NIST Technical Note 1930

Economic Analysis of Restricting 

Aggregate-Surfaced Roofing Systems

in Tornado-Prone Areas of the U.S. 

Joshua Kneifel

Marc Levitan

Long Phan

Thomas Smith

David Butry

Douglas Thomas

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NIST Technical Note 1930

Economic Analysis of Restricting 

Aggregate-Surfaced Roofing Systems 

in Tornado-Prone Areas of the U.S. 

Joshua Kneifel 

David Butry 

Douglas Thomas 

Applied Economics Office 

Engineering Laboratory 

Marc Levitan

Long Phan 

Structures Group

Engineering Laboratory 

Thomas Smith

TLSmith Consulting Inc.

This publication is available free of charge from:

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September 2016 

U.S. Department of Commerce 

Penny Pritzker, Secretary

National Institute of Standards and Technology 

Willie May, Under Secretary of Commerce for Standards and Technology and Director
Certain commercial entities, equipment, or materials may be identified in this

document in order to describe an experimental procedure or concept adequately.

Such identification is not intended to imply recommendation or endorsement by the

National Institute of Standards and Technology, nor is it intended to imply that the

entities, materials, or equipment are necessarily the best available for the purpose. 

National Institute of Standards and Technology Technical Note 1930

Natl. Inst. Stand. Technol. Tech. Note 1930, 41 pages (September 2016) 

CODEN: NTNOEF

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Abstract

Due to the significant life-safety and economic loss resulting from extreme, “low 

probability, high consequence” weather events such as Hurricane Katrina, Hurricane 

Sandy, and the Joplin tornado, greater focus has been placed on increasing the resiliency 

of buildings to decrease these impacts. Investigations of building performance following 

tornadoes and hurricanes have shown that wind-borne debris, including loose aggregate, 

gravel, and stone surfacing on roofs are significant contributors to building damage and 

occupant injuries. Buildings may often experience little to no structural damage, but 

suffer failure of exterior glazing, causing catastrophic damage to building interiors and 

building contents that can also result in injuries and fatalities. A recent proposed code 

change to the International Building Code (IBC) would prohibit installation of loose 

aggregate surfacing on roofs of Risk Category III or IV buildings located in the most 

tornado-prone region of the country (covering portions of the Great Plains, Midwest, and 

Deep South) in order to reduce the wind-borne debris hazard, particularly to glazed 

openings. 

Existing building codes already have provisions prohibiting the use of roof aggregate to 

reduce wind-borne debris hazards in hurricane-prone regions (IBC 2015, Section 1504.8), 

and expansion of similar provisions in the tornado-prone region are those currently being 

considered. However, the economic impacts of these proposed code changes to prohibit 

the use of roof aggregate in tornado-prone regions are not currently well understood. 

Needed is an assessment of the impacted building stock and the magnitude of associated 

costs. 

The purpose of this study is to analyze the potential economic impacts from adoption of 

this proposed change to the IBC by identifying the fraction of roof construction that could 

be impacted from the restriction on aggregate surfaced roofs, characterizing alternatives 

to aggregate-surfaced roofing systems, and estimating the costs associated with these 

alternative roofing systems. The results are presented using both a national and regional 

perspective, including a comparison of the impacts from the restrictions on aggregate in 

the hurricane-prone regions. 

The results demonstrate that the code change would potentially impact less than 0.1 % of 

all roof construction in the U.S., and less than 3.0 % of all non-low rise residential roof 

construction in the tornado-prone region. This includes roofs of newly constructed 

buildings and reroofing projects. The previously adopted code change that similarly 

prohibited aggregate-surfaced roofs in the hurricane-prone region impacted more than 

four times as much roof construction. 

It is found that few roof construction projects will be negatively impacted from a 

construction cost perspective by the proposed restriction on aggregate in the 

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tornado-prone region (<0.2 %) because cost-effective alternatives are available for most 

assemblies. Common types of aggregate-surfaced roofing that would be prohibited under 

the proposed code change can be more expensive than similar alternatives. For example, 

built-up roofs with aggregate surfacing are found to cost more than the alternative system 

that replaces the aggregate with a cap sheet. For single-ply roof systems over a steel roof 

deck, aggregate ballasted EPDM is also found to be more expensive than mechanically 

attached EPDM. Only in the case of the single-ply systems over a concrete roof deck are

aggregate ballasted EPDM systems found to be less expensive than the alternative EPDM

systems. These results hold for all 33 cities studied within the tornado-prone region. 

Keywords

Building economics; roofing systems; aggregate surfacing; economic analysis; 

commercial buildings; building codes; building standards

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Preface

This study was jointly conducted by the Applied Economics Office (AEO) and the 

Structures Group of the Materials and Structural Systems Division in the Engineering 

Laboratory (EL) at the National Institute of Standards and Technology (NIST). The study 

is to support NIST’s overall effort to implement recommendations NIST made as a result 

of the National Construction Safety Team Act investigation of the May 22, 2011 Joplin 

tornado (http://nvlpubs.nist.gov/nistpubs/NCSTAR/NIST.NCSTAR.3.pdf) and designed 

to identify the potential impacts on the commercial building sector from restricting 

aggregated surfaced roofing systems in areas of the United States with high tornado risk. 

The intended audience is researchers, standards and codes development organizations, 

policy makers in the commercial building sector, and others interested in building 

resiliency. 

Disclaimers

The policy of the National Institute of Standards and Technology is to use metric units in 

all of its published materials. Because this report is intended for the U.S. construction 

industry that uses U.S. customary units, it is more practical and less confusing to include 

U.S. customary units as well as metric units. Measurement values in this report are 

therefore stated in metric units first, followed by the corresponding values in U.S. 

customary units within parentheses. 

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Acknowledgements

The authors wish to thank all those who contributed ideas and suggestions for this report. 

They include Dr. Jennifer Helgeson of EL’s Applied Economics Office, Mr. Matthew 

Boyd of EL’s Energy and Environment Division, and Dr. Nicos S. Martys of EL’s 

Materials and Structural Systems Division. A special thanks to Ms. Shannon Craig for 

collecting the RS Means building cost data. 

Author Information

David Butry 

Chief, Applied Economics Office (AEO) 

National Institute of Standards and Technology 

Engineering Laboratory

100 Bureau Drive, Mailstop 8603 

Gaithersburg, MD 20899-8603 

Tel.: 301-975-6136 

Email: david.butry@nist.gov 

Joshua D. Kneifel 

Economist 

National Institute of Standards and Technology 

Engineering Laboratory

100 Bureau Drive, Mailstop 8603 

Gaithersburg, MD 20899-8603 

Tel.: 301-975-6857 

Email: joshua.kneifel@nist.gov 

Marc Levitan

Acting Director, National Windstorm Impact Reduction Program (NWIRP)

National Institute of Standards and Technology 

Engineering Laboratory

100 Bureau Drive, Mailstop 8611 

Gaithersburg, MD 20899-8611 

Tel.: 301-975-5340 

Email: marc.levitan@nist.gov 

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Long Phan 

Leader, Structures Group of the Materials and Structural Systems Division (MSSD)

National Institute of Standards and Technology 

Engineering Laboratory

100 Bureau Drive, Mailstop 8611 

Gaithersburg, MD 20899-8611 

Tel.: 301-975-6077 

Email: long.phan@nist.gov 

Thomas Lee Smith, AIA, RRC, F.SEI 

TLSmith Consulting Inc.

16681 Boswell Road 

Rockton, IL 61072 

Tel.: 815-629-2455 

Email: tlsmith@hughes.net 

Douglas Thomas 

Economist 

National Institute of Standards and Technology 

Engineering Laboratory

100 Bureau Drive, Mailstop 8603 

Gaithersburg, MD 20899-8603 

Tel.: 301-975-4918 

Email: douglas.thomas@nist.gov 

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Table of Contents

Abstract ............................................................................................................................... i

Preface ............................................................................................................................... iii

Acknowledgements ........................................................................................................... v

Author Information .......................................................................................................... v

List of Figures ................................................................................................................... ix

List of Tables .................................................................................................................... ix

List of Acronyms .............................................................................................................. xi

1 Introduction ............................................................................................................... 1

1.1 Background .......................................................................................................... 1

1.2 Impacts from Wind-Related Weather Events....................................................... 1

1.3 Defining Tornado-Prone Region .......................................................................... 2

1.4 Applicable Risk Category of Structures ............................................................... 4

1.5 Damages from Roof Aggregate............................................................................ 5

1.6 2006 IBC Change related to Roof Aggregate ...................................................... 6

1.7 Current IBC Proposal for Tornado-Prone Region Related to Roof Aggregate.. 10

1.8 Purpose and Approach ....................................................................................... 12

2 Estimating Potentially Impacted Roof Construction .......................................... 13

2.1 Hazus Building Stock Data ................................................................................ 13

2.2 Building Stock in Tornado-Prone Region .......................................................... 14

2.3 Comparison to the Hurricane-Prone Region ...................................................... 22

3 Costs of Alternative Roofing Systems ................................................................... 29

3.1 Alternative Roof Systems................................................................................... 29

3.1.1 Built-Up Roof Systems ............................................................................. 29

3.1.2 Single-Ply Membrane Roof Systems ........................................................ 30

3.2 Roofing Cost Data, Comparisons, and Implications .......................................... 32

3.3 Examples: 5-Story Hospital and 2-Story High School....................................... 35

4 Summary .................................................................................................................. 37

4.1 Potential Impacts of Code Change ..................................................................... 37

4.2 Limitations ......................................................................................................... 38

