1975.

FEMA, Coastal Construction Manual (FEMA 55, 3rd Edition), June 2000 (reissued August 2005 

under a new cover with no change in technical content). 

FEMA, Elevated Residential Structures (FEMA 54), March 1984.

FEMA, Engineering Principles and Practices of Retrofitting Flood-Prone Residential Structures (FEMA 

259), June 2001.

FEMA, Homeowner’s Guide to Retrofitting (FEMA 312, 2nd Edition), 2009.
A- PROTECTING Manufactured Homes FROM FloodS AND Hazards 

A Multi-Hazard Foundation and Installation Guide

A REFERENCES

FEMA, Installing Seismic Restraints for Mechanical Equipment (FEMA 412), December 2002. 

FEMA, Managing Floodplain Development in Approximate A Zone Areas, A Guide for Obtaining and Developing Base (100-year) Flood Elevations, (FEMA 265), July 1995. 

FEMA, Mitigation Assessment Team Report: Hurricane Ike in Texas and Louisiana, Building Performance Observations, Recommendations, and Technical Guidance (FEMA P-757), April 2009.

FEMA, Mitigation Assessment Team Report: Hurricane Ivan in Alabama and Florida, Observations, Recommendations, and Technical Guidance (FEMA 489), August 2005.

FEMA, Mitigation Planning How-to Guides (FEMA 386), May 2005.

FEMA, NEHRP Recommended Provisions for Seismic Regulations for New Buildings and Other Structures

(FEMA 450), 2003 Edition.

FEMA, Protecting Building Utilities from Flood Damage (FEMA 348), November 1999. 

FEMA, Taking Shelter From the Storm: Building A Safe Room for Your Home or Small Business (FEMA 

320, 3rd Edition), August 2008.

FEMA, Technical Bulletin 1, Openings in Foundation Walls and Walls of Enclosures Below Elevated 

Buildings in Special Flood Hazard Areas in accordance with the National Flood Insurance Program, August 2008.

FEMA, Technical Bulletin 2, Flood Damage-Resistant Materials Requirements for Buildings Located in 

Special Flood Hazard Areas in accordance with the National Flood Insurance Program, August 2008.

FEMA, Technical Bulletin 5, Free-of-Obstruction Requirements for Buildings Located in Coastal High 

Hazard Areas in accordance with the National Flood Insurance Program, August 2008.

FEMA, Technical Bulletin 9, Design and Construction Guidance for Breakaway Walls Below Elevated 

Buildings Located in Coastal High Hazard Areas in accordance with the National Flood Insurance Program, August 2008.

FEMA, Technical Bulletin 10-01, Ensuring That Structures Built on Fill In or Near Special Flood Hazard Areas Are Reasonably Safe from Flooding in accordance with the National Flood Insurance Program, 

May 2001.

Federal Manufactured Housing Consensus Committee (MHCC), Model Manufactured Home Installation Standards, 2003 Edition.

Geddes, J.D., Murray, E.J. “Passive Inclined Anchorages in Sand,” Journal of Geotechnical Engineering, ASCE, Vol. 117 (5), 810-814. 1991.

Ghaly, A.M., Hanna, A., Hanna, M. “Uplift Behavior of Screw Anchors in Sand I: Dry Sand,” 

Journal of Geotechnical Engineering, ASCE, Vol. 117 (5), 773 – 783. 1991.
PROTECTING Manufactured Homes FROM FloodS AND Hazards 

A Multi-Hazard Foundation and Installation Guide

AREFERENCES A

Ghaly, A.M., Hanna, A., Hanna, M. “Uplift Behavior of Screw Anchors in Sand II: Hydrostatic 

and Flow Conditions,” Journal of Geotechnical Engineering, ASCE, Vol. 117 (5), 784 – 808. 1991.

Ghaly, A.M., Clemence, S.P. “Pullout Performance of Inclined Helical Screw Anchors in Sand,” 

Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 124(7), 617-627. 1998.

FHWA NHI 01-001, Evaluating Scour at Bridges, 4th Edition, May 2001, Hydraulic Engineering Circular No. 18.

International Code Council, Inc. (ICC), International Residential Code 2009, Falls Church, VA. 

2009.

International Council of Building Officials (ICBO), Acceptance Criteria for Expansion Anchors in 

Concrete and Masonry Elements (AC01), July 1991.

Longinow, A., Guidance for Support of Manufactured Housing and Other Relocatable Structures for the 

Idaho Bureau of Disaster Services, for FEMA, under the National Earthquake Technical Assistance Contract EMW-92-C-3852, Task #45, June 22, 1995.

Longinow, A., Meinheit D. F., and Pearson, J. F., Laboratory Testing of Soil Anchors for the U.S. Department of Housing and Urban Development, Washington, DC, WJE No. 931299, December 

15, 1994.

Manufactured Housing Research Alliance (MHRA), Guide to Foundation and Support Systems for 

Manufactured Homes, March 2002.

Marshall, Richard D., Manufactured Homes – Probability of Failure and the Need for Better Windstorm 

Protection Through Improved Anchoring Systems, United States Department of Commerce, National 

Institute of Standards and Technology, November 1994.

Marshall, Richard D., and Yokel, Felix Y., Recommended Performance-Based Criteria for the Design of 

Manufactured Home Foundation Systems to Resist Wind and Seismic Loads (NISTIR 5664), U.S. Department of Commerce, National Institute of Standards and Technology, Gaithersburg, MD, 

August 1995.

Marshall, Richard D., Wind Load Provisions of the Manufactured Home Construction and Safety Standards – A Review and Recommendations for Improvement, United States Department of Commerce, 

National Institute of Standards and Technology, May 1993.

National Fire Protection Association (NFPA), Building Construction and Safety Code, (NFPA 5000), 

Quincy, MA, 2009.

National Fire Protection Association (NFPA), Model Manufactured Home Installation Standard, 

(NFPA 225), Quincy, MA, 2009.

National Fire Protection Association (NFPA), Standard for Fire Safety Criteria for Manufactured 

Home Installations, Sites, and Communities, (NFPA 501A), Quincy, MA, 2009.
A- PROTECTING Manufactured Homes FROM FloodS AND Hazards 

A Multi-Hazard Foundation and Installation Guide

A REFERENCES

National Fire Protection Association (NFPA), Standard on Manufactured Housing, (NFPA 501), 

Quincy, MA, 2005.

Peck, R.A., Hanson, W.E., and Thornburn, T.H., Foundation Engineering, Second Edition, John 

Wiley & Sons, Inc., 1973. 

Pearson, John E., Longinow, Anatol, and Meinheit, Donald F., “Wind Protection Tie-Downs for 

Manufactured Homes,” Practice Periodical on Structural Design and Construction, 1996.

State of California, Department of Housing and Community Development, The Effectiveness of 

Manufactured Home Support Systems During Earthquakes, April 1992.

State of California, Department of Housing and Community Development, Division of Codes 

and Standards, Manufactured Homes: A Handbook for Local Government, July 2001.

Systems Building Research Alliance, Guide to Foundation and Support Systems for Manufactured 

Homes. 2002.

United States Code (USC), The National Manufactured Housing Construction and Safety Standards 

Act of 1974, (42 USC 5404, Section 605).

U.S. Army Corps of Engineers, Shore Protection Manual, 4th Edition. 1984.

U.S. Department of Housing and Urban Development, The Effect of Earthquakes on Manufactured 

Home Installation (HUD-7004), December 1995.

U.S. Department of Housing and Urban Development, NAHB Research Center, Factory and SiteBuilt Housing: A Comparison for the 21st Century, October 1998.

U.S. Department of Housing and Urban Development, NAHB Research Center, Manual for the 

Construction of Residential Basements in Non-Coastal Flood Environs (CR-997), March 1977.

U.S. Department of Housing and Urban Development, Manufactured Home Construction and Safety Standards (24 CFR Part 3280), 1999. 

U.S. Department of Housing and Urban Development, Manufactured Home Producer’s Guide to the 

Site-Built Market, May 2000.

U.S. Department of Housing and Urban Development, Minimizing Damage and Repair Costs to 

Manufactured Homes During an Earthquake, February 1996.

U.S. Department of Housing and Urban Development, Model Manufactured Home Installation 

Standards (24 CFR 3285), 2008. 

U.S. Department of Housing and Urban Development, Northridge Earthquake Effect on Manufactured Housing in California (HUD-0006466), Prepared by the NAHB Research Center, Inc., June 

1994.
PROTECTING Manufactured Homes FROM FloodS AND Hazards 

A Multi-Hazard Foundation and Installation Guide

AREFERENCES A

U.S. Department of Housing and Urban Development, Office of Policy Development and Research, Permanent Foundations Guide for Manufactured Housing (HUD-007487), September 1996.

U.S. Department of Housing and Urban Development, Office of Policy Development and Research, Residential Structural Design Guide: 2000 Edition, Prepared by the NAHB Research Center, 

Inc., February 2000.

U.S. Department of Transportation, Federal Highway Administration, Evaluating Scour at Bridges, 

4th Edition (FHWA NHI 01-001), Hydraulics Engineering Circular No. 18, May 2001.

Yokel, F. Y. “A Study of Reaction forces on Mobile Home Foundations by Wind and Flood Loads.” 

NBS Building Science Series 132. U.S. Department of Commerce, National Bureau of Standards, 

Washington, DC, 1981.

PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards 

A Multi-Hazard Foundation and Installation Guide

B-

B.	 Sources for Flood

Information

To develop an effective manufactured home installation strategy applicable to a flood hazard 

area, several hydrological factors must be evaluated. These include the regulatory floodplain 

boundaries and the anticipated flooding characteristics for the site, such as location of the 

floodway, depth, velocity, duration, rate-of-rise, and frequency. Various sources for this information are available. Also, there are methods by which portions of such information can be 

individually developed.

As part of the NFIP, FEMA develops FIS reports that contain the most current and detailed information available for a community. Such studies can include a FIRM and a Flood Boundary 

and Floodway Map (FBFM). Although studies completed prior to 1986 are no longer separate 

from FIRMs, many communities still have pre-1986 studies and an FIS report from which the 

following information can be obtained:

n Floodplain and floodway boundaries

n Stream profiles that show the elevations of the 100-year or “base flood” (a flood that has a 

1-percent chance of being equaled or exceeded in any given year) and other flood events

n Mapped base flood elevations that can be used to develop flood depths for a specific site in 

combination with other data

n Flood velocity data

n Flood frequency data

n Flood discharge data

n Historical flood information

FIS reports can be extremely valuable sources for much of the information necessary to evaluate 

a potential manufactured home site. The specific hydrologic data elements, and ways to obtain 

them, are described below.

B.1	 Map Modernization and Risk Mapping, Assessment, and

Planning (Risk MAP)

FEMA developed a plan in 1997 to modernize the FEMA flood mapping program. The plan 

outlined the steps necessary to update FEMA’s flood maps for the nation to digital format and 

streamline FEMA’s operations in raising public awareness of the importance of the maps and 

responding to requests for revision. Since 1997, the plan has continually evolved as new 
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A Multi-Hazard Foundation and Installation Guide

B SOURCES FOR FLOOD INFORMATION

products, processes, and technical specifications have been developed and implemented within 

current funding levels. The goal of FEMA’s Map Modernization Plan is to upgrade the 00,000 

panel flood map inventory by the following methods:

n Develop up-to-date flood hazard data for all flood-prone areas nationwide to support 

sound floodplain management and prudent flood insurance decisions

n Provide the maps and data in digital format to improve the efficiency and precision with 

which mapping program customers can use this information

n Fully integrate FEMA’s community and State partners into the mapping process to build 

on local knowledge and efforts

n Improve processes to expedite creating and updating the maps

n Improve customer services to speed processing of flood map orders and raise public 

awareness of flood hazards

Building on the Map Modernization Program, FEMA’s Risk Mapping, Assessment, and Planning (Risk MAP) is a seamless program aimed at reducing losses of life and property through 

effective local mitigation activities enabled by quality flood hazard data, risk assessments, and 

mitigation planning. Risk MAP will provide an integrated national assessment of risks based 

on digital flood hazard data and web-accessible data. Risk MAP information and tools will help 

communities develop informed mitigation plans that will reduce losses from natural hazards. 

The goals of Risk MAP are:

n Flood Hazard Data: Address gaps in flood hazard data to form a solid foundation for risk 

assessments, floodplain management, and actuarial soundness of the National Flood 

Insurance Program.

n Public Awareness/Outreach: Ensure that a measurable increase of the public’s awareness 

and understanding of risk results in a measurable reduction of current and future 

vulnerability.

n Hazard Mitigation Planning: Lead and support State, Local, and Tribal communities to 

effectively engage in risk-based mitigation planning resulting in sustainable actions that 

reduce or eliminate risks to life and property from natural hazards.

n Enhanced Digital Platform: Provide an enhanced digital platform that improves 

management of Risk MAP and also improves communication and sharing of risk data and 

related products to all levels of government and the public.

n Alignment and Synergies: Align Risk Analysis programs and develop synergies to enhance 

decision-making capabilities through effective risk communication and management.