5 References ................................................................................................................ 39

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List of Figures

Figure 1-1 EF3 and Greater Intensity Tornado Paths (1950 to 2013) ................................ 3

Figure 1-2 Map Showing Proposed Tornado Prone Region ............................................... 4

Figure 1-3 Window (left) and Interior (right) Damage to Hospital Following the Joplin, 

MO Tornado........................................................................................................................ 6

Figure 1-4 Ultimate Design Wind Speeds for Risk Category II Buildings and Other 

Structures ............................................................................................................................ 7

Figure 1-5 Map Showing Wind Speeds Equivalent to the Nominal Stress Design Wind 

Speeds Referenced in Table 1-3 ....................................................................................... 10

Figure 2-1 Counties Within the Tornado-Prone Region ................................................... 14

Figure 2-2 Estimated Percentage of Risk Category III and IV Buildings, by County ..... 17

Figure 2-3 Census Divisions ............................................................................................. 20

Figure 2-4 Map of Existing Hurricane-Prone and Proposed Tornado-Prone Regions ..... 23

Figure 2-5 Building Stock Counts by County with Hurricane and Tornado-Prone Regions

........................................................................................................................................... 24

Figure 3-1 Example Built-Up Roof Asphalt with Aggregate (left) and Cap Sheet (right) 30

Figure 3-2 Example Single-Ply Roof with Aggregate Ballast (left), Fully Adhered 

(center), and Mechanically Attached (right) ..................................................................... 30

Figure 3-3 Representations of 5-Story Hospital: DOE (left) and RS Means (right) ........ 36

Figure 3-4 Representations of 2-Story High School: DOE (left) and RS Means (right) .. 36

List of Tables

Table 1-1 U.S. Weather-Related Fatalities, Injuries, and Property Damage in 2015 ........ 2

Table 1-2 Structure Risk Categories .................................................................................. 5

Table 1-3 Maximum Allowable Mean Roof Height Permitted for Buildings with 

Aggregate on the Roof in Areas Outside a Hurricane-Prone Region ................................. 8

Table 1-4 States Impacted by Proposed Code Change related to Roof Aggregate ......... 12

Table 2-1 Hazus Occupancy Types and Assignment of Risk Categories ........................ 16

Table 2-2 Existing Building Stock by Hazus Occupancy Type – Tornado-Prone Region

........................................................................................................................................... 18

Table 2-3 Percentages of Low-Slope Roofing Projects by Roof System Type by Census 

Division in the Tornado-Prone Region ............................................................................. 20

Table 2-4 Upper Bound Estimate of Percentage of Roof Construction Potentially 

Impacted by Proposed Code Change ................................................................................ 21

Table 2-5 Existing Building Stock by Occupancy – Tornado- vs Hurricane-Prone Region

........................................................................................................................................... 25

Table 2-6 Percentages of Low-Slope Roofing Projects by Roof System Type by Census 

Division in Hurricane-Prone Region ................................................................................ 26

Table 3-1 Roofing Assembly Alternatives ...................................................................... 29

Table 3-2 Roofing Systems and Roof Components in RS Means Databases .................. 31

Table 3-3 Roofing System Cost Data – National Average .............................................. 32

Table 3-4 Assembly Cost by Location in Tornado-Prone Region ................................... 34

Table 3-5 Example: Roofing System Costs of 5-Story Hospital – National Average ..... 35

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List of Acronyms 

AEO Applied Economics Office

BCAC Building Code Action Committee

BUR Built-Up Roof

DOE Department of Energy

EL Engineering Laboratory

EPDM ethylene propylene diene monomer

ERA EPDM Roofing Association

FEMA Federal Emergency Management Agency

IBC International Building Code

ICC International Code Council

MSSD Materials and Structural Systems Division

NIST National Institute of Standards and Technology

NRCA National Roofing Contractors Association

NWIRP National Windstorm Impact Reduction Program

PVC polyvinyl chloride

SEAKM Structural Engineers Association of Kansas and Missouri

SFCE Square Foot Cost Estimator

TPO Thermoplastic polyolefin

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1 Introduction

1.1 Background 

Due to the significant life-safety and economic loss resulting from extreme, “low probability, 

high consequence” weather events such as Hurricane Katrina, Hurricane Sandy, and the Joplin 

tornado, greater focus has been placed on increasing the resiliency of buildings to decrease these

impacts. Investigations of building performance following tornadoes and hurricanes have shown 

that wind-borne debris, including loose aggregate, gravel and stone surfacing on roofs are 

significant contributors to building damage and occupant injuries. Buildings often experience 

little to no structural damage, but suffer failure of exterior glazing, causing catastrophic damage 

to building interiors and building contents that can also result in injuries and fatalities. A recent 

proposed code change to the International Building Code (IBC) would prohibit aggregate 

surfacing on roofs of Risk Category III or IV buildings located in the most severe portion of the 

tornado-prone region of the country in order to reduce the wind-borne debris hazard, particularly 

to glazed openings. 

Existing building codes already have provisions prohibiting the use of these roof aggregate to 

reduce wind-borne debris hazards in hurricane-prone regions (IBC 2015, Section 1504.8), and 

expansion of similar provisions in the tornado-prone region are currently being considered. 

However, the economic impacts of these proposed code changes to prohibit the use of roof 

aggregate in tornado-prone regions are not currently well understood. Needed is an assessment of 

the impacted building stock and the magnitude of associated costs. 

1.2 Impacts from Wind-Related Weather Events

Wind-related weather events are some of the most impactful when it comes to fatalities and 

injuries as well as property damage. Table 1-1 shows that the combination of tornadoes, 

hurricanes, thunderstorm wind, and other high winds claimed 115 lives and caused 1195 injuries 

and $706 million in property damage in 2015 (NOAA 2016a). Of all wind-related weather 

events, tornadoes result in some of the greatest impacts across all three categories of damages. 

Property damage from tornadoes totaled $317 million in 2015, 7.5 times that of hurricanes. Of 

the 2143 weather-related injuries and illnesses in 2015, tornadoes were responsible for 924, or 

43 % of those reported in the U.S., which is 5.8 times greater than those caused by hurricanes. 

Tornadoes have led to 110 fatalities annually, on average, over the last 10 years while other 

windstorms and hurricanes have led to 56 and 43 fatalities annually, respectively (NOAA 

2016b). Since the beginning of official tornado record keeping (1950) through 2011, U.S. 

tornadoes have caused about 5600 fatalities 

(www.nws.noaa.gov/om/hazstats/resources/weather_fatalities.pdf). This number well exceeds 

the toll for U.S. hurricanes and earthquakes over the same period, 3102 (NOAA 

(www.noaanews.noaa.gov/2011_tornado_information.html) and 459 

(http://earthquake.usgs.gov/earthquakes/states/us_deaths.php), respectively. The May 22, 2011 

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Joplin, MO, tornado in particular was ranked the deadliest and costliest single tornado on record. 

It caused 161 fatalities, more than 1000 injuries, and damaged nearly 8000 structures and 

incurred close to $3 billion in insured losses (NIST 2014). 

Table 1-1 U.S. Weather-Related Fatalities, Injuries, and Property Damage in 2015

Impact

Categories

Tornado Tropical Storm 

/ Hurricane

Thunderstorm 

Wind 

High 

Wind 

U.S. 

Total

Fatalities 36 14 41 24 522

Injuries 924 50 159 62 2143

Property Damage (million $) 316.8 41.5 252.0 65.5 4202.0

Source: NOAA 2016a

1.3 Defining Tornado-Prone Region 

Tornadoes can occur in all 50 states, but the strongest and most frequent tornadoes occur in the 

Great Plains, Midwest, and Deep South. Tornadoes are rated in intensity using the Enhanced 

Fujita (EF) Scale, which ranges from 1 to 5 with 5 being the strongest, Figure 1-1 maps the EF3 

(green), EF4 (yellow), and EF5 (red) tornadoes reported between 1950 and 2013. For additional 

information on the EF scale, see NOAA 2016e. 

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Figure 1-1 EF3 and Greater Intensity Tornado Paths (1950 to 2013)1

Based on analysis of tornado frequency and intensity data, the map shown in Figure 1-2 was 

created to identify areas in the U.S. that are at risk of winds from tornadoes, for use in design of 

storm shelters (ICC 2014c). The proposed IBC code change designates the tornado prone region 

to be an area of the country within the 250 mph wind speed zone, as shown in dark gray in 

Figure 1-2. 

 

1 Source: FEMA (2015) 

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Figure 1-2 Map Showing Proposed Tornado Prone Region2

1.4 Applicable Risk Category of Structures

For structural design purposes, IBC (Table 1604.5) classifies buildings into four risk categories 

according to their occupancy type. The building types associated with each of the four Building 

Risk Categories can be seen in Table 1-2 from IBC 2015. This study focuses on Risk Category 

III and IV buildings, which are the subject of the proposed code change. 

 

2 ICC (2014c). Enhancements by NIST.

Tornado Prone Region 

is the 250 mph zone

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Table 1-2 Structure Risk Categories3

Risk

Category

Nature of Occupancy

I Buildings and other structures that represent a low hazard to human life in the event of failure, 

including but not limited to: 

• Agricultural facilities, certain temporary facilities, minor storage facilities, and screen enclosures.

II Buildings and other structures except those listed in Risk Categories I, III and IV.

III Buildings and other structures that represent a substantial hazard to human life in the event of 

failure, including but not limited to:

• Buildings and other structures whose primary occupancy is public assembly with an occupant load 

greater than 300

• Buildings and other structures containing elementary school, secondary school or daycare facilities 

with an occupant load greater than 250.