B.2	 Flood Study Software Programs

FEMA has approved the use of several modeling programs to support the NFIP and flood hazard 

mapping. These programs were created by various agencies and organizations, including the 
PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards 

A Multi-Hazard Foundation and Installation Guide

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SOURCES FOR FLOOD INFORMATION B

U.S. Natural Resources Conservation Service (NRCS), U.S. Army Corps of Engineers (USACE), 

U.S. Department of Agriculture (USDA), U.S. Geological Survey (USGS), and FEMA mapping 

contractors. Information about approved software is available at the FEMA website at http://

www.fema.gov/plan/prevent/fhm/frm_soft.shtm. The models can be organized into several 

categories:

Nationally Accepted Models

n Coastal Models: coastal storm surge, coastal wave height, and coastal wave effect models, 

including FEMA Surge, AdCIRC, WHAFIS 3.0, and RUNUP 2.0.

n Hydrologic Models: single event, continuous event, and interior drainage analysis models, 

including HEC-1, HEC-HMS, and TR-20. 

n Hydraulic Models: one-dimensional steady flow, one-dimensional unsteady flow, twodimensional steady/unsteady flow, and floodway analysis models, including HEC-RAS, 

HEC-2, and WSPRO.

Coastal model outputs include stillwater elevations (FEMA Surge and AdCIRC), wave height 

profiles along a transect (WHAFIS 3.0), and wave run-up elevations (RUNUP 2.0). Hydrologic model outputs are generally discharge values. Hydraulic model outputs include floodway 

analyses (where applicable) and flood elevations for 10-, 2-, 1-, and 0.2-percent exceedance 

probability floods.

Locally Accepted Models

n Hydrologic Models: single event models 

n Hydraulic Models: one- and two-dimensional unsteady flow models

The FEMA website also provides guidance on the use of automated hydrologic and hydraulic 

(H&H) techniques as part of the NFIP Map Modernization efforts at http://www.fema.gov/

plan/prevent/fhm/mm_main.shtm. 

B.3	 Flood Hazard Boundaries

Flood hazard boundaries must be identified for the different degrees and types of flooding, including floodways, floodway fringe areas, coastal high hazard areas, coastal fringe areas, and 

shallow flooding areas. These boundaries are significant for determining the specific flood 

hazard zones that would be part of a proposed development site and, thereby, influence site 

development. In addition, boundaries indicate where floodplain management regulations and 

flood insurance requirements apply. Flood hazard boundary data can be obtained from FBFMs, 

FIRMs, and floodplain maps, or can be developed from topographic maps, zoning maps, aerial 

photographs, and related hydrologic data.
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B SOURCES FOR FLOOD INFORMATION

B.4	 Flood Depth

Flood depths are determined by the difference between water surface elevations at times of 

flooding and normal ground surface elevations. This information is important both in determining the elevations at which floodwaters will likely cause damage and in defining the appropriate 

elevations for flood insurance and floodplain management regulations. Flood depths also influence the hydrostatic forces in effect during flooding, including the horizontal loads that can 

cause lateral displacement or overturning and the vertical loads that can cause uplift and flotation. Therefore, it is important to know the flood depth to determine appropriate foundations 

for the site. 

Flood depths for a particular site can be derived using a FIRM showing BFEs in combination 

with a topographic map depicting ground elevations. Flood depth data are also available from 

various technical studies that include flood elevations, water surface profiles, or stream and 

coast cross-sections. In the absence of official reports, information on flood depths can be obtained from site surveys and historical records.

B.5	 Floodwater Velocity

The average and maximum velocity of floodwater determines the hydrodynamic forces that influence horizontal loads in excess of hydrostatic loads. Velocity also affects the magnitude of 

debris impact loads (i.e., force of flotation objects carried by floodwaters), and can increase erosion and affect soil stability on slopes. Data on water velocity are listed in Floodway Data Tables 

and are often included in FIS reports. Data might also be available from various floodplain technical studies or determined by special hydrological studies. FIS Floodway Data Tables include 

the mean velocity of floodwaters (for the base flood event) within the floodway. These mean 

velocities can be used to estimate the upper limit of floodwater velocity in the adjacent flood 

fringe portion of the floodplain.

Velocity can also be calculated by assuming floodwaters are at a uniform flow, estimating some 

floodplain characteristic, and using Manning’s Equation:

Where: V = Average flow velocity (fps)

k = 1.49, unit conversion 

A = Cross-sectional flow area (square feet) (shaded area, Figure B-1)

P = Wetted perimeter of A (feet) (labeled in Figure B-1)

 = Hydraulic radius (feet) A

P

k	 A 2/3

n	 P

V = (S)1/2
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SOURCES FOR FLOOD INFORMATION B

S = Gradient (feet/feet) (use average ground surface slope within a reach from 

approximately 0.5 mile upstream to approximately 0.5 mile downstream)

n = Manning’s channel roughness coefficient, empirical value developed through 

lab testing of flow through a pipe 

B.6	 Frequency

Frequency of flooding is a major consideration in evaluating potential installation sites. Frequency of flooding is the probability (in percent) that a random flood event will equal or exceed a 

specified magnitude in a given time period. Manufactured homes sited at lower elevations near 

a flooding source will likely have higher frequency of flooding than those located at higher elevations. Flood frequency information is included in FIS reports and other technical floodplain 

studies. Frequency of flooding can also be statistically determined using historical records.

If manufactured homes are installed on sites vulnerable to high frequency floods (i.e., floods 

that occur often), there is a greater risk of damage and there will be a need for more frequent 

evacuations. Not only do frequent evacuations present an increased risk to owners and/or tenants, but also to a community’s emergency management officials who provide assistance during 

evacuations. 

B.7	 Rate of Rise

The rate of rise is how rapidly water depth increases during a flooding event. This factor is important in evaluating buoyancy hazards and investigating the feasibility of an evacuation plan. 

The rate can be derived from a stream flow hydrograph that compares flooding depth to time 

for the area under consideration. Determining the rate of rise requires information that may 

be obtained from existing hydrological studies, on-site investigations, historical records, and 

nearby gauge records.

B.8	 Duration

The duration of a flood is a function of the rate of rise and fall of water. Duration influences the 

saturation of soils and building materials, the amount of seepage, and the length of time that a 

Figure B-1. Stream cross-section for Manning’s Equation
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A Multi-Hazard Foundation and Installation Guide

B SOURCES FOR FLOOD INFORMATION

manufactured home might be inaccessible. Various floodplain technical studies and historical 

records are sources of information concerning duration of flooding. 

B.9	 Sources of Information

Tables B-1 and B-2 provide sources of various types of information and assistance.

Table B-1. Floodplain Management Information

Floodplain Agencies

Data Type

Floodproofing InformationPreserve Channel Capacity

Development Regulations

Land Use Controls

Floodwater ControlPublic InformationPost-Flood ReliefFlood Warning System

Federal Emergency Management Agency n n n n n

U.S. Army Corps of Engineers n n n n n n n

Natural Resources Conservation Service n n n n

Department of Housing and Urban 

Development n n

National Oceanic and Atmospheric 

Administration n n

U.S. Geological Survey n

Federal Highway Administration n n n

State Floodplain Management Coordinating 

Agency n n n n n

Regional Authorities n n n n n n

Local Government Planning Agencies n n n
PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards 

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BSOURCES FOR FLOOD INFORMATION B

Table B-2. Summary of Hydrologic Data Sources

Agencies

Data Type

Coastal Surveys and Reports

Flood Control MeasuresFlood Boundary MapsFlood Insurance Rate MapsFloodplain Information Reports and Technological StudiesFlood Records and Probabilities

Hydrologic Atlases

National Flood Insurance Program Regulations

State Floodplain Regulations

Technical AssistanceTopographic Maps

Zoning Ordinances and Maps

Coastal Modeling Software

Hydrologic Modeling Software

Statistical Modeling Software

Hydraulic Modeling Software

Federal Emergency 

Management Agency n n n n n n n n n

U.S. Army Corps of 

Engineers n n n n n n n n n n n n n

National Resources 

Conservation Service n n n n

National Oceanic 

and Atmospheric 

Administration

n n n n n

U.S. Geological Survey n n n n n n n

State Floodplain 

Management 

Coordinating Agency

n n n n n n n n

Regional Authorities 

(e.g., Tennessee Valley 

Authority)

n n n n n n

Local Government 

Planning Agency or 

Municipal Engineer

n n n n n n n n n n n n n n

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C.	 Flood Velocity

Determination

C.1	 Velocity Data

Determining the design floodwater velocity (velocity associated with base flooding) is important 

to properly design manufactured home foundations. Only hydraulic modeling or measurements from historic events can produce actual estimated velocity values. In addition to those 

resources, this section presents some of the tools and sources of information available that can 

help to provide an estimate of the floodwater velocity.

C.1.1	 Hydraulic Modeling

Hydrologic and hydraulic (H&H) modeling is done as part of floodplain mapping. Through 

hydraulic models such as the USACE’s Hydrologic Engineering Center’s River Analysis System 

(HEC-RAS), riverine systems are modeled to determine the elevation of a given flood, such as 

the base flood, and the extent of flooding. Hydraulic modeling tools also provide additional 

helpful information about flood scenarios, including average channel and overbank floodwater 

velocities at particular cross-section locations. These velocities can be used as design velocities 

for structures when they correspond to a cross-section at or very near the development site. 

Figure C-1 shows an example of output from a HEC-RAS model run that includes overbank 

velocities. While these data can be very helpful, communities are expected to use the best available data, and overbank flood velocities may not be readily available in FIS reports.

Generally, the most easily accessible tool to use in estimating velocities will be the community’s 

FIS. As described in Section 5.2.4, mean floodway velocities are determined during hydraulic 

modeling and can be obtained from the FIS using the following 3-step process:

n Step 1: Locate the manufactured housing site on the FIRM or DFIRM. This first step is 

illustrated in Figure C-2 using an example DFIRM.

n Step 2: Locate the stream cross-section on the FIRM or DFIRM that is located closest to the 

manufactured housing site. This second step is illustrated in Figure C-3 using an example 

DFIRM.

n Step 3: Match the cross-section for a particular stream on the FIRM or DFIRM with the 

cross-section in the stream’s Floodway Data Table found in the FIS. This third step is 

illustrated in Figure C-4 using an example Floodway Data Table. At cross-section A, the 

mean floodway velocity is 0.4 fps. 

It is important to note that the floodway’s mean velocity is not a good measure of the actual 

flood velocity within the flood fringe, but it can be used as a general measure to determine 
C-2 PROTECTING MaNufaCTuREd HOMEs fROM flOOds aNd OTHER HazaRds 

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C FLOOD VELOCITY DETERMINATION

cross-section locations within the floodplain where floodwaters will move relatively faster or 

slower, and to provide an upper limit for velocities in the flood fringe.

Hydraulic modeling with the potential to provide overbank flood velocities might also be available. Modeling used to complete a community’s FIS can be ordered through FEMA’s Project 

Library by completing a FIS data request form (http://www.fema.gov/plan/prevent/fhm/st_

order.shtm). These data are usually, but not always, available for communities with existing FISs. 

Depending on the data format (e.g., paper, diskettes, or microfiche), it may be possible to determine overbank velocities at cross-sections without significant effort. While this modeling can be 

helpful, the community is not expected to use it to create any new data; use of the best available 

data is all that is necessary. However, there is a possibility that this modeling will be available locally, perhaps through the municipal engineer, especially if new or updated modeling for the 

FIS has been recently completed.

H&H modeling might also be available regionally or locally if other agencies have done studies. Table 5- lists several Federal, State, and local agencies that might have information about 

past flooding events. These same agencies are the ones most likely to have performed flood 

studies, separate from the communities’ FIS reports, for a localized area. Such studies might be 

performed in order to further characterize flooding in a particularly vulnerable area or to 

Figure C-1. Example

HEC-RAS output.

profile corresponds to event 

frequency (e.g., 50- or 100-year)

overbank velocities
PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards 

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CFLOOD VELOCITY DETERMINATION C

provide updated information about an area that may have experienced significant development 

since the FIS was completed. 

C.1.2	 Documented Historic Information

Another potential source of documented floodwater velocities are post-flood reports. Refer to 

Table 5-1 for Federal, State, and local agencies that often publish post-event flood information. 

Such documentation might include information about observed damages, flood elevations, and 

other flood characteristics, including floodwater velocities. One such example of these types of 

reports is the Delaware Geologic Survey (http://www.udel.edu/dgs/index.html) Open File Reports completed after coastal storms in 1992 and 1998. These reports provide synopses of the 

events, high water mark survey results, rainfall and stream gauge records, and wave height records. Another example is The South Florida Water Management District, which offers reports 

documenting past flood events via its website (http://www.sfwmd.gov). Additionally, the Harris 

County (Texas) Flood Control District provides a summary of past flooding, including observed 

damages noting areas that were hardest hit, on their website (http://www.hcfcd.org). Local officials (e.g., engineers, floodplain managers, building inspectors, planners) are probably the best 

resource for determining if these types of reports exist and where they can be found.

One agency listed in Table 5-1, the U.S. Geological Survey (USGS), measures stream flow 

characteristics at established gauges throughout the U.S. during flood events. The specific characteristics measured depend on the gauge’s capabilities, but can include discharge and mean 

channel velocity. These data are available at the USGS website via the National Water Information System (NWIS) Web Water Data system (http://waterdata.usgs.gov/nwis). The USGS also 

frequently takes measurements of depth and velocity at discrete cross-section locations along 

gauged streams during flood events. This information is not available online, but can be obtained from the appropriate USGS office.