• Buildings and other structures containing adult education facilities, such as colleges and 

universities, with an occupant load greater than 500.

• Group I-2 occupancies with an occupant load of 50 or more resident patients but not having 

surgery or emergency treatment facilities.

• Group I-3 occupancies.

• Any other occupancy with an occupant load greater than 5000.

• Power-generating stations, water treatment facilities for potable water, waste water treatment 

facilities and other public utility facilities not included in Risk Category IV.

• Buildings and other structures not included in Risk Category IV containing sufficient quantities of 

toxic or explosive substances to be dangerous to the public if released.

IV Buildings & other structures designated as essential facilities, including but not limited to:

• Group I-2 occupancies having surgery or emergency treatment facilities.

• Fire, rescue, ambulance and police stations and emergency vehicle garages.

• Designated earthquake, hurricane or other emergency shelters.

• Designated emergency preparedness, communications and operations centers and other facilities 

required for emergency response.

• Power-generating stations and other public utility facilities required as emergency backup facilities 

for Risk Category IV structures.

• Structures containing highly toxic materials as defined by Section 307 where the quantity of the 

material exceeds the maximum allowable quantities of Table 307.1(2).

• Aviation control towers, air traffic control centers and emergency aircraft hangars.

• Buildings and other structures having critical national defense functions.

• Water storage facilities and pump structures required to maintain water pressure for fire 

suppression.

1.5 Damages from Roof Aggregate 

There are many types of debris generated during high wind events, all of which create risk of 

property damage, fatalities, and injuries. Much of that debris would be difficult, or impossible, to 

mitigate because of the randomness of its location. However, one particular type of debris that 

could be directly mitigated with code changes is aggregate from roofs. 

 

3 ICC (2014b) Table 1604.5 Risk Category of Buildings and Other Structures

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Investigations of building performance following tornadoes and hurricanes have shown that 

loose aggregate, gravel and stone used as surfacing on roofs are significant contributors to 

building damage and injuries. These buildings often experience little to no damage to the 

structural systems, but nevertheless suffer catastrophic damage to their envelopes and exterior 

glazing, leading to significant damage to building interiors and contents that can also result in 

injuries and fatalities. In particular, buildings in Risk Category III and IV (see Table 1-2), such 

as schools and hospitals, have often experienced significant glazing damage due to aggregate 

“blow-off” from their own roofs, and/or roofs of nearby buildings during tornadoes (e.g., NIST 

2014, FEMA 2007, FEMA 2010, and FEMA 2012) and hurricanes (e.g., NIST 2006 and FEMA 

2005). For example, Figure 1-3 shows the glazing failures (left) and interior damage (right) in 

the East Tower at St John's Regional Medical Center following the Joplin tornado (NIST 2014). 

Note the extensive amount of roof aggregate inside the building. The Federal Emergency 

Management Agency (FEMA) has also documented instances where people have been injured 

after being struck directly by roof aggregate in tornadoes in Illinois (FEMA 2010) and Texas 

(FEMA 2007). These studies have led to a number of adopted or proposed changes to building 

codes to prevent these damages in the future. 

Figure 1-3 Window (left) and Interior (right4) Damage to Hospital Following the Joplin, 

MO Tornado

1.6 2006 IBC Change related to Roof Aggregate 

The 2006 edition of the International Building Code (IBC) prohibited the use of aggregate roof 

surfacing in hurricane–prone regions while allowing its use in other regions based on mean roof 

height and exposure category. Such restrictions remain in the 2015 IBC with the following 

language:

IBC 1504.8 Aggregate. Aggregate used as surfacing for roof coverings and 

aggregate, gravel or stone used as ballast shall not be used on the roof of a 

 

4 Copyright 2011 Malcolm Carter. Used with Permission.

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building located in a hurricane-prone region as defined in Section 202, or on any 

other building with a mean roof height exceeding that permitted by Table 1504.8 

based on the exposure category and basic wind speed at the site. 

IBC defines the hurricane-prone region (Section 202) as

Areas vulnerable to hurricanes defined as: 

1. The U.S. Atlantic Ocean and Gulf of Mexico coasts where the ultimate 

design wind speed, Vult, for Risk Category buildings is greater than 115 

mph (51.4 m/s); 

2. Hawaii, Puerto Rico, Guam, Virgin Islands and American Samoa.

using wind speeds shown in Figure 1-4. 

Figure 1-4 Ultimate Design Wind Speeds for Risk Category II Buildings and Other 

Structures5 

 

5 ICC (2014b) Figure 1609.3(1)

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The restrictions on aggregate roof surfacing outside of the hurricane prone region are shown in 

Table 1-3, adapted from IBC Table 1504.8 (ICC 2014b). The nominal stress design wind speed 

referred to in this table is equal to the ultimate design wind speed shown in Figure 1-4 times the 

square root of 0.6, which is equivalent to the basic wind speed mapped in the 2012 International 

Residential Code (ICC 2011), reproduced in Figure 1-5. Examination of this map and Figure 1-2

shows that a 145 km/hr (90 mph) wind speed applies to most of the proposed tornado-prone 

region (with the exception of the southeastern-most part of the tornado-prone region where wind 

speeds increase due to proximity to the hurricane prone region along the Gulf Coast – see also 

Figure 2-4). 

Table 1-3 Maximum Allowable Mean Roof Height Permitted for Buildings with Aggregate 

on the Roof in Areas Outside a Hurricane-Prone Region

Nominal Stress 

Design Wind 

Speed*** (mph)

Maximum Mean Roof Height* (ft)

Exposure Category

B C D

85 170 60 30

90 110 35 15

95 75 20 NP

100 55 15 NP

105 40 NP NP

110 30 NP NP

115 20 NP NP

120 15 NP NP

> 120 NP NP NP

*Mean roof height as defined in ACSE 7.

** NP = Gravel and stone not permitted for any roof 

height.

*** The nominal stress design wind speed is equal to 

the ultimate design wind speed as shown in Figure 

1-4 times √0.6. 

Conversions: 1 mph = 0.44704 m/s; 1 ft = 0.3048 m

Bold shows roof heights above which aggregate 

surfaced roofs are already prohibited in most of the 

tornado-prone region

Roof aggregate is not allowed for buildings above a given height for all three exposure 

categories at a nominal stress design wind speed of 145 km/hr (90 mph). This requirement is due 

to the increase of wind speed with height, which varies in winds flowing over different 

exposures. As shown in Figure 1-5, almost the entire contiguous U.S., excluding the states on the 

West Coast (California, Oregon, and Washington) and seaward portions of the states along the 

Gulf of Mexico and East Coast, is encompassed in the 145 km/hr (90 mph) region. Therefore, 

roof aggregate is currently not allowed for buildings taller than 33.5 m (110 ft), 10.7 m (35 ft), 

and 4.6 m (15 ft) that are in exposure categories B, C, and D, respectively. Exposure categories

are defined in IBC Section 1609.4.3 (ICC 2014b) and summarized below, which has specific 

application to IBC Table 1-3: 

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• Exposure B = Urban and suburban areas, wooded areas, areas with many closely spaced 

obstructions, extending for at least 457 m (1500 ft) in all directions for buildings with mean 

roof height of 9.1 m (30 ft), and for taller buildings, extending at least 792 m (2600 ft) or 20 

times the height of the building in all directions. 

• Exposure C = Open terrain with scattered obstructions. Includes airports and areas that are 

generally flat open country.

• Exposure D = Flat, unobstructed areas and water surfaces. This category includes smooth 

mud flats, salt flats, and unbroken ice that extend 1524 m (5000 ft) or 20 times the building 

height in any direction. 

Buildings located in urban, suburban, or wooded areas are subject to the Exposure B restriction 

in Table 1-3, which does not allow roof aggregate for buildings taller than 33.5 m (110 ft). 

Therefore, buildings of about 10 stories or less (assuming 3.4 m (11 ft) per story) in Exposure B 

currently have no restriction on roof aggregate throughout most of the tornado-prone region.

However, for buildings located in Exposure C, which is flat open terrain that is more common 

across much of the western part of the tornado-prone region, roof aggregate is already restricted 

when the mean roof height exceeds 10.7 m (35 ft). High-bay one-story buildings (such as used 

in some manufacturing and warehousing applications) commonly exceed this 35 ft threshold, so 

are already subject to roof aggregate restrictions, as are some three-story and all four-story and 

taller buildings located in Exposure C. All buildings with mean roof height exceeding 4.6 m 

(15 ft) in Exposure D (less common) are subject to the existing roof aggregate restrictions, which 

would exclude only some single story buildings.

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Figure 1-5 Map Showing Wind Speeds Equivalent to the Nominal Stress Design Wind 

Speeds Referenced in Table 1-3

1.7 Current IBC Proposal for Tornado-Prone Region Related to Roof Aggregate 

Similar to the change to the building code adopted in the IBC to address damage from roof 

aggregate in hurricane-prone regions and for buildings taller than 33.5 m (110 ft) located 

elsewhere (excluding West Coast states), a recently proposed change would also restrict use of 

aggregate for Risk Category III and IV buildings in the tornado-prone region (defined in Section 

1-3). The specific language of this code change proposal, jointly developed by the ICC’s

Building Code Action Committee (BCAC) and the National Institute of Standards and 

Technology (NIST), in consultation with experts from the roofing industry, is as follows: (ICC 

2016a, proposal S22-16) 

1504.9 Surfacing and ballast materials in tornado-prone regions. Aggregate 

shall not be used as surfacing for roof coverings and aggregate, gravel or stone 

shall not be used as ballast on the roof of a Risk Category III or IV building 

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located in areas where the wind speed is 250 MPH in accordance with Figure 

304.2(1) of ICC 500. 