C.2 	 Alternate Methodologies for Estimating Velocities

In the absence of hard data such as those found in stream modeling or historic measurements 

and reports, local expertise and evaluation of site indicators can be used to determine relative 

velocities. To estimate site floodwater velocities, local officials, such as engineers and floodplain 

managers, with knowledge of past flood events can be consulted. Additionally, there are some 

site indicators that local officials can use to estimate relative velocities.

C.2.1	 Knowledge of Past Flooding

Local floodplain management and building officials are encouraged to use their knowledge of 

past flood events and exercise their judgment in determining relative flood velocities in the absence of hard data. Some questions to consider include the following:

n Have base flood conditions consisted of very shallow (less than 1 foot) or ponding (slow 

draining, no velocity) flooding?
C- PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards 

A Multi-Hazard Foundation and Installation Guide

C FLOOD VELOCITY DETERMINATION

If the answer to this question is yes, then a pier foundation with dry-stacked blocks would 

be appropriate. With no velocity and/or very shallow floodwaters, the stability and strength 

provided by either surface bonded mortar or grouted/rebar/anchored piers may not be 

necessary.

n Have structures at or near the site been damaged by high velocity flows in the past?

If there are any indications that high velocity floodwaters have caused damage to structures 

in the past, none of the pre-engineered foundations described in Appendix H should 

be used. Some examples of foundation failure as the result of high velocity floodwaters 

include: 1) structures shifting from their foundations (sliding failure), 2) scour occurring 

at the foundation, and 3) uplift of the structure from the foundation (in conjunction with 

buoyancy forces). Section 5.2.4 provides further information on hydrodynamic forces and 

foundation failure.

n Have moving floodwaters damaged appurtenant structures such as decks and porches?

Floodborne debris often consists of lumber and other pieces of decking and porches. 

High velocity floodwaters damage attachments, such as carports, decks, porches, and 

awnings, and/or their connections and can lead to structural failure of the attachment. 

Field observations have noted that the primary cause of this failure has been inadequate 

support; the attachments’ support systems were simply not designed or constructed 

to withstand the hydrodynamic loadings imposed upon them. At or near sites where 

appurtenant structures have been severely damaged or destroyed by fast moving 

floodwaters, only designs for high velocity foundations (not Appendix H foundations) 

should be used.

C.2.2	 Site Indicators

Another set of tools to use in estimating relative velocity are characteristics of the site and/or 

the surrounding watershed. These indicators provide a broader frame of reference for estimating relative velocities. 

The proximity of a site to the flood source (stream or river) is one of the most basic gauges of 

floodwater velocity. Generally, the velocity of floodwater will decrease from the channel center 

out to the edges of the flood fringe as illustrated in Figure C-5. The floodway is the area of the 

stream where floodwaters are likely to be the deepest and have the highest velocities. Therefore, in the outer edges of the flood fringe, velocities can be expected to be relatively low. Sites 

located well within the flood fringe closer to the floodway will tend to have higher floodwater 

velocities. Figure C-6 shows an aerial view of a stream with the floodway highlighted in dark gray 

and the floodplain outlined in white. Within the floodway, velocities will be higher. As you move 

outwards to the edges of the flood fringe, velocities will decrease.
PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards 

A Multi-Hazard Foundation and Installation Guide

CFLOOD VELOCITY DETERMINATION C

Figure C-2. Manufactured housing site location on DFIRM.

Figure C-3. DFIRM detail

showing stream crosssection located closest

to the manufactured

housing site location.

Nearest Cross-Section: 

Country Club Creek, Cross-Section A

MH Location
C- PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards 

A Multi-Hazard Foundation and Installation Guide

C FLOOD VELOCITY DETERMINATION

Figure C-4. Floodway Data

Table and mean velocity.

Figure C-6. Floodwaters in

the floodway (dark gray)

will have higher velocities

than those in the flood

fringes (between black

and gray).

Figure C-5. Flood velocity

profiles.

Floodplain

Floodway
PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards 

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C-

FLOOD VELOCITY DETERMINATION C

Terrain characteristics can also help to provide some clues about floodwater velocities. Flood 

velocity is largely dependent on the slope and roughness of the stream channel and overbank 

terrain. Water moves downstream faster when the channel and overbank areas are steeply 

sloped. Water also flows faster over paved or smooth surfaces such as roadways or parking lots 

as opposed to densely vegetated stream banks. Streams in steep or mountainous areas tend to 

have higher floodwater velocities capable of more damage than those in flatter areas. Figure 

C-7 shows a damaged home along a stream that is surrounded by steep, mountainous terrain 

and subject to flash flooding.

Velocity patterns are also affected by the sinuosity of a waterway. Within streams with some 

meandering characteristics, channel velocities are higher along the exterior bends of streams. 

Stream banks along the exterior bends of meandering streams are subject to erosion due to 

forces exerted by moving waters while deposition of material often occurs along interior bends 

due to lower velocities. Therefore, sites along the exterior bends of streams should be considered at risk for higher floodwater velocities. Figure C-8 illustrates a meandering stream system 

where there are several exterior bends. 

Stream hydraulics can be greatly impacted by flow constrictions within the waterway, including 

bridges, culverts, and levees. Higher floodwater velocities are often observed both within the 

constricted area and directly downstream of these constrictions. Velocity within an open stream 

is equal to the discharge rate divided by the flow area. When a constriction reduces the area 

that water may flow through, water must either flow faster or pond up (thereby reducing the 

discharge rate); it often does both.

Typically there is some ponding or storage upstream of a constriction during major flooding, but the velocity is also likely to increase dramatically through and just downstream of the 

Figure C-7. Home

damaged along a stream

subject to flash flooding

due to surrounding

mountainous terrain.
C-8 PROTECTING MaNufaCTuREd HOMEs fROM flOOds aNd OTHER HazaRds 

A Multi-Hazard Foundation and Installation Guide

C FLOOD VELOCITY DETERMINATION

constriction. As the flow area decreases, the depth of the water will also increase through the 

constriction and continue to be elevated for some time downstream of the constriction as the 

flow area gradually expands into more natural terrain. Therefore, areas just downstream of constrictions may be prone to high velocity waters during flooding.

Figure C-8. Meandering

stream system. Velocities

are generally higher

along the outer bends of

streams.
D. Definitions

GENERAL NOTE: NFIP definitions in quotes taken from 44 CFR 59.1 unless otherwise noted.

Base flood – Defined by the NFIP as “The flood having a one percent chance of being equaled 

or exceeded in any given year.” The base flood is also called the 100-year flood.

Base flood elevation (BFE) – The elevation of the base flood as given in the community’s FIRM 

and FIS.

Breakaway wall – Defined by the NFIP as “A wall that is not part of the structural support of the 

building and is intended through its design and construction to collapse under specific lateral 

loading forces, without causing damage to the elevated portion of the building or supporting 

foundation system.” Breakaway walls are used in V1-30, VE, and V zones to enclose parking, 

building access, and storage areas below buildings.

Coastal A Zone – See Limit of Moderate Wave Action (LiMWA).

Coastal High Hazard Area – Defined by the NFIP as “An area of special flood hazard extending 

from offshore to the inland limit of a primary frontal dune along an open coast and any other 

area subject to high velocity wave action from storms or seismic sources.” Coastal High Hazard 

Areas are identified as V1-30, VE, and V zones on FIRMs.

Community – Defined by the NFIP as “Any State or area or political subdivision thereof, or any 

Indian tribe or authorized tribal organization, or Alaska Native village or authorized native organization, which has authority to adopt and enforce floodplain management regulations for 

the areas within its jurisdiction.”

Design flood elevation (DFE) – The elevation to which development in the regulatory floodplain is built. The minimum requirement for this elevation in NFIP communities is the BFE. In 

areas where a higher degree of protection is promoted or required, a freeboard is added; in this 

case, the DFE is some height (1, 2, or more feet) above the BFE. The 2009 International Residential Code Sections R 322.2.1 and R 322.3.2 require a minimum DFE equal to the BFE plus 1 

foot for elevation projects located in Coastal A zones and Coastal High Hazard Areas.

Enclosed areas – The portion of an elevated building below the BFE that is partially or fully 

surrounded by solid (including breakaway) walls. These enclosed areas can only be used for 

parking, building access, or storage and must meet other construction requirements.

Encroachment – Any type of development within the floodplain that results in a loss of its flood 

carrying capacity. Encroachments in floodways are particularly important in floodplain development because, under NFIP minimum requirements, any development within the floodway that 

results in an increase in the BFE elevation is not permitted.

PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards 

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D-1
D DEFINITIONS

Existing manufactured home park or subdivision – Defined by the NFIP as “A manufactured 

home park or subdivision for which the construction of facilities for servicing the lots on which 

the manufactured homes are to be affixed (including, at a minimum, the installation of utilities, the construction of streets, and either final site grading or the pouring of concrete pads) 

is completed before the effective date of the floodplain management regulations adopted by a 

community.”

Expansion to an existing manufactured home park or subdivision – Defined by the NFIP as “The 

preparation of additional sites by the construction of facilities for servicing the lots on which the 

manufactured homes are to be affixed (including the installation of utilities, the construction 

of streets, and either final site grading or the pouring of concrete pads).”

Flood damage-resistant material – Any building material capable of withstanding direct and prolonged contact with floodwaters without sustaining significant damage. The term “prolonged 

contact” means at least 72 hours, and the term “significant damage” means any damage requiring more than low-cost cosmetic repair.

Flood depth – The difference in height between the water surface elevation at the time of flooding and the normal grade elevation of the flooded area.

Flood Hazard Boundary Map (FHBM) – Defined by the NFIP as “An official map of a community, issued by the Administrator, where the boundaries of the flood and/or mudslide (i.e., 

mudflow) related to erosion areas having special hazards have been designated as Zones A, M, 

and/or E.”

Flood Insurance Rate Map (FIRM) – Defined by the NFIP as “An official map of a community, 

on which the Administrator has delineated both the SFHAs and the risk premium zones applicable to the community.”

Flood Insurance Study (FIS) – Defined by the NFIP as “An examination, evaluation, and determination of flood hazards and, if appropriate, corresponding water surface elevations, or 

an examination, evaluation, and determination of mudslide and/or flood-related erosion hazards.”

Floodplain – Defined by the NFIP as “Any land area susceptible to being inundated by water 

from any source.”

Floodway – Defined by the NFIP as “The channel of a river or other watercourse and the adjacent land areas that must be reserved in order to discharge the base flood without cumulatively 

increasing the water surface elevation more than a designated height.”

Freeboard – An additional elevation requirement some height above the BFE; this additional 

height is added to the BFE to produce the DFE. Freeboard provides a margin of safety above 

the estimated BFE and against extraordinary or unknown risks. As in all natural hazard events, 

the design event can only be predicted in probabilistic terms and some uncertainties remain in 

any analysis. Freeboard is intended to allow for those uncertainties.

D-2 PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards

A Multi-Hazard Foundation and Installation Guide
DEFINITIONS D

Limit of Moderate Wave Action (LiMWA) – The inland limit of the area affected by waves greater than 1.5 feet. The area between this inland limit and the V zone boundary is known as the 

Coastal A zone.

Manufactured home – Defined by the NFIP as “A structure, transportable in one or more sections, that is built on a permanent chassis and is designed for use with or without a permanent 

foundation when attached to the required utilities. The term ‘manufactured home’ does not 

include a ‘recreational vehicle.’”

Manufactured home park or subdivision – Defined by the NFIP as “A parcel (or contiguous parcels) of land divided into two or more manufactured home lots for rent or sale.”

New manufactured home park or subdivision – Defined by the NFIP as “A manufactured home 

park or subdivision for which the construction of facilities for servicing the lots on which the 

manufactured homes are to be affixed (including, at a minimum, the installation of utilities, the 

construction of streets, and either final site grading or the pouring of concrete pads) is completed on or after the effective date of the floodplain management regulations adopted by a 

community.”

Permanent foundation – A foundation constructed of durable materials that may include masonry, reinforced concrete, corrosion resistant steel, or pressure treated timber. Note that this 

definition of a permanent foundation is provided for the purposes of this publication. The 

foundation designs provided in Appendix H are considered permanent foundations in accordance with this definition.

Regulatory floodplain – The floodplain to which a community applies their floodplain development regulations. A community participating in the NFIP is required to adopt the floodplain 

on their FIRM, or use a more restrictive (larger aerial extent or higher floodplain water surface 

elevations) floodplain.

Special Flood Hazard Area (SFHA) – Defined by the NFIP as “The land in the floodplain within 

a community subject to a 1-percent or greater chance of flooding in any given year. The area 

may be designated as Zone A on the FHBM. After detailed ratemaking has been completed in 

preparation for publication of the FIRM, Zone A is usually refined into Zones A, AO, AH, A1-30, 

AE, A99, AR, AR/A1-30, AR/AE, AR/AO, AR/AH, AR/A, VO or V1-30, VE, or V.” The SFHA is 

also called the base floodplain, 100-year floodplain, or 1-percent annual chance floodplain.

Structural fill – Engineered fill, compacted to 90 percent compaction, Modified Proctor Test, 

ASTM D1557. It should have a minimum bearing capacity as recommended by geotechnical engineer, and be free of organic material such as weeds, or grasses, or other organic matter.