The proposed language was modified slightly in a Public Comment as shown below (ICC 

2016b), which was jointly developed by the BCAC and NIST in response to concerns raised at 

the Committee Action Hearings held in Louisville in April, 2016 

1504.9 Surfacing and ballast materials in tornado-prone regions. Aggregate 

shall not be used as surfacing for roof coverings and aggregate, gravel or stone 

shall not be used as ballast on the roof of a Risk Category III or IV building 

located in the region having the greatest wind speed in Figure 304.2(1) of ICC 

500. 

The proposed code change is consistent with findings and recommendations from the NIST 

technical investigation of the 2011 tornado in Joplin Missouri (NIST 2014, Finding 19, and 

Recommendation 10). This change proposal is also consistent with FEMA recommendations, 

developed from observations of building performance in tornadoes. FEMA recommends that 

aggregate roof surfacing not be specified for critical facilities in tornado-prone regions (FEMA 

2012). The NIST and FEMA recommendations are intended to reduce the wind-borne debris 

hazard by reducing the potential number of “missiles” generated by a tornado, and hence reduce 

the potential for building damage and injury to people. Additionally, the proposed code change is 

consistent with recommendations made by the Structural Engineers Association of Kansas and 

Missouri following their investigation of the Joplin tornado (SEAKM 2012, Recommendation 

#9). 

The probability of aggregate “blow-off” from roofs outside of hurricane-prone regions is 

generally small except in the case of a tornado. Since the probability of a site-specific tornado 

strike is also low, the proposal is limited to Risk Category III and IV buildings. Although 

tornadoes generate many types of debris, an aggregate surfaced roof has a tremendous number of 

potential missiles. For example, an aggregate ballasted 1858 m2

 (20 000 ft2) single-ply roof 

would have approximately 1.6 million loose aggregates. A similarly sized built-up roof would 

have approximately 4.5 million to 9.0 million loose aggregates, depending on gradation (based 

on aggregate samples collected from a number of roofs reported by FEMA (2006). The proposed 

code change will eliminate these millions of potential projectiles. 

As shown in Figure 1-2 and listed in Table 1-4, the proposed code change would impact Risk 

Categories III and IV buildings in 22 states in the central U.S., including the entirety of 

Arkansas, Illinois, Indiana, Iowa, Missouri, and Ohio. Significant portions of Alabama, 

Kentucky, Louisiana, Michigan, Minnesota, Mississippi, Nebraska, Oklahoma, Tennessee, 

Texas, and Wisconsin, and small portions of Georgia, Pennsylvania, New York, South Dakota, 

and West Virginia would also be impacted. Of the 22 states, 17 would have a significant portion 

of their state potentially impacted by this proposed code change.

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Table 1-4 States Impacted by Proposed Code Change related to Roof Aggregate

Portion of State Land Mass States Impacted

Entire State AR IL IN IA MO OH

Nearly Entire State KY OK TN

Significant Portion of State AL LA MI MN MS NE TX WI

Small Portion of State GA NY PA SD WV

1.8 Purpose and Approach

The purpose of this study is to analyze the potential economic impacts from adoption of this 

proposed change to the IBC by identifying the fraction of the building stock that could be 

impacted from the restriction on aggregate roofs, characterizing alternatives to aggregatesurfaced roofing systems, and estimating the costs associated with these alternative roofing 

systems. The number of buildings potentially impacted will be identified by combining building 

stock data from Hazus (FEMA 2009) by occupancy type, roofing system construction market 

share data from the National Roofing Contractors Association (NRCA 2015) and EPDM Roofing 

Association (ERA 2016), and RS Means roofing system cost data (RS Means 2016a). The results 

will be put into perspective from both a national and regional level, including a comparison of 

the impacts from the existing IBC restrictions on roof aggregate in hurricane-prone regions. 

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2 Estimating Potentially Impacted Roof Construction

The proposed change in the IBC will only impact a small fraction of U.S. roof construction, 

either through re-roofing projects or for roofing of new buildings. The only buildings that are 

eligible to be impacted are buildings that meet four requirements: (1) located within the tornadoprone region, (2) classified as Risk Category III or IV buildings, (3) built with a low slope roof, 

and (4) would be constructed or re-roofed using aggregate-surfaced roofing systems if not for the 

new restriction. This chapter uses Hazus building stock and building occupancy type data to 

estimate the percentage of existing buildings meeting requirements 1 and 2, and uses NRCA and 

ERA roof system construction data to control for requirements 3 and 4 together (no data is 

explicitly available on roof slopes of the existing building stock). 

There is a fifth requirement that cannot be controlled for given the available data. Under 

provisions that were introduced in the 2006 IBC (discussed previously), buildings in the tornadoprone region with mean roof heights of 4.6 m (15 ft) to 33.5 m (110 ft), or approximately one to 

10 stories, depending on terrain exposure, are already restricted from using roof aggregate. The 

building stock data used in this analysis does not include building height or exposure 

information. Not controlling for building height and exposure will bias the final results towards 

overestimation of the impacts of the proposed code change, as many buildings that are already 

restricted from using aggregate roof surfacing will not be excluded from the pool of potentially 

impacted buildings. As described later, assumptions made during the analysis of each of the first 

four requirements also lead to an overestimation of potential impacts. Therefore, the results 

presented in this chapter are effectively upper bounds; the anticipated impacts of the code change 

are expected to be smaller, and perhaps substantially so. 

The percentage of potentially impacted roof construction will be identified by combining 

building stock data by occupancy type from Hazus (FEMA 2009) that are likely to be Risk 

Category III or IV buildings with roofing system construction market share data from the 

National Roofing Association (NRCA 2015) and EPDM Roofing Association (ERA 2016). RS 

Means roofing system cost data (RS Means 2016a) will be used to estimate the cost of 

commonly used aggregate roofing systems, along with non-aggregate alternative systems.

2.1 Hazus Building Stock Data

Hazus provides a standardized methodology to assess losses from earthquakes, hurricane winds, 

and floods (FEMA 2014). It leverages data from the Census Bureau, among other sources, to 

inventory the building stock for the U.S., and provides the data at the census tract level. For this 

study, the census tract data is aggregated to calculate a building count by occupancy type at the 

county level using data in Hazus-MH MR4 Version 1.4 (FEMA 2009). 

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2.2 Building Stock in Tornado-Prone Region

Of the 3219 counties in the Hazus database, 1456 counties are at least partially within the 

tornado-prone region, including 33 metropolitan areas as shown in Figure 2-1. Cities span from 

Dallas, TX (south) to Minneapolis, MN (north) and Amarillo, TX (west) to Cleveland, OH

(east). The following analysis assumes the building stock for the tornado-zone region includes 

the building stock data for these 1456 counties. The full number of buildings is included even for 

counties that are only partially in the tornado-prone region, so building stock counts are 

overestimates, which will slightly bias the impacts reported as a percentage of the U.S. market, 

but should not have much effect on estimated regional impacts.

Figure 2-1 Counties Within the Tornado-Prone Region 

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Buildings with Risk Category III and IV designations (defined in Table 1-2) could include public 

assembly buildings, schools, colleges, universities, daycare centers, hospitals, other patient care 

facilities, utility facilities, facilities that handle dangerous materials, emergency services, and any 

building with occupancy greater than 5000. Hazus occupancy types likely to be designated as 

Risk Category III or IV structures were mapped against the IBC requirements shown in Table 

2-1, which include: institutional residential facilities, nursing homes, hospitals, medical offices, 

entertainment, theaters, general government services, government emergency centers, schools, 

and colleges. 

Note that these selections are an approximation, as the occupancy categories in Hazus and the 

Risk Categories in the IBC are not identical, and no information is available on the occupant load 

of the building stock, which is a factor in the IBC table. For example, only schools with 

occupant loads greater than 250 and college buildings with occupant loads greater than 300 are 

required to be Risk Category III buildings per IBC, but since no occupant load information is 

available in the Hazus building stock data, all school and college buildings in the Hazus data 

were assumed to be Risk Category III. Similarly, minimum occupant loads for other 

occupancies, such as places of public assembly and medical care facilities (I-2 occupancy in 

Table 1-2), could not be controlled for, so all of the Hazus buildings having occupancy types of 

entertainment, theater, hospital, and medical office were assigned to Risk Category III/IV. 

Although less common than the preceding problem of over-assigning building stock to Risk 

Category III/IV, it would also be expected that some existing buildings which are Risk Category 

III/IV would not be mapped as such in Table 2-1. For example, churches and other places of 

religious worship with occupant loads greater than 250 would be considered places of public 

assembly in the IBC and assigned to Risk Category III. While low-slope, aggregate-surfaced 

roofs are not commonly used for churches, there are likely a modest number of religious 

occupancy buildings that would fit this category. Another example would include certain 

industrial and agricultural buildings that contain sufficient quantities of toxic or explosive 

substances that are dangerous to the public if released. These would be assigned Risk Category 

III or IV by the IBC. 

The net effect of data limitations on the assignment of Risk Category to Hazus building stock 

data may bias the final results towards overestimation of the impacts of the proposed code 

change, because a larger percentage of the existing building were assigned Risk Category III/IV 

when they are in fact in other risk categories.

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Table 2-1 Hazus Occupancy Types and Assignment of Risk Categories

Occupancy

Type

Risk 

Category 

III or IV

Occupancy

Type

Risk 

Category 

III or IV

RES1I Residential Single-Family COM6I Hospital X

RES2I Residential Manufactured Housing COM7I Medical Office X

RES3AI Residential Duplex COM8I Entertainment X

RES3BI Residential 3-4 Units COM9I Theaters X

RES3CI Residential 5-9 Units COM10I Parking

RES3DI Residential 10-19 Units IND1I Heavy Industrial

RES3EI Residential 20-49 Units IND2I Light Industrial

RES3FI Residential 50+ Units IND3I Food/Drug

RES4I Residential Temp Lodging IND4I Metals

RES5I Residential Institutional X IND5I High Tech

RES6I Residential Nursing Home X IND6I Construction

COM1I Retail Trade AGR1I Agriculture

COM2I Wholesale Trade REL1I Religious

COM3I Personal Service GOV1I General Services X

COM4I Professional GOV2I Emergency Center X

COM5I Banking EDU1I Schools X

EDU2I Colleges X

Figure 2-2 shows the estimated percentage of existing Risk Category III and IV buildings in the 

tornado-prone region based on the definition of building stock and applicable building 

occupancy types from Table 2-1. The percent of Risk Category III and IV buildings varies 

significantly, from 0 to 5 %, depending on the county, with “hotspots” in and around the 

metropolitan areas mentioned previously. 

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Figure 2-2 Estimated Percentage of Risk Category III and IV Buildings, by County

Aggregation of the building stock for the tornado-prone region compared to the U.S. building 

stock is shown in Table 2-2. The tornado-prone region accounts for 36 % of the building stock, 

ranging from 22 % to 43 % depending on the occupancy type. Of the occupancy types identified 

as Risk Category III and IV buildings, the tornado-prone region accounts for 29 % to 37 % 

depending on the occupancy type, with an overall average of 33 %. The Risk Category III and IV 

buildings are estimated to comprise 1.5 % of the building stock in the tornado-prone region. 

Low-rise residential buildings, including single-family homes, manufactured housing, duplexes, 

and small apartment buildings, typically have non-low-slope roofs and should not be considered 

in the same roofing market. Excluding all residential buildings with fewer than 50 units, the total 

U.S. building stock is reduced from 110.3 million to 10.1 million (91 % reduction) with similar 

effects in the tornado-prone region. The building stock occupancy types for Risk Category III 

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and IV buildings in the tornado-prone region becomes 6.0 % of the entire U.S. building stock 

(excluding residential with less than 50 units) and 17.6 % of the building stock in the tornadoprone region (excluding residential with less than 50 units). For the remainder of this study, 

residential buildings with fewer than 50 units will be considered synonymous with “low-rise 

residential buildings.” 

Table 2-2 Existing Building Stock by Hazus Occupancy Type – Tornado-Prone Region

Occupancy Type Building Count

Total U.S. Tornado -

Prone Region

Fraction of U.S. in 

Tornado-prone Region

RES1I Residential Single-Family 77 341 549 27 821 964 36 %

RES2I Residential Manufactured Housing 8 585 222 2 874 686 33 %

RES3AI Residential Duplex 4 701 077 1 748 133 37 %

RES3BI Residential 3-4 Units 3 693 939 1 362 735 37 %

RES3CI Residential 5-9 Units 2 649 603 1 011 118 38 %

RES3DI Residential 10-19 Units 1 838 264 683 031 37 %

RES3EI Residential 20-49 Units 1 389 157 503 242 36 %

RES3FI Residential 50+ Units 1 059 443 349 569 33 %

RES4I Temp Lodging 86 318 23 711 27 %

RES5I Institutional 205 116 67 527 33 %

RES6I Nursing Home 40 295 14 381 36 %

COM1I Retail Trade 983 783 335 682 34 %

COM2I Wholesale Trade 707 373 243 929 34 %

COM3I Personal Service 1 039 452 362 089 35 %

COM4I Professional 1 565 278 501 562 32 %

COM5I Banking 141 214 53 019 38 %

COM6I Hospital 27 440 10 196 37 %

COM7I Medical Office 404 628 124 365 31 %

COM8I Entertainment 763 363 250 217 33 %

COM9I Theaters 25 326 7273 29 %

COM10I Parking 0 0 NA

IND1I Heavy Industrial 278 759 112 012 40 %

IND2I Light Industrial 307 080 100 918 33 %

IND3I Food/Drug 73 066 24 089 33 %

IND4I Metals 43 899 18 029 41 %

IND5I High Tech 8 706 1 917 22 %

IND6I Construction 931 380 302 842 33 %

AGR1I Agriculture 503 485 215 968 43 %

REL1I Religious 504 437 187 182 37 %

GOV1I General Services 154 613 56 603 37 %

GOV2I Emergency Center 30 576 10 966 36 %

EDU1I Schools 176 226 57 946 33 %

EDU2I Colleges 19 987 7078 35 %

TOTAL 110 280 054 39 443 979 36 %

TOTAL (Excluding Residential < 50 units) 10 081 243 3 439 070 34 %

TOTAL (Risk Cat. III & IV Occ. Types) 1 847 570 606 552 33 %

Note: Bold showing Risk Category III and IV buildings

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The above building stock estimates include all buildings in each occupancy type. However, not 

all buildings have low-slope roofs, and not all buildings with low-slope roofs are surfaced with 

roof aggregate. The National Roofing Contractors Association Annual Market Survey (NRCA 

2015) estimates that built-up roofs (BURs) (hot-applied asphalt or coal tar, and cold process) 

accounted for 5.2 % of all new low-slope roof construction and 9.9 % of all re-roofing of lowslope roof projects nationwide in 2014. Ethylene propylene diene monomer (EPDM), which may 

include a layer of aggregate surfacing, accounted for 27 % of all new construction and 24.1 % of 

all re-roofing projects nationwide. According to the EPDM Roofing Association (ERA 2016) 

approximately 35 % of new EPDM roofing systems are ballasted, which is assumed in this study 

to be aggregate surfaced.

6

 In combination, BURs and EPDM with aggregate ballast account for 

an estimated 14.7 % of new construction and 18.3 % of re-roofing for low-slope roof 

construction.7 However, usage of roofing system type varies throughout the country, so regional 

data will be used to provide estimates specific to the tornado-prone region. 

Table 2-3 shows the breakdown of BUR and EPDM roofs (NRCA 2015) for new construction 

and re-roofing projects for all census divisions (see Figure 2-3) that are partially included in the 

tornado prone region. The mix of roofing system types can be seen to vary in different parts of 

the country. Averages are computed by weighting the values for each census division by their 

relative fraction of Risk Category III and IV buildings in the tornado-prone region. BUR and 

EPDM roofs with aggregate ballast account for an estimated 12.2 % of new construction and 

16.9 % of re-roofing for low-slope roofs in the tornado-prone region. These values are 

somewhat lower than the estimated national averages of 14.7 % and 18.3 % for new construction 

and re-roofing, respectively.

 

6 Single-ply ballasted systems include both aggregate surfaced and paver surfaced. The assumption that all ballasted 

systems use aggregate is a conservative estimate for this analysis as the paver surfaced systems would not be 

impacted by the proposed code change. 

7 Although built-up roofs can be surfaced with aggregate, a cap sheet or liquid-applied coating, the analysis 

conservatively assumes that all built-up roofs are surfaced with aggregate.

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Table 2-3 Percentages of Low-Slope Roofing Projects by Roof System Type by Census 

Division in the Tornado-Prone Region

Census

Division

Fraction 

Risk Cat. 

III & IV 

Buildings# 

(%)

New Construction (%) Re-roofing (%)

BUR EPDM Total* BUR EPDM Total*

US 100 5.2 27.1 14.7 9.9 24.1 18.3

Mid-Atlantic 5 4.1 40.0 18.1 7.0 33.3 18.7

South Atlantic 1 9.6 14.4 14.6 13.0 11.3 17.0

East North Central 43 1.4 35.2 13.7 4.5 33.0 16.1

East South Central 16 3.8 11.0 7.7 5.5 11.1 9.4

West North Central 19 3.3 45.0 19.1 12.1 52.2 30.4

West South Central 15 1.4 2.5 2.3 9.0 1.2 9.4

Tornado-Prone Region Avg 12.2 Avg 16.9

#Percentage of tornado-prone region Risk Category III/IV buildings

*Assumes 35 % of EPDM systems are ballasted 

Note: TPO and PVC membranes are excluded because these systems are not typically 

surfaced with aggregate.

Figure 2-3 Census Divisions8

 

8 Source: NOAA (2016d). Enhancements by NIST.

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Based on these estimates of market share for aggregate-surfaced roof types and the number of 

Risk Category III and IV buildings as a percentage of the total building stock9, approximately 

0.07 % of all new U.S. roof construction would be potentially impacted and approximately 0.1 % 

of all U.S. re-roofing projects would potentially be impacted by the proposed code change (see 

Table 2-4). These numbers change to 0.7 % and 1.0 % of all U.S. roofing construction when 

excluding residential buildings with less than 50 units, respectively, 0.2 % and 0.3 % of all 

buildings in the tornado-prone region, respectively, and 2.2 % to 3.0 % of all buildings in the 

tornado-prone region excluding residential buildings with less than 50 units, respectively. 