Substantial damage – Defined by the NFIP as “Damage of any origin sustained by a structure 

whereby the cost of restoring the structure to its before damaged condition would equal or exceed 50 percent of the market value of the structure before the damage occurred.”

PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards 

A Multi-Hazard Foundation and Installation Guide

D-3
D DEFINITIONS

Substantial improvement – Defined by the NFIP as “Any reconstruction, rehabilitation, addition, or other improvement of a structure, the cost of which equals or exceeds 50 percent of 

the market value of the structure before the ‘start of construction’ of the improvement. This 

term includes structures that have incurred ‘substantial damage,’ regardless of the actual repair 

work performed. The term does not, however, include either: (1) any project for improvement of a structure to correct existing violations of State or local health, sanitary, or safety code 

specifications that have been identified by the local code enforcement official and that are the 

minimum necessary to ensure safe living conditions, or (2) any alteration of a ‘historic structure,’ provided that the alteration will not preclude the structure’s continued designation as a 

‘historic structure.’” 

D-4 PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards

A Multi-Hazard Foundation and Installation Guide
PROTECTING Manufactured Homes FROM FloodS AND OTHER HazardS

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E-

E. Acronyms and 

Abbreviations

A

ABS	 	 acrylonitrile butadiene styrene

ACI	 	 American Concrete Institute

ACQ	 	 alkaline copper quaternary

ACZA	 	 ammoniacal copper zinc arsenate

AdCIRC Advanced Circulation Model

AF&PA American Forest and Paper Association

AISC	 	 American Institute of Steel Construction

ANSI	 	 American National Standards Institute

APA American Plywood Association

ASCE	 	 American Society of Civil Engineers

ASD	 	 allowable stress design

ASFPM	 Association of State Floodplain Managers

ASTM	 	 American Society for Testing Materials

AWC American Wood Council

AWPA		 American Wood Preservers Association

B

BFE	 	 base flood elevation

BMP	 	 braced masonry pier

BPAT	 	 Building Performance Assessment Team

C

C&C	 	 components and cladding

CCA	 	 chromated copper arsenate
E- PROTECTING Manufactured Homes FROM FloodS AND OTHER HazardS

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E Acronyms and Abbreviations

CFR	 	 Code of Federal Regulations

CMU	 	 concrete masonry unit

COSAA	 Council of State Administrative Agencies

CRS	 	 Community Rating System

D

DAPIA		 Design Approval Primary Inspection Agency

DFE	 	 design flood elevation

DFIRM	 Digital Flood Insurance Rate Map

DHS	 	 Department of Homeland Security

DMA 2000	 Disaster Mitigation Act of 2000

E

ERBS	 	 earthquake-resistant bracing system

F

FBFM	 	 Flood Boundary and Floodway Map

FEMA	 	 Federal Emergency Management Agency

FHBM		 Flood Hazard Boundary Map

FHWA		 Federal Highway Administration

FIA	 	 Federal Insurance Administration

FIRM	 	 Flood Insurance Rate Map

FIS	 	 Flood Insurance Study

FMHA		 Florida Manufactured Housing Association	

fps	 	 feet per second

ft	 	 foot or feet

FY	 	 fiscal year

G

g	 	 acceleration due to gravity

GA	 	 ground anchor
PROTECTING Manufactured Homes FROM FloodS AND OTHER HazardS

A Multi-Hazard Foundation and Installation Guide

E-

Acronyms and Abbreviations E

GAATP	 Ground Anchor Assembly Test Protocol

GASP	 	 ground anchor with stabilizer plate

GIS	 	 Geographic Information System

GPO	 	 Government Printing Office

H

H&H	 	 hydrologic and hydraulic

HAZUS-MH	 Hazards U.S. – Multi-Hazard

HEC Hydrologic Engineering Center (USACE)

HEC-HMS	 Hydrologic Engineering Center's Hydrologic Modeling System

HEC-RAS	 Hydrologic Engineering Center's River Analysis System

HMS	 	 Hydrologic Modeling System

HUD	 	 U.S. Department of Housing and Urban Development

HVAC	 	 heating, ventilation, and air conditioning

I

IBC	 	 International Building Code

ICBO 	 	 International Council of Building Officials

ICC	 	 International Code Council

ID	 	 identification

in	 	 inch or inches	 	

IRC	 	 International Residential Code

ITW	 	 Illinois Tool Works

L

lb	 	 pound or pounds

LiMWA Limit of Moderate Wave Action

LOMA		 Letter of Map Amendment

LOMR 	 Letter of Map Revision
E- PROTECTING Manufactured Homes FROM FloodS AND OTHER HazardS

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E Acronyms and Abbreviations

M

MAT	 	 Mitigation Assessment Team

MH	 	 manufactured housing

MHCC		 Manufactured Housing Consensus Committee

MHCSS	 Manufactured Home Construction and Safety Standards

MHI	 	 Manufactured Housing Institute

MHIA 2000	 Manufactured Housing Act of 2000

MHRA 	 Manufactured Housing Research Alliance (now Systems Building Research

	 	 Alliance)

mm	 	 millimeter

mph 	 	 miles per hour

MWFRS	 main wind force resisting system

N

NAHB	 	 National Association of Home Builders

NAVD	 	 North American Vertical Datum

NCSBCS	 National Conference of States on Building Codes and Standards

NDS	 	 National Design Standard

NEHRP	 National Earthquake Hazards Reduction Program

NFHL	 	 National Flood Hazard Layer

NFIA	 	 National Flood Insurance Act

NFIP	 	 National Flood Insurance Program

NFPA	 	 National Fire Protection Association

NGVD	 	 National Geodetic Vertical Datum

NIST	 	 National Institute of Standards and Technology

NRCS	 	 National Resources Conservation Service

NWIS 	 National Water Information System
PROTECTING Manufactured Homes FROM FloodS AND OTHER HazardS

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E-

Acronyms and Abbreviations E

O

o.c.	 	 on center

OC	 	 Oversight Committee

P

PATH	 	 Partnership for Advancing Technology in Housing

pcf	 	 pounds per cubic foot

PFGMH	 Permanent Foundations Guide for Manufactured Housing

plf	 	 pounds per linear foot

psf	 	 pounds per square foot

psi	 	 pounds per square inch

R

RiskMAP	 Risk Mapping, Assessment, and Planning

RM	 	 reinforced masonry

RUNUP	 wave runup module

S

SBRA	 	 Systems Building Research Alliance (formerly Manufactured Housing Research

	 	 Alliance)

SD	 	 substantial damage

SEI	 	 Structural Engineering Institute

SFHA	 	 Special Flood Hazard Area

SI	 	 substantial improvement

STP	 	 Standard Torque Probe

T

TVA	 	 Tennessee Valley Authority
E- PROTECTING Manufactured Homes FROM FloodS AND OTHER HazardS

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E Acronyms and Abbreviations

U

USACE	 U.S. Army Corps of Engineers

U.S.C.		 United States Code

USDA	 	 United States Department of Agriculture

USGS	 	 United States Geological Survey

W

WF	 	 wood framed

WHAFIS	 Wave Height Analysis for Flood Insurance Studies

WSPRO	 water surface profile computation module
F.	 Example Calculations

Design a CMU pier and ground anchor foundation for a manufactured home to be placed in 

an SFHA Zone AE having a flood velocity of 2 fps. The BFE is 9 feet and existing ground elevation is approximately 7 feet. The flood depth is 2 feet and the freeboard is 1 foot, which yields 

a DFE depth of 3 feet. The manufactured home dimensions for these example calculations are 

shown in Figure F-1. The manufactured home is a single unit, 16 feet wide and 60 feet long 

with a 30-degree gable roof with a 1-foot overhang. Roofing members are spaced 16 inches on 

center (o.c.). The manufactured home weighs 20 psf. Assume an NFPA 5000 soil classification 

of soft, sandy clay, or clay (allowable bearing pressure qa

 =1,000 psf ; ultimate bearing pressure 

qu = 2,000 psf). Use ASCE 7 to calculate loads. 

Foundation loads selected for this example of a manufactured home in an SFHA differ from 

those that may be found in HUD standard 24 CFR 3280. Design loads in this example are in accordance with ASCE 7-05 and other standards.

These example calculations assume transverse wind loads produce the controlling loading. 

Wind in the direction parallel to the roof ridge may produce greater loads for certain cases and 

must be evaluated during final design.

Figure F-1. Manufactured

home dimensions.

ASCE 7-05 

PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards

A Multi-Hazard Foundation and Installation Guide

F-1
F EXAMPLE CALCULATIONS

Step 1: 	 Determine Design Criteria

NORMAL LOADS

Dead Load (D)

D = 20 psf Given in the example statement

Live Load (L)

L is based on one- and two-family dwellings

L = 40 psf

Roof Live Load (Lr)	

Lr = 20R1R2 =20(1)(0.85)=17 psf 

R1

 = 1 for At ≤ 200 ft2

At = 2(9.2 ft)(16 in) =24.5 ft2

F = number of inches of rise per foot

F = 1ft tan 30 ̊ = 7 in

Note that the roof live load falls between the limits given: 

12 ≤ Lr ≤ 20

ENVIRONMENTAL LOADS

Wind Loading

Structure is a regular shape, located in a windborne debris region with 

terrain classification of Exposure C and surrounded by flat terrain.

Mean roof height (h)

h = 3 ft + 10 ft + 0.5(4 ft)

= 15 ft

h < 16 ft (least horizontal dimension)

Calculations are for a foundation system, which is a main wind force resisting system (MWFRS).

Velocity Pressures

Velocity pressures are determined using 

Method 2: Analytical Procedure

ASCE 7-05 

Table 4-1

ASCE 7-05

Section 6.2

Section 6.5

1 ft

12 in ( )

12 in

1 ft ( )

F-2 PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards

A Multi-Hazard Foundation and Installation Guide
EXAMPLE CALCULATIONS F

(A simplified alternative is to use ASCE 7, Section 6.4, Method 1. Wind 

pressures are tabulated for basic conditions. The wind pressure must be 

adjusted for mean roof height and exposure category.) 

Velocity Pressure Coefficient (qz

)

qz

 = 0.00256KzKztKdV2I

Velocity pressure exposure coefficient evaluated at height z (the 

height above ground level in feet) (Kz

)

Kz

= 0.85

Topographical factor (Kzt)

Kzt=1 (assume a flat surface)

Wind directionality factor (Kd)

Kd = 0.85

Basic Wind Speed (V)

V = 110 mph (3-second gust)

I = 1 (Category II building: Table 1-1 (ASCE 7)) 

Therefore, qz

 = 0.00256(0.85)(1)(0.85)(110)2(1)

= 23 psf

Design Pressures for MWFRS

Internal Pressure Coefficient (GCpi) 

GCpi = ± 0.18 

External Pressure Coefficient (Cp) 

h = Mean roof height, in feet

L = Horizontal dimension of building, in feet, measured parallel to wind direction

B = Horizontal dimension of building, in feet, measured normal to wind direction

Table F-1 shows the External Pressure Coefficients calculated for the 

windward, leeward and side walls. Computations of the External Pressure Coefficients for the windward and leeward roof are shown Table 

F-2. 

Section 6.5.10

Eq. 6-15

Section 6.5.6 

Table 6-3

Section 6.5.6

Section 6.5.4.4 

Section 6.5.5

Table 6-1

Section 6.5.4

Section 6.5.11.1

Figure 6-5

Section 6.5.11.2.1

Figure 6-6

Figure 6-6

PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards

A Multi-Hazard Foundation and Installation Guide

F-3
F EXAMPLE CALCULATIONS

Table F-1. External Wall Pressure Coefficients 

Surface Wind Direction L/B Cp

Windward Wall n/a n/a 0.8

Leeward Wall Perpendicular to 

roof ridge -0.5

Side Wall n/a n/a -0.7

Table F-2. External Roof Pressure Coefficients 

Surface Wind Direction h/L Cp

Windward Roof

Perpendicular to 

roof ridge

-0.3

0.2

Leeward Roof -0.6

Foundation systems are considered rigid, therefore, G = 0.85.

Design Pressure (p)

The basic pressure equation (ASCE 7 6-17), which includes the internal 

pressure coefficient is as follows:

p = qGCp – qi

(GCpi) 

However, this would only be used if designing individual components 

whose effective tributary area is equal to or greater than 700 sf (ASCE 

7-05 6.5.12.1.3 and IBC 2006 1607.11.2.1). When determining loads on 

the global structure (i.e., shear walls or foundation design), the internal pressure components will act in equal and opposite directions on 

the roof/floor and the leeward/windward walls, thereby algebraically canceling each other. The resulting simplified form of the pressure 

equation is:

p = q x GCp

Table F-3 summarizes the design pressures calculated using this simplified wind design pressure equation. Figure F-2 shows the application of 

these design pressures on the structure. For foundation design, internal 

pressures need not be considered since internal pressure on windward 

walls, leeward walls, floors, and roofs cancel each other. For example, 

internal pressures acting on a windward wall are equal and opposite to 

those acting on a leeward wall and the net force on the foundation from 

internal pressures is zero. 

15 ft

16 ft = 0.94

16 ft

60 ft = 0.27

Figure 6-6

Figure 6-6

Section 6.5.8

Eq. 6-17

F-4 PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards

A Multi-Hazard Foundation and Installation Guide
EXAMPLE CALCULATIONS F

While internal pressures cancel, internal pressures for a partially enclosed building have been included in the example. This is to provide 

an example of more general wind load calculations. 

Table F-3. Design Pressures for Wind Perpendicular to the Roof Ridge

Surface Design Wind Pressure

Calculations

pressure

(psf)

Windward Wall p = 23 psf(0.85)(0.8) 15.7

Leeward Wall p = 23 psf(0.85)(-0.5) -9.8

Side Walls p = 23 psf(0.85)(-0.7) -13.7

Windward Roof

p = 23 psf(0.85)(-0.3) -5.9

p = 23 psf(0.85)(0.2) 4.0

Leeward Roof p = 23 psf(0.85)(-0.6) -11.8

MWFRS Roof Overhang Pressures

ASCE 7 only addresses the windward overhang, specifying the use of a 

positive pressure coefficient of Cp = 0.8. Acting on the bottom surface of 

the overhang in combination with pressures acting on the top surface. 

For the leeward overhang, the coefficient for the leeward wall (Cp = -0.5) 

could be used, but the coefficient has been conservatively taken as zero.

p = 23 psf(0.85)(0.8)

= 15.7 psf

pOH = -5.9 psf – 15.7 psf = -21.6 psf

A conservative simplification is to use the wind pressure acting away from 

the roof case for uplift roof pressure simultaneously with the wind pressure toward the roof for the lateral roof pressure.

ASCE 7-05

Section 6.5.11.4.1

PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards

A Multi-Hazard Foundation and Installation Guide

F-5
F EXAMPLE CALCULATIONS

Figure F-2. Maximum

uplift and lateral wind

loads on roof.

SNOW LOADING

Ground Snow Load (pg)

pg = 20 psf

Flat Roof Snow Load (pf)

pf = 0.7Ce

CtIpg

pf = 0.7(1.0)(1.0)(1.0)(20) 

= 14 psf

But not less than pf =(I)pg = 20 psf

ASCE 7-05

Section 7.2

Figure 7-1

Section 7.3

Eq. 7-1

F-6 PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards

A Multi-Hazard Foundation and Installation Guide
EXAMPLE CALCULATIONS F

Section 7.3.1

Table 7-2

Section 7.3.2

Table 7-3

Section 7.3.3

Table 7-4

Section 7.4

Eq. 7-2

Section 7.4.1

Figure 7-2

Section 7.6

Section 7.6.1

Figure 7-9

Eq. 7-3

PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards

A Multi-Hazard Foundation and Installation Guide

F-7

Exposure Coefficient (Ce)

Ce = 1.0 (partially exposed roof)

Thermal Factor (Ct)

Ct = 1.0

Importance Factor (I)

I = 1.0 (Category II building: Table 7-4 (ASCE 7))

Sloped Roof Snow Load (ps)

ps = Cs

pf

= (1.0)(20 psf)

= 20 psf

Warm Roof Slope Factor (Cs)

Cs = 1.0 (asphalt shingle not slippery)

Unbalanced Roof Snow Load (pu)

Since the roof’s eave to ridge distance ≤ 20 ft, unbalanced uniform snow 

loads shall be applied as follows:

Pwindward= 0.3 ps

= 6 psf

Pleeward.1 = ps

= 20 psf

Pleeward.2 = (hd)(γ)/√(S)

= (1.44 ft)(16.6 pcf)/√(1.73)

= 18.2 psf

From the ridge toward the leeward eave a distance of:

x = (8/3)(hd)√(S)

= 5.1 ft

hd = 1.44 ft

γ = 0.13 pg + 14 ≤ 30 pcf

= 16.6 pcf
F EXAMPLE CALCULATIONS

FLOOD LOADING

Hydrostatic Load (Fh)

If the manufactured home is elevated above the BFE on 

an enclosed foundation, venting must be provided in all 

manufactured homes placed in a SFHA; the hydrostatic 

forces on either side of the foundation wall will cancel. 

However, the hydrostatic load is calculated because it is 

used in the hydrodynamic load calculation.

Fh

 = 1⁄2 PhH

Hydrostatic Pressure (Ph

)

Ph

 = γH

Specific Weight of Fresh Water (ω)

γ = 62.4 pcf

Floodproofing Design Depth (H)

H = 2 ft (base flood depth) + 1 ft

Hydrodynamic Load 

The hydrodynamic load is calculated by converting it to an equivalent 

hydrostatic load by increasing the flood depth. The increase in flood 

depth is referred to as dh.

Drag Coefficient (Cd)

In the above equation, a value of 2.0 was assumed 

for Cd. This is a conservative estimate; the actual 

value for Cd could be anywhere between 1.2 and 

2.0.

Acceleration Due to Gravity (g)

g = 32.2 ft/s2

with a hydrodynamic pressure of

Phydr = γ(dh

) = 62.4 pcf (0.13 ft) = 8.2 psf 

The equivalent hydrostatic load (Fh/ad) taken into consideration the hydrodynamic load is :

Fh/ad = Phydr x H = 8.2 psf x3 ft = 24.6 plf

ASCE 7

CdV Eq. 7-5 2

 2g

dh = = = 0.13 ft 2.0(2 ft/s)2

2(32.2 ft/s) 

FEMA’s Coastal Construction Manual (FEMA 55) recommends a value 

of 2.0 for square or rectangular piles 

and 1.2 for round piles.

For additional guidance regarding 

drag coefficients, refer to Volume II 

of the U.S. Army Corps of Engineers’ 

Shore Protection Manual (USACE 

1984), FEMA 55, and the Engineering 

Principles and Practices for Retrofitting Flood-Prone Residential Structures (FEMA 259).

Note: A 1-foot freeboard is added 

to the BFE depth to provide a 

protection above the BFE; 3 feet 

becomes the “design” depth or the 

DFE.

F-8 PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards

A Multi-Hazard Foundation and Installation Guide
EXAMPLE CALCULATIONS F

Since piers are 16 inches wide, the total hydrodynamic force on 

the pier is

CHECK SCOUR

Reference: Publication No. FHWA NHI 00-001, Evaluating Scour at Bridges, 4th Edition, May 2001, 

Hydraulic Engineering Circular No. 18.

Ys = 2.0 x (K1

)x (K2) x(K3) x(K4) x (a/Y1)0.65 x Fr1

0.43

Y1

Where: Y = Scour depth

Y1 = Flow depth directly upstream of pier

a = Pier width (ft.)

L = Pier length (ft)

Fr1 = Froude number

 Fr1= V1/(gY1)1/2

 Where V1 = Mean velocity of flow directly upstream of pier

g = acceleration due to gravity (32.2 feet/sec2)

lb

ft

= 24.6 16 in 32 lbs per pier 1 ft

12 in

L = 16"

a = 8"

L = 16"

PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards

A Multi-Hazard Foundation and Installation Guide

F-9
F EXAMPLE CALCULATIONS

K1 = Factor for pier nose shape. For square nose 

K2 = Factor for Angle of attack . K2 = (cosine φ + (L/a) x sine φ)0.65

K3 = Factor for bed condition/. K3 = 1.1

K4 = Factor for armoring by bed material size. 

Project parameters:

Flood low = 2 fps

Flood depth = 3 ft

Assume flood angle of attack = 0o

So that:

K1 = 1.1 (Table 6.1)

K2 = [cosine 0o +(L/a)x sine 00]0.65 = [1.00 + (1.33’/0.67”) x 0]0.65 = 1.00

K3 = Factor for bed condition/. K3 = 1.1 (Table 6.3)

K4 = Factor for armoring by bed material size. K4 = 1.0 unarmored

Fr1 = V1/(gY1)1/2 = 2/[32.2 x 3]1/2 =2/9.84 = 0.203

And

Ys

 = (2) x (1.1) x (1.0) x (1.1) x (1.0) x (0.67/2)0.65 x (0.203)0.43

Y1

Ys

 = (2.42) x (0.491) x (.504) = 0.6 Y1

Ys = (0.6) x (Y1) = (0.6)x(3) = 1.8 ft

Scour protection or increased footing embedment required.

F-10 PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards

A Multi-Hazard Foundation and Installation Guide
EXAMPLE CALCULATIONS F

Step 2:	 Select a Design Methodology and Assess Load

Combinations and Failure Modes

Figure F-3 illustrates the loads applied to the manufactured home. Table F-4 lists the nomenclature of the applied loads shown in Figure F-3.

Table F-4. Load Nomenclature

Nomenclature Load Description

D dead load

L live load

LR roof live load

RH horizontal reaction

RLV leeward vertical reaction

RWV windward vertical reaction

SB balanced snow load

WH horizontal wall wind pressure

WRH roof horizontal wind pressure

Figure F-3. Loading on a

manufactured home.

PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards

A Multi-Hazard Foundation and Installation Guide

F-11
F EXAMPLE CALCULATIONS

Nomenclature Load Description

WLRV leeward roof vertical wind pressure

WWRV windward roof vertical wind pressure

Note that snow load governs over roof live load and wind downward load, and wind lateral 

load governs over earthquake lateral load. Load combinations for non-governing cases are not 

shown.

For the purposes of these calculations, the worst case wind load is taken to be perpendicular to 

the roof ridge for all failure modes. Wind in the direction parallel to the roof ridge may produce greater loads for certain failure modes.

Uplift and Downward Failure Mode

Uplift failure is a vertical force phenomenon. The loads that act vertically are wind, snow, dead, 

and live loads. Table F-5 summarizes the loads that influence uplift and downward failure mode. 

Table F-6 assesses uplift and downward failure load combinations. Note that uplift is based on 

MWFRS pressures for the global foundation design. Design of the connections to the foundation may require components and cladding (C&C) pressures to be used.

Table F-5. Vertical Load Values

Load

Type Total load acting on the structure and, therefore, must be supported by the foundation

D D = [dead load per square foot][width of the manufactured home]

D = [20 psf][16 ft]

D = 320 lbs per linear ft of manufactured home length

L L = [live load per square foot][width of the manufactured home]

L = [40 psf][16 ft]

L = 640 lbs per linear ft of manufactured home length

Lr Lr

 = [roof live load per square foot][width of the manufactured home]

Lr

 = [17 psf][18 ft]

Lr

 = 306 lbs per linear ft of manufactured home length

Table F-4. Load Nomenclature (continued)

F-12 PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards

A Multi-Hazard Foundation and Installation Guide
EXAMPLE CALCULATIONS F

Load

Type Total load acting on the structure and, therefore, must be supported by the foundation

W Maximum wind uplift loads occur for winds parallel to the roof ridge at the windward end.

W = WWRV+WLRV = [(vertical component roof wind pressures)(area roof)]/manufactured home length 

W = [-17.6 psf][(9 ft)(15 ft)(2)] 0 ft to 15 ft + 

[-9.8 psf][(9 ft)(15 ft)(2)] 15 ft to 30 ft + 

[-5.9 psf][(9 ft)(30 ft)(2)] 30 ft to 60 ft

W = -10,584 lbs/60 ft = -176 lbs per linear foot of manufactured home length (average)

In this case, vertical uplift loads are low and so this simplification is acceptable. However, to 

account for the unbalanced uplift if wind loads were higher, either overturning in this direction 

would need to be considered, or the windward uplifts conservatively made symmetrical about the 

middle. 

Maximum wind downward loads occur for wind perpendicular to the roof ridge; however, they are 

much less than, and do not govern over, roof live or snow loads.

S S = [snow pressure][horizontal projected roof area]

S = [20 psf][(9 ft)]SW + [20 psf][(9 ft)]SL

S = 360 lbs per linear ft of manufactured home length

Table F-6. Vertical Failure Mode ASD Load Combinations

Load Combinations

4 D + 0.75L + 0.75S

320 lbs + 0.75(640 lbs) + 0.75(360 lbs) = 1,070 lbs per linear ft of manufactured home length

7 0.6D + W

0.6(320 lbs) - 176 lbs = 16 lbs per linear ft of manufactured home length acting downward

Note that, for load combination 7, the 0.6 load factor should be applied to the dead load that 

would actually be present over the whole structure. Additions to the dead load tabulation such 

as mechanical and miscellaneous or shingles should not be included in this value as they may 

not be present in all areas or during a high-wind event and their inclusion would not be conservative.

Sliding or Shearing Failure Mode

Sliding failure is a lateral force phenomenon. The loads that act laterally are wind and flood 

loads. Table F-7 summarizes the lateral loads and their values. Maximum lateral wind loads occur when the wind is perpendicular to the roof ridge. Note that lateral wind loads act on the 

overall structure (i.e., foundation), whereas flood loads act on the individual piers. Table F-8 

gives the load combinations for sliding failure. Once the number of piers is defined, the hydrodynamic forces on these piers are to be added to load combination 4, and the foundation 

design will have to be checked to make sure it can resist the added hydrodynamic loads. 