Table 2-4 Upper Bound Estimate of Percentage of Roof Construction Potentially Impacted 

by Proposed Code Change 

Number of Risk Category III and IV Buildings in the 

Tornado-Prone Region with Low-Slope Roofs having 

Aggregate Surfacing

New 

Construction

Reroofing 

Fraction of U.S. Building Stock 0.07 % 0.10 %

Fraction of U.S. Building Stock Excluding Residential 

Buildings with < 50 units 

0.7 % 1.0 %

Fraction of Tornado-Prone Region Building Stock 0.2 % 0.3 %

Fraction of Tornado-Prone Region Building Stock Excluding 

Residential Buildings with < 50 units 

2.2 % 3.0 %

Limiting the building stock data to those buildings that are expected to have a low-slope roof, 

located within the tornado-prone region, with the Risk Category III or IV designation, and would 

be constructed using aggregate-surfaced roofing systems if not for the new restriction reduces the 

estimated fraction of the building stock potentially impacted by the proposed code change to 

less than 0.1 % of the total U.S. roof construction and less than 3.0 % of the non-low rise 

residential roof construction within the tornado-prone region. 

These results are effectively upper bound estimates; the actual values would be smaller, perhaps 

substantially so. This is due to limitations of the data and assumptions made in the analysis, all of 

which tend to bias the final results towards overestimation of the impacts of the proposed code 

change. These biases, which have been described throughout this chapter, are summarized in the 

following paragraphs. 

 

9 Assuming the future rate of construction of Risk Category III and IV buildings retains the same proportional mix 

with other Risk Category buildings as in the existing building stock within the tornado-prone region.

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At the beginning of this chapter, five requirements were identified for buildings to be potentially 

impacted by the proposed code change: (1) located within the tornado-prone region; (2) 

classified as Risk Category III or IV structures; (3) built with a low-slope roof; (4) not already 

restricted from using aggregate surfacing based on building height and exposure; and (5) would 

be constructed or re-roofed using aggregate surfaced roofing systems if not for the new 

restriction. Biases introduced in each step of the analysis are: 

(1) Location within the tornado-prone region: A small part of the tornado-prone region in 

Mississippi and Alabama is also in the hurricane-prone region. Roofs of all buildings in this 

overlap region are already prohibited from using aggregate surfacing. 

(2) Classification of Risk Category III and IV buildings: The number of buildings from the 

Hazus database assigned to Risk Categories III and IV is an overestimate due to differences 

between the occupancy categories defined by IBC and those used in Hazus, and the 

procedures used to map between the two sets of occupancy definitions.

(3) Built with a low-slope roof: Having no data on the percentage of Risk Category III and 

IV buildings with low-slope roofs, it was assumed that all of these buildings had low-slope 

roofs.

(4) Existing IBC prohibition on aggregate surfacing: As described in Section 1.6, IBC 

Section 1504.8 restricts the use of aggregate surfacing outside of the hurricane-prone region 

based on design wind speed, mean roof height, and terrain exposure. In the tornado-prone 

region, this limitation translates to restrictions ranging from buildings of one to 10 stories in 

height, depending on the exposure category. Since no data are available on the heights and 

exposures of the Risk Category III and IV buildings, it was assumed that all buildings were 

low enough not to be subject to the existing restrictions. 

(5) Current market share for aggregate surfaced roofs: The available roofing industry 

market data does not fully differentiate roofing system types by aggregate surfacing. 

Although built-up roofs can be surfaced with aggregate, a cap sheet or liquid-applied coating, 

it was assumed that all BUR were surfaced with aggregate. Similarly, while EPDM roofs 

can use aggregate or pavers for ballast, it was assumed that all singly-ply ballasted systems 

were surfaced with aggregate (see footnotes 7 and 8). 

As demonstrated, the analysis tended to overestimate the number of buildings meeting each of 

the five requirements, meaning the potential impacts are less than would be inferred from the 

values in Table 2-4. These results are upper bounds on the estimated impacts. 

2.3 Comparison to the Hurricane-Prone Region 

In addition to comparisons to the U.S. building stock, it is of interest to compare the potential 

impacts of the proposed change to the impacts of the change to the 2006 IBC that prohibited

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aggregate-surfaced roofs for all Risk Category buildings in hurricane-prone regions. Figure 2-4

shows both the proposed tornado-prone region as well as the hurricane-prone region (hatch 

marked contour). Although the tornado-prone region covers a larger land area than the hurricaneprone region, the map does not account for the densities of building stock or the fact that all 

building types in the hurricane-prone region are impacted by the roof aggregate prohibition, 

whereas the proposed code change in the tornado-prone region applies only to Risk Category III 

and IV buildings A small part of the tornado prone region, in central Mississippi and Alabama, 

is also part of the hurricane-prone region, so the proposed code change would have no effect 

there as aggregate-surfaced roofs are already prohibited. 

Figure 2-4 Map of Existing Hurricane-Prone and Proposed Tornado-Prone Regions

Figure 2-5 shows the county level total building stock data for the contiguous U.S. along with the 

outline of both regions. The hurricane-prone region (seaward to the red dotted line) includes 415 

counties compared to the tornado-prone region’s 1456 counties. However, the hurricane-prone 

region appears to have a heavier building density. Given a potentially higher building density 

along the coasts combined with the prohibition on aggregate for all risk categories, not just Risk 

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Category III and IV, it would be expected that the number of potentially impacted buildings may 

be as large or larger for the hurricane-prone region than those in the tornado-prone region. 

Figure 2-5 Building Stock Counts by County with Hurricane and Tornado-Prone Regions

Table 2-5 shows that the hurricane-prone region has fewer Risk Categories III and IV buildings 

(457 483) than the tornado-prone region (606 552). However, the prohibition on aggregate 

surfaced roofs applies to all buildings of all four risk categories in the hurricane-prone region, 

which can be estimated using the total building stock excluding residential buildings with fewer 

than 50 units. The number of buildings potentially impacted by the roof aggregate ban in 

hurricane-prone regions totals 2 461 185. 

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Table 2-5 Existing Building Stock by Occupancy – Tornado- vs Hurricane-Prone Region

Occupancy Type Building Count

Tornado -

Prone 

Hurricane -

Prone Region

RES1I Residential Single-Family 27 821 964 16 263 471

RES2I Residential Manufactured Housing 2 874 686 2 308 025

RES3AI Residential Duplex 1 748 133 1 146 156

RES3BI Residential 3-4 Units 1 362 735 820 090

RES3CI Residential 5-9 Units 1 011 118 539 744

RES3DI Residential 10-19 Units 683 031 381 593

RES3EI Residential 20-49 Units 503 242 312 753

RES3FI Residential 50+ Units 349 569 272 393

RES4I Temp Lodging 23 711 20 180

RES5I Institutional 67 527 45 950

RES6I Nursing Home 14 381 9368

COM1I Retail Trade 335 682 244 115

COM2I Wholesale Trade 243 929 170 361

COM3I Personal Service 362 089 251 151

COM4I Professional 501 562 396 219

COM5I Banking 53 019 32 709

COM6I Hospital 10 196 6786

COM7I Medical Office 124 365 109 496

COM8I Entertainment 250 217 194 108

COM9I Theaters 7273 7234

COM10I Parking 0 0

IND1I Heavy Industrial 112 012 58 669

IND2I Light Industrial 100 918 73 385

IND3I Food/Drug 24 089 16 158

IND4I Metals 18 029 9245

IND5I High Tech 1 917 1714

IND6I Construction 302 842 240 953

AGR1I Agriculture 215 968 96 428

REL1I Religious 187 182 120 022

GOV1I General Services 56 603 31 749

GOV2I Emergency Center 10 966 6553

EDU1I Schools 57 946 41 219

EDU2I Colleges 7078 5020

TOTAL US 39 443 979 24 233 017

TOTAL US (excl. Residential < 50 units) 3 439 070 2 461 185

TOTAL Risk Cat. III and IV 606 552 457 483

As in the case of the tornado-prone region, the accurate number of impacted buildings needs to 

account for the fact that aggregate surfacing is used for only a fraction of all low-slope roofing 

projects, which varies by Census Division. Table 2-6 shows the percentages of low-slope roofing 

work by BUR and EDPM roof systems. Assuming approximately 35 % of EDPM roof systems 

are aggregate ballasted, Table 2-6 also shows that the average fraction of roofing construction in 

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the hurricane-prone region that would have used aggregate if not for the adopted ban in the IBC 

is 14.0 % for new construction and 16.9 % for re-roofing, leading to the total number of 

impacted buildings of 344 566 and 415 940, respectively, which is 0.31 % and 0.38 %, 

respectively, of the entire U.S. building stock (3.4 % and 4.1 %, respectively, if excluding 

residential buildings with less than 50 units). In comparison, similar estimates of the potential 

effects of the proposed code change in the tornado-prone region (from Table 2-5) are much 

smaller, 0.07 % of all new U.S. roof construction and 0.1 % of all U.S. re-roofing projects (0.7 % 

and 1.0 % respectively, if excluding residential buildings with less than 50 units). 

Table 2-6 Percentages of Low-Slope Roofing Projects by Roof System Type by Census 

Division in Hurricane-Prone Region

Census

Division

Fraction 

All 

Buildings#

(%)

New Construction (%) Reroofing (%)

BUR EPDM Total* BUR EPDM Total*

US 100 5.2 27.1 14.7 9.9 24.1 18.3

New England 20.2 1.5 51.5 19.5 2.0 58.3 22.4

Mid-Atlantic 16.9 4.1 40.0 18.1 7.0 33.3 18.7

South Atlantic 43.5 9.6 14.4 14.6 13.0 11.3 17.0

East South Central 4.4 3.8 11.0 7.7 5.5 11.1 9.4

West South Central 15.0 1.4 2.5 2.3 9.0 1.2 9.4

Hurricane-Prone Region Avg 14.0 Avg 16.9

#Percentage of all hurricane-prone region buildings

*Assumes 35 % of EPDM systems are aggregate ballasted

Assuming new construction and re-roofing occur at rates proportional to the applicable elements 

of existing building stock in each of the two regions, the existing ban on aggregate-surfaced 

roofs in the hurricane-prone region applies to more than four times as much roof construction as 

the proposed restrictions in the tornado-prone region. This is a lower bound estimate of the ratio 

between the impacts in the hurricane prone versus tornado prone region, the actual ratio is likely 

much higher, for several reasons. Let’s again consider the biases in analyses of the five 

requirements for estimating impacts in the tornado-prone region, but in comparison with how the 

data limitations and assumptions bias the estimates in the hurricane-prone region.