Table F-5. Vertical Load Values (continued)

PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards

A Multi-Hazard Foundation and Installation Guide

F-13
F EXAMPLE CALCULATIONS

Table F-7. Lateral Load Values

Load

Type Total load acting on the structure and, therefore, must be supported by the foundation

W Maximum lateral wind loads occur for winds ┴ to the roof ridge

 W = WRH + WH =

[lateral roof pressures][roof height] + [wall pressures][wall height]

W = [4 psf – (-11.8 psf)] (4.7 ft) 

[(15.7 psf + 9.8 psf)(10 ft)] 

W = 329.3 lbs per linear ft of manufactured home length

Fa Hydrodynamic load per pier

Fa

 = [hydrodynamic force][pier length]

Fa

 = 32.8 lbs per pier

Assume total of 9 piers x 2 rows for 1st iteration

Fa

 = (32.8 lbs per pier)(9 piers per row)(2 rows)

Fa

 = 590.4 lbs / 60 ft = 9.84 lbs per linear ft of manufactured home length

Table F-8. Sliding Load Combinations

Load Combinations

5 W + 1.5Fa

329.3 lbs + 1.5(9.84 lbs) = 344 lbs per linear ft of manufactured home length

Note: The vertical gravity loads are not considered to be conservative. Thus, the frictional resistance of the footings under the piers has been neglected. This component may be used in 

borderline situations at the discretion of the engineer.

Overturning Failure Mode

Overturning failure results from loads that act on the whole structure and pivot about the bottom of the leeward pier. Dead, live, wind, and snow loads can all influence the overturning 

moment. Table F-9 summarizes the moments that affect overturning due to wind in this case. 

Table F-10 assesses the moment load combinations. Only the portions of the roof and floor 

live loads that are over the part that cantilevers out past the leeward pier will contribute to the 

overturning. Since this is the worst overturning case for each, only these conditions will be calculated.

F-14 PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards

A Multi-Hazard Foundation and Installation Guide
EXAMPLE CALCULATIONS F

Table F-9. Moment Load Values

Moment

Type

Total moment about the bottom of the leeward foundation support

(positive moment is counter clockwise)

D D = [dead load per square foot][home width][moment arm]

D = [20 psf][(16 ft)(4 ft)]

D = +1,280 ft-lbs per linear ft of manufactured home length

L L1

 = [live load per square foot][home width][moment arm]

L1

 = [40 psf][(16 ft)(4 ft)]

L1

 = 2,560 ft-lbs per linear ft of manufactured home length

L2

 = [live load per square foot][cantilever width][moment arm]

L2

 = [40psf][4 ft][-1 ft]

L2

 = -160 ft lbs per linear ft of manufactured home length

Lr Lr

 = [roof live load per square foot][roof width][moment arm]

Lr

 = [17 psf][4 ft][1 ft]

Lr

 = -68 ft-lbs per linear ft of manufactured home length

W WIND PERPENDICULAR TO THE ROOF RIDGE

WWRV = [vertical component roof wind pressures][roof width][moment arm]

WWRV (-21.6 psf)(1 ft)(12.5 ft) + (-5.9 psf)(8 ft)(8 ft)

WWRV = -648 ft-lbs per linear ft of manufactured home length

WLRV = [vertical component roof wind pressures][roof width][moment arm]

WLRV (-11.8 psf)(9 ft)(-0.5 ft)

WLRV = +53 ft-lbs per linear ft of manufactured home length

WRH = [horizontal component roof wind pressures][roof height][moment arm]

WRH = [4 psf – (-11.8 psf)](-4.67 ft)](15.3 ft)

WRH = -1,129 ft-lbs per linear ft of manufactured home length

WW+L = [windward wall pressure + leeward wall pressure][home’s height from ground to roof 

eave][moment arm]

WW+L = [15.7 psf + 9.8 psf](10 ft)(-8 ft)

WW+L = -2,040 ft-lbs per linear ft of manufactured home length

Fa Hydrodynamic load on piers

Fa

 = [horizontal component][moment arm]

 = (9.84 plf)(-3 ft/2) = -15 ft-lbs per linear foot of manufactured home length

S SB = [balanced snow pressure][horizontal projected roof area][moment arm]

SB = [20 psf][18 ft][-4 ft]

SB = 1,440 ft-lbs per linear ft of manufactured home length

PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards

A Multi-Hazard Foundation and Installation Guide

F-15
F EXAMPLE CALCULATIONS

Table F-10. Overturning Load Combinations

Moment Load Combinations (positive moment is counter clockwise)

6 D + 0.75W + 0.75L + 0.75Lr + 1.5Fa (Partial live loading to produce max OT)

(1,280 ft-lbs) + (0.75)(-648 ft-lbs + 53 ft-lbs – 1,129 ft-lbs – 2,040 ft-lbs) + (0.75)(-160 ft-lbs)+ 

(0.75)

(-68 ft-lbs) + (1.5)(-15 ft-lbs) = -1,737 ft-lbs per linear ft of manufactured home length

D + 0.75W + 0.75L + 0.75S + 1.5Fa

 (Full live and snow to produce max downward reaction)

(1,280 ft-lbs) + (0.75)(-648 ft-lbs + 53 ft-lbs – 1,129 ft-lbs – 2,040 ft-lbs) + (0.75)(-2,560 ft-lbs) + 

(0.75)(1,440 ft-lbs) + (1.5)(-15 ft-lbs) = 1,435 ft-lbs per linear ft of manufactured home length

7 0.6D + W + 1.5Fa

(0.6)(1,280 ft-lbs) + (-648 ft-lbs + 53 ft-lbs – 1,129 ft-lbs – 2,040 ft-lbs) + (1.5)(-15 ft-lbs) 

= -3,019 ft-lbs per linear ft of manufactured home length

Table F-11 summarizes the load combinations that govern for each of the three failure modes. 

The maximum roof vertical and lateral load cases are assumed to act simultaneously as a conservative simplification.

Table F-11. ASD Load Combinations for Example Problem (loads are in pounds per linear foot)

Failure

Modes

Load Combinations

4 5 7

Uplift 1,070 lbs n/a1 15 lbs

Sliding n/a1 313 lbs n/a1

Overturning n/a1 n/a1 -979 ft-lbs

1

 Load combination does not govern.

Load combinations 1-3 do not govern. Load combination 6 does not comply with HUD 24 CFR 3280.

Step 3: 	 Select Foundation Type and Materials

The example statement specified a CMU pier and ground anchor foundation type. Since the 

flood velocity is 2 fps, CMU piers must have surface bonded mortar that meets ASTM C887-79a 

(2001) and ASTM C946-91 (2001) and maintain bonding between CMUs. 

Step 4: 	 Determine Forces at Connections and on Foundation

Components

CMU piers transfer the compressive loads from the manufactured home into footings and 

then into the ground. The masonry piers are not considered to provide any lateral or uplift 

F-16 PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards

A Multi-Hazard Foundation and Installation Guide
EXAMPLE CALCULATIONS F

resistance. The governing load combination for downward forces is the vertical failure mode 

(load combination 4), which produces a total downward force from the manufactured home 

equal to

(downward force)(length of manufactured home)

Therefore, (1,070 lbs)(60 ft) = 64,200 lbs

This downward force governs the number of footings and, therefore, piers needed to transfer 

the downward load into the ground.

Following the braced masonry pre-engineered foundation design for flood velocities over 2 fps 

specification given in Chapter 10 of the use of a dry-stack 16-inch by 8-inch block pier with a 

minimum of an 1/4-inch thick surface bonded mortar and a 24-inch square, 10-inch deep footing, calculate the number of footings needed to adequately transfer the downward loads to the 

ground.

Required footing area = 

Consult the geotechnical engineer for the ultimate soil bearing capacity value. An approximate 

method to calculate the ultimate soil bearing capacity is to multiply the allowable soil bearing 

capacity by a safety factor. The maximum pressure given in the NFPA 5000 Soil Classification 

Table can also be used as the ultimate soil bearing capacity.

Required footing area = = 64 ft2

Individual footing area = (24 in x 24 in) = 4 ft2

Number of footings =

= = 16 footings/piers 

Therefore, provide 8 piers per side of the manufactured home

Pier spacing =

= = 8.6 ft 

The maximum spacing of the piers is set to 8 feet to provide effective floodborne debris protection. To protect against floodborne debris, it is assumed that 1 pier will be lost due to floodborne 

debris. 

Minimum number of piers = +1= 8.5 piers, say 9 piers per side (i.e., 8 spaces at 7.5 

feet). Therefore, the home will be supported by a total of 18 piers (9 piers on each side) 

spaced at 7.5 feet.

comprehensive load

allowable soil bearing capacity

64,200 lbs

1,000 psf

1 ft2

144 in ( )2

total required footing area

individual footing area

64 ft2

4 ft2

manufactured home length

(number piers per side-1)

60 ft

8-1

60 ft2

8 ft

PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards

A Multi-Hazard Foundation and Installation Guide

F-17
F EXAMPLE CALCULATIONS

Lateral wind loads are resisted by the strapping and ground anchors. The final number of piers 

equals the initial guess; therefore, the lateral load on the piers does not have to be updated.

Calculate the number of anchors needed to resist sliding failure. 

The recommended design stiffness of the anchors in Table 7-5 in this guide is given for 5-foot 

anchors installed at 45 degrees and axially loaded is 1,200 lb/in (Figure 7-4). The horizontal 

component of the ground anchors strength is equal to

(1,200 lbs/in)(cos 45°) = 848 lb/in 

The manufactured home industry gives an allowable lateral manufactured home movement of 

3 inches. So the total lateral strength of a ground anchor is (3 in)(848 lbs) = 2,544 lbs.

Number of ground anchors needed = 

Number of ground anchors needed = = 8 anchors per side 

Calculate ground anchor spacing = = 8.5 ft

The anchor strapping should attach into a wall stud; therefore, anchor spacing must be adjusted 

to 16-inch increments. 

Both uplift and overturning failure modes are resisted by the vertical strength of ground anchors. The uplift forces will be resisted by all the ground anchors and the overturning moment 

will be resisted only by the windward ground anchors.

For the worst uplift of the vertical failure mode, load combination 7 (refer to Table F-6) governs. 

However, the maximum net uplift is 16 plf downward, which means that overturning will govern 

the uplift requirements.

For the overturning failure mode, load combination 7 (refer to Table F-10) governs for wind 

perpendicular to the roof ridge. Overturning moment is only resisted by the windward anchors. 

Therefore, the total vertical load each anchor will have to resist is

= 612 lbs per anchor

The vertical component of the anchor stiffness equals

(1,200 lbs/in)(cos 45) = 848 lbs/in 

The manufactured home industry gives an allowable vertical movement of 2 inches. This results 

in a vertical strength per anchor equal to

(2 in)(848 lbs/in) = 1,697 lbs per anchor 

60 ft

(8-1)

(979 ft-lbs)(60 ft)

12 ft

8 anchors

(overturning moment)(length of home)

moment arm

number of anchors per side =

lateral load

anchors lateral capacity

(313 lb)(60 ft)

2.544 lbs

F-18 PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards

A Multi-Hazard Foundation and Installation Guide
EXAMPLE CALCULATIONS F

This is more than the strength needed by each anchor to resist the overturning moment. 

The anchor strapping should attach into a wall stud and, therefore, anchor spacing must be adjusted to 16-inch increments. Place anchors at each end of the home and space at 72 inches.

For the overturning case, the connection of the straps to the stud and the ground anchor embedment is based on MWFRS pressures. However, although it would likely not govern, to be 

thorough, the uplift only condition using C&C pressures should be checked for these two anchorages (straps to studs and anchors in ground).

Step 5: 	 Specify Connections and Framing Methods Along with

Component Dimensions to Satisfy Load Conditions

The CMU pier and ground anchor foundation will consist of 16 dry-stack, 16-inch by 8-inch 

block piers with a minimum of a 1/4-inch thick surface bonded mortar and 24-inch square, 

10-inch deep footings. Ground anchors will be placed at 45-degree angles at each end of the 

manufactured home and spaced at 72 inches. 

Step 6: 	 Note All Design Assumptions and Details on Drawings

Refer to pre-engineering foundation design drawings contained in Appendix H and specifications presented in Chapter 10 herein as to how to adequately document assumptions and detail 

drawings.

PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards

A Multi-Hazard Foundation and Installation Guide

F-19

G.	 Wind Zone Comparisons

(HUD's MHCSS and

FEMA 85)

G.1	 Background

HUD’s Manufactured Home Construction and Safety Standards (MHCSS; 24 CFR 3280) and its Model 

Manufactured Home Installation Standards (24 CFR 3285) categorize the United States and many 

of its territories and protectorates into three wind zones: Zone I, Zone II, and the highest wind 

zone, Zone III. 

FEMA 85, NFPA 225 Model Manufactured Home Installation Standard, and NFPA 501 Standard on 

Manufactured Housing categorize the U.S. into four wind zones: Zone I, Zone II, Zone III, and 

the highest wind zone, Zone IV. However, to avoid confusion, in this appendix Zones 1, 2, 3, and 

4 will be used to distinguish the NFPA zones from the HUD zones.

This appendix explains the differences between the wind provisions contained in the HUD standards and those contained in FEMA 85 and the NFPA standards.

G.2	 Wind Provisions

The HUD standards, FEMA 85, and the NFPA standards all contain wind provisions based on 

ASCE 7, Minimum Design Loads for Buildings and Other Structures; however, they are not all based 

on the same edition of ASCE 7. HUD documents are based on a modified version of the 1988 

edition of ASCE 7. FEMA 85 and the NFPA standards are based on more recent editions. The 

wind loads used to design FEMA 85 foundations comply with ASCE 7-02; the NFPA standards 

reference ASCE 7-05. 

Only minor differences exist between the wind provisions of ASCE 7-05 and ASCE 7-02; however, 

the wind provisions of both ASCE 7-05 and ASCE 7-02 differ greatly from the wind provisions 

of ASCE 7-88.

Two major differences exist between ASCE 7-88 and ASCE 7-05. The basic wind speed map (Figure 6-1) in ASCE 7-88 was completely redrawn in 1995 and revised in 1998. The newer maps 

show high-wind zones extending much farther inland than those shown on the 1988 map. Also, 

the basic wind speeds are based on different averaging times. The 1988 edition of ASCE 7 uses 

PROTECTING Manufactured Homes FROM FloodS AND OTHER HazardS

A Multi-Hazard Foundation and Installation Guide

G-1
g WIND ZONE COMPARISONS (HUD'S MHCSS AND FEMA 85)

fastest mile wind speeds; ASCE 7-02 and 7-05 use 3-second gust wind speeds. The wind speeds in 

the older standard are numerically less than those in the contemporary standards. For example, 

ASCE 7-88 required buildings in the interior portion of the U.S. to be designed for a 70-mph 

(fastest mile) wind speed; ASCE 7-02 and 7-05 place the interior portions of the U.S. in a 90-mph 

(3-second gust) wind speed area. While the wind speeds are numerically greater, the design wind 

loads are often not greater and, in some areas, the wind loads are less.

G.3	 Wind Zones

HUD regulates wind zones as:

HUD Zone II: Areas where the basic wind speed is 100 mph (fastest mile) 

HUD Zone III: Areas where the basic wind speed is 110 mph (fastest mile) 

HUD Zone I includes all areas not identified as either HUD Zone II or HUD Zone III and is not 

associated with a specific wind speed.

FEMA 85 and the NFPA Codes Zones identify wind zones as: 

Zone 1: Areas bounded by the 90-mph isotach1

 in ASCE 7-02 (ASCE 7-05 for the NFPA codes)

Zone 2: Areas between the 90-mph and the 110-mph isotachs

Zone 3: Areas between the 110-mph and the 130-mph isotachs

Zone 4: Areas between the 130-mph and the 150-mph isotachs

Figure G-1 shows the HUD wind speed map contained in 24 CFR 3280. Figure G-2 shows the 

wind speed map used for FEMA 85 and the NFPA standards. 

Note that the 2006 IBC Section §1609.3.1 shows the relationship between fastest mile and 3-second gust wind speeds.

Vfm = (V3 sec – 10.5)/1.05

Where: Vfm = Fastest mile wind speed (mph)

 V3 sec = 3-second gust wind speed (mph)

1	 An isotach is a line in a given surface connecting points with equal wind speeds.

G-2 PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards

A Multi-Hazard Foundation and Installation Guide
WIND ZONE COMPARISONS (HUD'S MHCSS AND FEMA 85) G

Figure G-1. Basic wind zone map for manufactured housing.

PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards 

A Multi-Hazard Foundation and Installation Guide

G-3
g WIND ZONE COMPARISONS (HUD'S MHCSS AND FEMA 85)

Figure G-2. NFPA 501 wind speed map.

G-4 PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards

A Multi-Hazard Foundation and Installation Guide
WIND ZONE COMPARISONS (HUD'S MHCSS AND FEMA 85) G

G.4	 Wind Speeds and County Lists

Wind speeds for communities are typically determined when building codes are adopted. The 

wind speeds should agree with those specified by ASCE 7, but are often decided upon by local 

officials particularly in large municipalities where several wind speeds apply. 

For manufactured homes, wind zones are specified by the MHCSS and are listed by State and 

County. The MHCSS lists Counties contained in Wind Zones II and III. By default, all Counties 

not listed as either in Zone II or III are in HUD Zone I.

The following list identifies Counties by State for the various wind zones used in FEMA 85. 

Counties that are not listed or that are within the 90-mph isotach are in Zone 1.

Zone 4 (150 mph)

Florida: Broward, Martin, Miami-Dade, Monroe, Palm Beach

Louisiana: Parishes of Cameron, Lafourche, Plaquemines, St. Bernard, St. Mary, Terrebonne, 

Vermillion

Mississippi: Jackson

North Carolina: Brunswick, Carteret

Texas: Cameron

The U.S. Territories of Guam, Northern Mariana Islands, Puerto Rico, Trust Territory of the Pacific Islands, and the United States Virgin Islands 

Zone 3 (130 mph)

American Samoa

Alabama: Baldwin, Mobile

Alaska: The coastal regions seaward of the 110-mph isotach on the ASCE 7 map

Connecticut: Middlesex, New London

Florida: Bay, Brevard, Calhoun, Charlotte, Citrus, Collier, DeSoto, Dixie, Duval, Escambia, Flagler, Gulf, Hendry, Hernando, Hillsborough, Holmes, Indian River, Jackson, Lee, Levy, Liberty, 

Manatee, Nassau, Okaloosa, Okeechobee, Pasco, Pinellas, Santa Rosa, Sarasota, St. Johns, St. Lucie, Taylor, Volusia, Wakulla, Walton, Washington

Georgia: Bryan, Camden, Chatham, Glynn, Liberty, McIntosh

PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards 

A Multi-Hazard Foundation and Installation Guide

G-5
g WIND ZONE COMPARISONS (HUD'S MHCSS AND FEMA 85)

Louisiana: Parishes of Assumption, Iberia, Lafayette, Orleans, St. Charles, St. James, St. John the 

Baptist, St. Martin, St. Tammany

Maryland: Somerset, Worchester

Massachusetts: Barnstable, Bristol, Dukes, Nantucket, Plymouth

Mississippi: George, Greene, Hancock, Harrison, Pearl River, Perry, Stone

New Jersey: Atlantic, Cape May, Ocean

New York: Nassau, Suffolk

North Carolina: Beaufort, Camden, Chowan, Columbus, Craven, Currituck, Dare, Hyde, Jones, 

New Hanover, Onslow, Pamlico, Pasquotank, Pender, Perquimans, Tyrrell, Washington

Rhode Island: Bristol, Kent, Newport, Washington

South Carolina: Beaufort, Charleston, Georgetown, Horry 

Texas: Brazoria, Calhoun, Chambers, Galveston, Jefferson, Kenedy, Kleberg, Matagorda, Nueces, Refugio, San Patricio, Willacy

Virginia: Accomack, Northampton

Zone 2 (110 mph)

Alabama: Autauga, Barbour, Bibb, Bullock, Butler, Chambers, Chilton, Choctaw, Coffee, Conecuh, Coosa, Covington, Crenshaw, Dale, Dallas, Elmore, Escambia, Macon, Montgomery, Perry, 

Pike, Russell, Sumter, Tallapoosa, Washington, Wilcox

Connecticut: Fairfield, Hartford, Litchfield, New Haven, Tolland, Windham

Delaware: Kent, Sussex

Florida: Alachua, Baker, Bradford, Clay, Columbia, De Soto, Gadsden, Gilchrist, Glades, Hamilton, Hardee, Highlands, Jefferson, Lafayette, Lake, Leon, Madison, Marion, Orange, Osceola, 

Putnam, Polk, Seminole, Sumter, Suwannee, Union

Georgia: Appling, Atkinson, Bacon, Baker, Baldwin, Ben Hill, Berrien, Bibb, Bleckley, Brantley, Brooks, Bulloch, Burke, Calhoun, Charlton, Chattahoochee, Clay, Clinch, Coffee, Colquitt, 

Columbia, Cook, Crawford, Crisp, Decatur, Dodge, Dooly, Dougherty, Early, Echols, Effingham, 

Emanuel, Evans, Glascock, Grady, Hancock, Harris, Houston, Irwin, Jeff Davis, Jefferson, Jenkins, Johnson, Jones, Lamar, Lanier, Laurens, Lee, Long, Lowndes, Macon, Marion, McDuffie, 

Meriwether, Miller, Mitchell, Monroe, Montgomery, Muscogee, Peach, Pierce, Pike, Pulaski, 

Quitman, Randolph, Richmond, Schley, Screven, Seminole, Stewart, Sumter, Talbot, Tattnall, 

G-6 PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards

A Multi-Hazard Foundation and Installation Guide
WIND ZONE COMPARISONS (HUD'S MHCSS AND FEMA 85) G

Taylor, Telfair, Terrell, Thomas, Tift, Toombs, Treutlen, Troup, Turner, Twiggs, Upson, Ware, 

Warren, Washington, Wayne, Webster, Wheeler, Wilcox, Wilkinson, Worth

Hawaii: the entire state

Louisiana: Parishes of Acadia, Allen, Ascension, Avoyelles, Beauregard, Calcasieu, Catahoula, 

Concordia, East Baton Rouge, East Feliciana, Evangeline, Iberville, Jefferson Davis, Jefferson, 

Livingston, Pointe Coupee, Rapides, St. Helena, St. Landry, St. Martin, Tangipahoa, Vermilion, 

Vernon, Washington, West Baton Rouge, West Feliciana

Maine: Androscoggin, Cumberland, Hancock, Kennebec, Knox, Lincoln, Sagadahoc, Waldo, 

York

Maryland: Caroline, Dorchester, Queen Annes, Talbot, Wicomico

Massachusetts: Bristol, Essex, Franklin, Hampden, Hampshire, Middlesex, Norfolk, Suffolk, 

Worcester

Mississippi: Adams, Amite, Claiborne, Clarke, Copiah, Covington, Forrest, Franklin, Hinds, Jasper, Jefferson, Jefferson Davis, Jones, Kemper, Lamar, Lauderdale, Lawrence, Leake, Lincoln, 

Marion, Neshoba, Newton, Noxubee, Pike, Rankin, Scott, Simpson, Smith, Walthall, Wayne, 

Wilkinson, Winston

New Hampshire: Cheshire, Hillsborough, Merrimack, Rockingham, Strafford

New Jersey: Bergen, Burlington, Camden, Cumberland, Essex, Gloucester, Hudson, Mercer, 

Middlesex, Monmouth, Morris, Passaic, Salem, Somerset, Union

New York: Bronx, Kings, New York, Putnam, Queens, Richmond, Rockland, Westchester

North Carolina: Bertie, Bladen, Cumberland, Duplin, Edgecombe, Gates, Greene, Halifax, Harnett, Hertford, Hoke, Johnston, Lenior, Martin, Nash, Northampton, Pitt, Robeson, Sampson, 

Scotland, Wayne, Wilson

Pennsylvania: none

Rhode Island: Providence

South Carolina: Aiken, Allendale, Bamberg, Barnwell, Berkeley, Calhoun, Chesterfield, Clarendon, Colleton, Darlington, Dillon, Dorchester, Fairfield, Florence, Hampton, Jasper, Kershaw, 

Lancaster, Lee, Lexington, Marion, Marlboro, Orangeburg, Richland, Sumter, Williamsburg

Texas: Angelina, Atascosa, Austin, Bastrop, Bee, Brooks, Burleson, Caldwell, Colorado, De Witt, 

Duval, Fayette, Fort Bend, Goliad, Gonzales, Grimes, Guadalupe, Hardin, Harris, Hidalgo, Jackson, Jasper, Jim Hogg, Jim Wells, Karnes, Lavaca, Lee, Liberty, Live Oak, McMullen, Montgomery, 

PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards 

A Multi-Hazard Foundation and Installation Guide

G-7
g WIND ZONE COMPARISONS (HUD'S MHCSS AND FEMA 85)

Newton, Orange, Polk, San Jacinto, Starr, Trinity, Tyler, Victoria, Walker, Waller, Washington, 

Webb, Wharton, Wilson, Zapata

Virginia: Gloucester, Isle of Wight, James City, Lancaster, Mathews, Middlesex, Northumberland, 

Southampton, Surry, York. Cities of Chesapeake, Hampton, Newport News, Norfolk, Suffolk, 

Virginia Beach 

G-8 PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards

A Multi-Hazard Foundation and Installation Guide
PROTECTING Manufactured Homes FROM FloodS AND OTHER Hazards 

A Multi-Hazard Foundation and Installation Guide

H-

H.	 Pre-Engineered and

Prescriptive Foundation

Designs

Drawing No. Title

GN-1.1 Recommended Foundation – General Notes

M-1 Single Unit Masonry Foundation Plan 

M-2 Double Unit Masonry Foundation Plan

M-3 Masonry Wall Foundation Detail

M-4 End Wall Foundation Detail

WF-1 Single Unit Wood Framed Foundation Plan 

WF-2 Double Unit Wood Framed Foundation Plan

WF-3 Wood Framed Foundation Detail

WF-4 Wood Framed Shear Wall Detail

BM-1.1 Single Unit Braced Masonry Pier Foundation Plan 

BM-1.2 Braced Masonry Pier Detail

BM-2.1 Double Unit Braced Masonry Pier Foundation Plan 

BM-2.2 Braced Masonry Pier Detail

HF-1.1 Single Unit Braced Wood H-Frame Foundation Plan 

HF-1.2 Single Unit Wood H-Frame Detail

HF-2.1 Double Unit Braced Wood H-Frame Foundation Plan 

HF-2.2 Double Unit Wood H-Frame Detail

GASP90-1.1 Single Unit Ground Anchor Foundation Plan 

GASP90-1.2 Ground Anchor and Pier Detail (Single Unit)