(1) Location within the tornado-prone region: A small part of the tornado-prone region in 

Mississippi and Alabama is also in the hurricane-prone region. Roofs of all buildings in this 

overlap region are already prohibited from using aggregate surfacing, so the estimated 

impacts in the tornado-prone regions are slightly too large. 

(2) Classification of Risk Category III and IV buildings: In the tornado-prone region, the 

mapping between Hazus occupancy types and IBC Risk Category requirements led to an 

overestimate of the number of Risk Category III and IV buildings. However, since the 

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prohibition on aggregate in the hurricane-prone region applies to all Risk Category buildings, 

the building stock data is used directly, with no overestimation bias. 

(3) Built with a low-slope roof: No difference between assumptions for tornado- and 

hurricane-prone regions. 

(4) Existing IBC prohibition on aggregate surfacing: In the tornado-prone region, due to 

data limitations, it was assumed that all buildings were low enough not to be subject to the 

existing restrictions based on height and terrain exposure, leading to an overestimate. 

However, in the hurricane-prone region, the prohibition on aggregate applies to all buildings 

regardless of height and exposure. Therefore, the building stock data is used directly, with no 

overestimation bias.

(5) Current market share for aggregate surfaced: The NRCA regional roofing industry data 

used to estimate the market share of aggregate-surfaced roofs includes the effects of the 

prohibition on aggregate in the hurricane prone region that was introduced in the 2006 IBC. 

Therefore, in census divisions which include parts of the hurricane-prone region, roofs that 

would otherwise have been constructed with aggregate-surfacing have switched to other roof 

surfacing systems. This perhaps explains why the total estimated market share for aggregatesurfaced roofs in new construction in the three census divisions that are partly in the 

hurricane-prone region is smaller than in any other census divisions east of the continental 

divide. This underestimation of market share introduces the same biases in both the 

hurricane- and tornado-prone region. However, most (62 %) of the Risk Category III and IV 

buildings in the tornado-prone region are located in the Midwest and Great Plains states, 

represented in the East North Central and West North Central census divisions (see Figure 2-

3 and Table 2-4), which are not subject to this particular bias. 

One additional assumption also biases the results towards relative overestimation of impacts 

in the tornado-prone region compared to the hurricane-prone region. It was assumed that the 

proportion of existing construction of potentially impacted buildings would continue in 

relation to such proportions in the existing building stock. However, no assumption was 

made as to the rate of this construction. The value for new construction along the East and 

Gulf Coasts coastal areas exceed those in the interior of the country, biasing the comparative 

analysis to underestimate impacts in hurricane- versus tornado-prone regions. The value of 

private non-residential construction put in place is greatest for the West South Central, South 

Atlantic, and Middle Atlantic census divisions, accounting for 24 %, 16 %, and 13 % of the 

U.S. total in 2015, respectively (Census 2016). 

Of the six factors described above, five are biased toward overestimation of the number of 

potentially impacted buildings in the tornado-prone region compared to the hurricane-prone 

region (and the remaining factor has the same relative biases for both regions). Therefore, the 

estimate that the impact of the potential code change in the tornado-prone region is less than one 

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fourth of the current impacts of aggregate restrictions in the hurricane-prone region is an upper 

bound estimate. The relative impact would be even smaller. 

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3 Costs of Alternative Roofing Systems

To quantify the potential economic impacts of adopting the proposed change to the IBC, it is 

necessary to estimate the costs of installing a low-slope roof with and without the use of 

aggregate surfacing. The cost comparisons are dependent on the roof deck, roofing system type, 

and the selected top layer of the roofing system. 

3.1 Alternative Roof Systems 

There are a wide range of low-slope roofing systems, including built-up, single-ply membrane, 

and liquid-applied systems. To condense the number of systems for comparison, only the most 

commonly used roofing systems were considered, those that use aggregate and commonly 

available alternatives that do not. This study considers three different types of roofing structural 

support with at least two roofing system options for each, one with aggregate surfacing as the 

baseline system and one or more alternatives that do not include aggregate. Table 3-1 shows the 

seven roofing systems considered, including two built-up roof deck, two concrete deck, and three 

steel deck systems. It is assumed that the roof deck is selected before selecting the roofing 

system.

Table 3-1 Roofing Assembly Alternatives

Surfacing Roof Deck Roofing System

Built-Up Roof Any Deck Type 4-ply membrane with aggregate surfacing

3-ply membrane with cap sheet

Single-Ply

Concrete Deck Aggregate ballasted

Fully adhered 

Steel Deck

Aggregate ballasted

Mechanically attached membrane

Fully adhered 

3.1.1 Built-Up Roof Systems

A built-up roof (BUR) system is composed of alternating layers of bitumen (typically asphalt, 

coal tar or cold-applied adhesives) and reinforcing fabrics (roofing felt) with a surfacing 

(aggregate or mineral surfaced cap sheets), or a smooth surfacing (hot asphalt glaze coat, 

aluminum-pigmented asphalt, or elastomeric coating). The two built-up roof systems selected for 

this study are a (1) traditional 4-ply hot asphalt with an aggregate surfacing and (2) traditional 3-

ply hot asphalt with a mineral surface cap sheet. Figure 3-1 shows an example of a hot asphalt 

system with aggregate surfacing (left) and a hot asphalt system with a cap sheet (right). 

However, a cap sheet surfacing decreases the weight needed to be supported by the roof deck. 

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Figure 3-1 Example Built-Up Roof Asphalt with Aggregate (left)10 and Cap Sheet (right)11

For a cost comparison of the BUR systems, only the number of ply sheets and surfacing are 

considered because the remainder of the assembly will be the same for the two options, including 

the deck and insulation.

12

3.1.2 Single-Ply Membrane Roof Systems 

A single-ply membrane roof system includes a single layer of material instead of multiple layers 

as in built-up roof systems. Single-ply membranes include thermosets and thermoplastics. The 

most common thermoset membrane is EPDM while common thermoplastics are polyvinyl 

chloride (PVC) and thermoplastic polyolefin (TPO). Single-ply membranes can be attached to 

the roof deck through several methods as shown in Figure 3-2: loosely laid and aggregate 

ballasted (left), fully adhered (center), and mechanically attached (right). 

Figure 3-2 Example Single-Ply Roof with Aggregate Ballast (left), Fully Adhered (center), 

and Mechanically Attached (right)13

The three single-ply systems selected for this study are a (1) EPDM with aggregate ballast, (2)

EPDM fully adhered, and (3) EPDM mechanically fastened. EPDM is selected because it is 

commonly ballasted, whereas PVC and TPO are typically adhered or mechanically attached. 

 

10 Image Source: http://www.staterfg.com/wp-content/uploads/2013/07/systemA21.gif 11 Image Source: http://www.fromridgetoeave.com/top-tips-for-mounting-pv-on-low-slope-roofing/ 12 Cap sheets can be traditional asphalt/mineral surfaced or modified bituminous. 13 Image Source: ERA 2016. 

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Aggregate Ballasted EPDM is loose laid with aggregate (or pavers) spread out on the membrane 

to keep it from being uplifted in high winds. Fully adhered EPDM is attached with adhesives. 

Mechanically attached EPDM is loose laid with fasteners attaching the membrane to the roof 

deck.

14 

Each of these options will be considered for both a concrete roof deck and steel roof deck. The 

EPDM membrane is assumed to be 1.143 mm (45 mil) for all three membrane securement

options and roof decks (i.e., concrete or steel). The aggregate ballasted option assumes 49 kg/m2

(10 lb/ft2

) of aggregate with a protection mat regardless of the roof deck. To make comparisons 

to the aggregate ballasted system, the fully adhered EPDM will need to include the adhesive 

between the membrane and insulation and attaching the insulation (with either adhesives or 

fasteners) in cost estimates.

Each of the five roof covering alternatives are mapped to the RS Means cost databases as shown 

in Table 3-2, which is based on UNIFORMAT II (standard for classifying building elements) for 

assembly cost data and MASTERFORMAT (standard for classifying building products) for unit 

cost data. 

Table 3-2 Roofing Systems and Roof Components in RS Means Databases

Roofing Assembly UNIFORMAT II RS Means Description

Asphalt 4-ply w/ aggregate 

surfacing

B30101051600 Roofing, asphalt flood coat, gravel, base 

sheet, 4 plies 15# asphalt felt, mopped

3-ply w/ cap sheet

B30101055700 Roofing, asphalt mineral surface, roll, 3 

plies glass fiber felt (Type IV), 1 ply 

mineral surfaced selvage roofing, lap 1.9", 

mopped

Single-Ply 

EPDM

Aggregate 

ballasted

B30101202100 Roofing, single ply membrane, EPDM, 45 

mils, loosely laid, stone ballast (10 PSF)

Fully adhered B30101202000 Roofing, single ply membrane, EPDM, 

45mils, fully adhered

Mechanically 

attached B30101202200 Roofing, single ply membrane, EPDM, 45 mils, mechanically fastened, batten strips

Roofing Component MASTERFORMAT RS Means Description

Protection 

Mat

075510100130 Protected membrane roofing components, 

filter fabric

Insulation 

Installation

Installation 

fastener

072216103010 4” Coated Screws (1 per ft2)

 

14 Mechanically attached EPDM to concrete decks is uncommon because there are less costly alternatives, and is 

typical used only under special circumstances.