GASP90-2.1 Double Unit Ground Anchor Foundation Plan 

GASP90-2.2 Ground Anchor and Pier Detail (Double Unit)

GASP110/130/150-1.1 Single Unit Ground Anchor Foundation Plan 

GASP110/130/150-1.2 Ground Anchor and Pier Detail (Single Unit)

GASP110/130/150-2.1 Double Unit Ground Anchor Foundation Plan 

GASP110/130/150-2.2 Ground Anchor and Pier Detail (Double Unit)

GA90-1.1 Single Unit Ground Anchor Foundation Plan 

GA90-1.2 Ground Anchor and Pier Detail (Single Unit)
H- PROTECTING MaNufaCTuREd HOMEs fROM flOOds aNd OTHER HazaRds 

A Multi-Hazard Foundation and Installation Guide

H Pre-enGIneered And PrescrIPtIve FoundAtIon desIGns 

Drawing No. Title

GA90-2.1 Double Unit Ground Anchor Foundation Plan

GA90-2.2 Ground Anchor and Pier Detail (Double Unit)

GA110/130/150-1.1 Single Unit Ground Anchor Foundation Plan 

GA110/130/150-1.2 Ground Anchor and Pier Detail (Single Unit)

GA110/130/150-2.1 Double Unit Ground Anchor Foundation Plan

GA110/130/150-2.2 Ground Anchor and Pier Detail (Double Unit)

APD-1.1 Alternative Pier Details for Flood Velocities Not Shown on Plans

AS-1.1 Lateral Ground Anchor Spacing

AS-1.2 Longitudinal Ground Anchor Spacing and Anchor Notes

SP-1/2.1 Single Unit Concrete Masonry Pier Foundation Plan

SP-1/2.2 Concrete Masonry Pier Details

SM-1/2.1 Single Unit Concrete Masonry Wall Foundation Plan

SM-1/2.2 Concrete Masonry Wall Foundation Detail

SWF-1/2.1 Single Unit Wood Framed Foundation Plan

SWF-1/2.2 Wood Framed Foundation Detail
N otes 

1. These recommended foundation designs c ontain guidance for making manufactured homes, placed

in Special Flood H azard Areas, resistant to natural hazards. The designs do not cover all

installations. Manufactured homes placed in the follow ing areas are excluded: 

a. V zones 

b. C oastal A zones: Areas identified on FIR M as areas of Limited Moderate W ave Action

(LIMW A) w here c onditions can produce breaking w ave heights of 1.5 feet or greater. 

c. F loodways 

d. Areas w ith Flood Velocities over 5 feet per second 

e. Areas w ith Flood Depths over 3 feet 

f. Areas w ith Seismic Spectral Ac celeration C onstants Ss greater than 0.5g or S1 greater than 

0.15g 

g. Areas w ith ground snow loads greater than 40 pounds per square foot 

h. Installations with pier heights over 36 inches 

i. Manufactured homes w eighing less than 25 psf 

j. Manufactured homes w ith frames spaced les s than 95 inches 

k. Manufactured homes w ith endwall heights over 8 feet 2 inches 

2. The foundation s ystems have been designed to resis t loads specified in ASC E 7 Minimum Design 

Loads for Buildings and Other Structures. Any locally adopted code or ordinance w ith more stringent

requirements shall govern. 

3. D esigns are provided as guidance for foundations that satisfy the recommended design criteria

specified in C hapter 10. Selection of specific foundation type and final design must be made on the

basis of individual site conditions, manufacturer installation instructions, and regulatory

requirements. 

4. R ecomm ended foundation design is based on a 14-foot w ide (single unit) and a 28-foot wide 

(double unit) manufactured home. T hese designs can be used with section widths up to 16 feet 

provided the spacing of fasteners connecting the hom e to the longitudinal foundation walls ar e

reduced by the factor (14 divided by the actual sec tion w idth). 

5. Seismic resistant foundation designs provided (drawings SP, SM, SW F) are a suitable for homes 

placed in areas w ith Seismic D es ign Criteria D 0, D 1, D 2 and E with a basic wind speed of 90 mph 

(3-second gust) or less. 

6. The foundations are depicted on flat s ites. W here manufactured homes are plac ed on sloped lots,

refer to N FPA 5000 Chapter 36.8 or IR C-2009 Sections R 403.1.5 and R 403.1.7 for additional 

requirements. 

7. Soil bearing capacity shall be as requir ed by the authority having jurisdiction. W here presumptive

values ar e allowed, they shall be per NF PA 5000-2009 Table 36.3.4(a) IRC 2009 Table R401.4.1 or

24CF R Table 3285.202 as directed by the Building official. 

8. Anchor performance shall be based on load testing and probabilistic analysis of anchors tested in 

both dry and saturated soils . Anchor design values shall be based on a maximum 10% lower 

exclusion limit and a minimum 90% confidence interval. Anchor spacing in these designs is based

on a minimum s tiffness of 1,200 pounds per inch for in-line anchors and 675 pounds per inch for 

anchors used with 11”x17” steel stabilizer plates. These stiffness values were determined from 

tests of 5-foot anchors c onducted in saturated and unsaturated uniform (poorly graded) medium to 

fine grained sandy soils (soil class 4 A) in Kissimmee, Florida, July 2002. Torque and/or Standard

penetration tests (ASTM D -1586-99) tests should be used in ground anchor s election. 

9. Preload anchors a m inimum of 500 pounds or as required to fully ac tivate the resistance of the 

stabilizer plates and the anchor helices. 

10 . Anchor straps shall be galvanized, tested per AST M D 3 953, and shall have a minim um allowable

w ork ing load of 3,150 pounds and a minimum tens ile strength of 4,725 pounds. 

11 . C oncrete anchors shall provide a minimum allowable working load of 3,150 pounds and a minim um

ultimate load of 4,725 pounds. Place concrete anchors a minimum of 3 inches away from all edges

of footings. 

12 . C oncrete masonry units shall be Type I or Type II per ASTM C 90. Mortar shall be Type M, N, or S

per AST M C270. 

13 . Polyurethane based masonry adhesives may be used in place of Portland cement/lime mortar.

Masonry adhesives used shall have been tested and certified by a nationally recognized agency as

equivalent to Type M, O, and S Portland cement/lime masonry. C ertification shall indicate that

masonr y adhesive is applicable to mas onry construction designed in accordanc e w ith the

requirements of IRC Section 606. Masonry adhesives shall be installed in strict accordance w ith the

manufacture's instruction. 

14. Grout shall be coarse aggregate and shall be apportioned per ASTM C476. 

15. Surface bonded mortar shall meet ASTM C887-79a (2001) and ASTM C946-91 (2001). 

16. Wind speed is a 3-second gust speed in miles per hour at 33 feet above ground in Exposure Category C. 

17. Maximum anchor spacing is the spacing designed to limit manufactured home movement during a design event to 3” 

horizontal or 2” vertical. This criterion has historically been used by the manufactured housing industry. 

18. End anchors shall be located no more than 2 feet from the ends of the home per 24 CFR Part 3280.306 (c)(2). 

19. Flood vents required by the NFIP, in accordance with NFIP Section 60.3(c)(5), may be omitted for homes with 

non-structural skirting providing: 

• The skirting system will prevent differential flood depths from reaching 12 inches. 

• Performance of skirting under flood conditions is certified by a licensed professional engineer or architect. 

Symbols and Abbreviations

ABS - acrylonitrile butadiene styrene 

CMU - concrete masonry unit 

fps - feet per second 

fb - Bending strength (wood) 

fc - compressive strength parallel to grain (wood) 

fc - compressive strength perpendicular to grain (wood) 

f’c - compressive strength (concrete) 

ft - tensile strength parallel to grain (wood) 

fv - shear strength parallel to grain (wood) 

fy - yield strength of reinforcing steel 

g - gravitational force 

max - maximum 

mph - miles per hour 

o.c. - on center 

psf - pounds per square foot 

psi - pounds per square inch 

PL - plate 

PT - preservative treated 

Ss - spectral response acceleration at 

 short periods 

S1 - spectral response acceleration at 

 a period of 1 second 

T and B - top and bottom 

Typ. - typical 

a > b - a is greater than or equal to b 

a < b - a is less than or equal to b 

a < b - a is less than b 

Material Specifications 

1. All exposed hardware shall be hot dipped galvanized per ASTM A-153. Nails and hardware in contact with framing or 

plywood with preservative treatment concentration exceeding 0.40 pound per cubic foot shall be stainless steel. 

2. All concrete shall be a mixture of Portland Cement, water, and aggregate and shall be proportioned to provide a 

minimum 3,000 psi 28-day compressive strength (f’c). Portland Cement shall be per ASTM C150. 

3. Reinforcing steel shall be deformed bars per ASTM A615, A617 or A706 and shall have a minimum 60 ksi yield strength 

(fy). Lap splices in reinforcing steel a minimum of 36 bar diameters. 

4. All wood below the BFE and not in contact with the soil (framing, shims, etc.) shall meet or exceed NFIP flood resistant 

material requirements. One acceptable method is to provide rot and insect resistance that meets or exceeds protection 

provided by ACQ (alkaline copper quaternary) at a retention of 0.25 pounds per cubic foot. See FEMA Technical 

Bulletin 2 for additional guidance. 

5. All wood shall be of a species recognized for structural use by the authority having jurisdiction and shall provide the 

minimum design values as follows: 

 fb > 975 psi 

 ft > 550 psi 

 fv > 90 psi 

 fc > 565 psi 

 fc > 1,450 psi 

6. Foundation wall sheathing shall be American Plywood Association (APA) rated panels and constructed with cross 

laminated full width veneers that conform to Voluntary Product Standard PSI-95. Sheathing shall be pressure 

preservative treated with ACQ-B (ammoniacal copper quat) Type B or D to a minimum retention of 0.25 pounds per 

cubic foot. 

_ 

_ 

_ 

_ 

_ 

_ 

_ 






Table 1 - Side Wall Framing Connector Schedule 1

Wind Speed Single Unit Double Unit 

90 mph 8’–0” 8’–0” 

110 mph 8’–0” 8’–0”

130 mph 4’–0” 4’–0”

150 mph 4’–0” 4’–0”

Braced Masonry Pier Foundation - Pier, Footing and Tie Schedule

(Minimum 1,000 lb/ft2

 Soil Bearing Capacity Required) 

Wind 

Speed 

Frame 

Footing 

(W by D) 

Perimeter 

Footing 

(W by D) 

Pier

Spacing1

Wall Tie 

Spacing2,3

Frame Tie 

Spacing2,3

Single Unit 

 90 mph 16” by 10” Not 

Required 10’-0” Not Required 8’-0” 

110 mph 16” by 12” 8” by 16” 10’-0” 10’-8” 5’-4” 

130 mph 16” by 12” 12” by 16” 8’-0” 8’-0” 4’-0” 

150 mph 16” by 12” 12” by 24” 8’-0” 5’-4” 2’-8” 

1

 Locate end piers within 2’-0” of the ends of the home. 

2

 Wall and frame ties may be connected to a single concrete anchor. 

3 Install wall and frame straps in other locations if required by the manufacturer.





















P i e r S p a c i n g T a b le





Table SP-1.2 of Pier to Chassis Seismic/Wind Anchorage Forces1

Width 

Pier 

Height 

(ft) 

Maximum 

Pier Spacing 

(ft) 

Horizontal 

Seismic/Wind 

Force Each 

Pier (lb) 

Net 

Seismic/Wind 

Uplift Force 

Each Pier (lb)2

4 320 720 

single 3 7 560 1250 

10 800 1790 

4 290 860 

double 3 7 440 1510 

10 600 2150 

4 350 720 

single 5 7 620 1250 

10 880 1790 

4 340 860 

double 5 7 490 1510 

10 650 2150 

Notes 

1. Listed loads are in accordance with ASCE/SEI 7 Section 2.4 Allowable Stress Design (ASD) 

2. Based on 14 ft section width. Multiply by 1.15 for 16 ft section width 

Table SM-1.1 Required Anchorage Capacity Between 

Foundation Wall and Floor Diaphragm. 

Home Width Wall Location 

Horizontal 

Seismic/Wind 

Force Per Foot 

Wall (Shear) 

(lb/ft)1 

Net 

Seismic/Wind 

Uplift Force 

Per Foot Wall 

Length 

(lb/ft)1, 

end 510 50 

single side 70 120 

end 260 50 

double 

side 140 150 

1 Based on anchorage over entire home perimeter. Forces shall be 

increase proportionally if anchorage length on any side is 

decreased. 

Table SM-1.2 Manufactured Home Minimum Perimeter to 

Foundation Wall Seismic/Wind Anchorage 

Home Width Wall 

Location 

Maximum 

Spacing for 

1/2-in Anchor 

Bolt Anchoring 

Sill to 

Foundation 

Wall (in) 

Maximum 

Spacing for 

Framing Plate 

with Shear and 

Uplift Capacity 

(in)1 

end 16 12 

single 

side 48 42 

end 32 18 

double 

side 32 18 

1

Not less than 18 ga. Galvanized plate, fastened to sill and home with not less 

than six 8d common short nails each. 



FEMA

FEMA P-85

Catalog No. 08211-1