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3.2 Roofing Cost Data, Comparisons, and Implications

The mapping of the seven roofing assemblies to the RS Means cost data (based on 

UNIFORMAT II and MASTERFORMAT) allows for comparisons across alternative systems 

given the assumed roof deck. Table 3-3 shows the national average cost per unit of area for each 

of the roofing systems. 

Table 3-3 Roofing System Cost Data – National Average

Roofing System $/m2 ($/ft2

)

Built-Up 

Roof

Traditional 

Hot Asphalt

4-ply membrane with aggregate surfacing $36.27 ($3.37)

3-ply membrane with cap sheet $31.75 ($2.95)

Single-Ply 

EPDM

Concrete Deck

EPDM - Aggregate ballasted $17.11 ($1.59)

EPDM - Fully adhered $25.83 ($2.40)

Steel Deck

EPDM - Aggregate ballasted $17.11 ($1.59)

EPDM - Mechanically attached membrane $16.58 ($1.54)

EPDM - Fully adhered $25.83 ($2.40)

For a built-up roof, the system with aggregate surfacing (B30101051600) leads to higher costs

than the alternative system that replaces the aggregate with a cap sheet (B30101055700). These 

additional costs are driven by slightly higher material costs ($0.54/m2 or $0.05/ft2

) and 

significantly higher labor costs ($3.98/m2 or $0.37/ft2). 

For the single-ply system for a building with a steel deck, the mechanically attached EPDM 

(B30101202200) is the less expensive system ($16.58/m2 or $1.54/ft2) followed by the aggregate 

ballasted EPDM system ($17.11/m2 or $1.59/ft2) because the ballasted system (B30101202100) 

uses a protection mat (075510100130), which adds $2.15/m2

 ($0.20/ft2). The ballasted system 

assumes no additional cost to the roof structure to handle the additional weight of the aggregate, 

which may vary depending on whether the deck is steel or concrete. The materials are less costly, 

but labor costs are higher for the mechanically attached assembly. 

The most expensive single-ply system is fully adhered EPDM ($25.83/m2 or $2.40/ft2

), which 

includes the fully adhered single-ply EPDM assembly (B30101202000) and the cost of attaching

the insulation under the single-ply (072216103010). Only the cost of attaching the insulation is 

required because the insulation itself would be included in the ballasted and mechanically 

attached systems, and would not be independently fastened to the roof deck. The most common 

insulation attachment method is foam ribbon adhesive. However, data is not available for the 

adhesive, leading to the assumption that insulation is attached using coated screws, which is 

expected to be more expensive than foam ribbon adhesive and should lead to a conservative 

(higher) cost estimate. The additional $8.72/m2 or $0.81/ft2 (51 %) relative to the ballasted 

system are a result of higher costs for both the materials and labor. 

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Given that material and labor costs may vary significantly by location, these same cost estimates 

are completed for the 33 metropolitan areas shown in Figure 2-1. Table 3-4 shows the cost for 

five roofing systems, two BUR and three EDPM, across 33 cities throughout the tornado-prone 

region. Regardless of the location, the optimal choices remain the same. The built-up system 

with the cap sheet saves between $3.01/m2 ($0.28/ft2) and $5.92/m2 ($0.55/ft2) relative to the 

built-up system with aggregate. The mechanically attached EPDM system lowers costs by 

$0.11/m2

 ($0.01/ft2) to $1.08/m2 ($0.10/ft2) while the fully adhered EPDM system increases 

costs by $6.14/m2 ($0.57/ft2) to $10.87/m2 ($1.01/ft2) relative to the aggregate ballasted EPDM 

system. Note that the mechanically attached EPDM system cost estimates only apply to 

assemblies with steel decks.

Based on these results, the only new roof construction or re-roofing that would be negatively

impacted from a construction cost perspective by the code change are those that would have 

otherwise installed a ballasted single-ply membrane on a concrete deck. Any installation of BUR 

systems would realize lower costs using a cap sheet instead of aggregate surfacing, which lowers 

the fraction of Risk Category III/IV buildings that could potentially realize higher costs from the 

code change proposal to under 10 % for both new construction (9.8 %) and reroofing projects 

(9.7 %). This is a drop from 0.2 % for new construction and 0.3 % for reroofing to less than 

0.2 % of the total building stock in the tornado-prone region and from 2.2 % to 3.0 % to 1.7 % of 

the non-low rise residential roof construction in the tornado-prone region. Additionally, any roof 

assemblies with steel decks would lower costs using a mechanically attached EPDM instead of 

EPDM with aggregate ballast, further lowering the number of roof construction negatively

impacted by the change in the code to restrict roof aggregate.

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Table 3-4 Assembly Cost by Location in Tornado-Prone Region

Location Cost ($/ft2)

Asphalt –

Aggregate 

Surfacing

Asphalt –

Cap Sheet 

EPDM –

Aggregate 

Ballasted

EPDM –

Mechanically 

Attached* 

EPDM –

Fully 

Adhered

National Average 3.37 2.95 1.59 1.54 2.40

AL Birmingham 3.18 2.79 1.54 1.47 2.30

AR Ft Smith 2.70 2.42 1.37 1.27 1.95

AR Little Rock 2.64 2.35 1.32 1.23 1.89

IA Des Moines 2.99 2.65 1.46 1.38 2.14

IA Sioux City 2.92 2.60 1.47 1.38 2.13

IL Chicago 4.05 3.49 1.82 1.81 2.83

IL Springfield 3.64 3.17 1.72 1.67 2.59

IN Indianapolis 3.10 2.74 1.53 1.44 2.23

KS Topeka 2.91 2.58 1.43 1.36 2.12

KS Wichita 2.64 2.36 1.33 1.24 1.92

KY Lexington 3.18 2.81 1.57 1.48 2.28

KY Owensboro 3.17 2.79 1.53 1.46 2.27

MI Detroit 3.57 3.11 1.66 1.62 2.54

MN Minneapolis 3.89 3.36 1.77 1.75 2.76

MN Rochester 3.52 3.09 1.69 1.62 2.52

MO Kansas City 3.33 2.90 1.52 1.50 2.34

MO Springfield 3.04 2.69 1.48 1.40 2.19

MO St. Louis 3.45 3.01 1.61 1.57 2.45

MS Jackson 2.82 2.49 1.38 1.31 2.03

NE Grand Island 3.03 2.69 1.49 1.41 2.18

NE Omaha 3.01 2.65 1.44 1.38 2.16

OH Cincinnati 3.13 2.76 1.52 1.45 2.25

OH Cleveland 3.66 3.21 1.74 1.68 2.62

OK Oklahoma City 2.88 2.56 1.42 1.34 2.08

OK Tulsa 2.82 2.51 1.41 1.32 2.05

TN Memphis 2.86 2.54 1.41 1.34 2.07

TN Nashville 2.78 2.47 1.36 1.29 2.00

TX Abilene 2.85 2.53 1.42 1.33 2.06

TX Amarillo 2.75 2.44 1.37 1.28 1.98

TX Dallas 2.74 2.42 1.34 1.27 1.97

TX Ft Worth 2.73 2.42 1.33 1.26 1.97

WI Green Bay 3.28 2.89 1.62 1.52 2.34

WI Milwaukee 3.57 3.11 1.67 1.63 2.54

Average 3.12 2.75 1.51 1.44 2.24

Minimum 2.64 2.35 1.32 1.23 1.89

Maximum 4.05 3.49 1.82 1.81 2.83

Standard Deviation 0.37 0.30 0.14 0.15 0.25

Conversion: $1/ft2 = $10.76/m2

* Only applicable to steel deck roof assemblies

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3.3 Examples: 5-Story Hospital and 2-Story High School

The costs per unit of roof area does not articulate the magnitude of the cost impacts of 

constructing an entire building. For this reason, two examples are developed to provide 

perspective. The Department of Energy (DOE) Commercial Reference Building for Hospitals 

and High Schools (DOE 2016), shown in Figure 3-3 and Figure 3-4, respectively, have been 

selected, with the resulting cost estimates shown in Table 3-5. 

Table 3-5 Example: Roofing System Costs of 5-Story Hospital – National Average

Building 

Prototype

Roofing System Roofing Costs by Building and Roofing System

Roofing Cost 

per m2 (ft2) 

Total 

Roofing Cost

Percent of 

Building Cost

Hospital* EPDM - Aggregate ballasted $17.11 ($1.59) $63 998 0.12 %

EPDM - Fully adhered $25.83 ($2.40) $96 600 0.19 %

High School** EPDM - Aggregate ballasted $17.11 ($1.59) $203 711 0.85 %

EPDM - Fully adhered $25.83 ($2.40) $307 488 1.28 %

*Total cost of constructing hospital = $51.7 million

**Total cost of constructing high school = $24.0 million

NA = Data Not Available

The hospital has 5-stories, 22 428 m2

 (241 410 ft2) of conditioned floor area, and 3739 m2

(40 250 ft2

) of roof area. Assuming a concrete deck with a single-ply membrane roofing system, 

the fully adhered EPDM system increases costs by $34 615 relative to the aggregate ballasted 

system. However, based on the RS Means Square Foot Cost Estimator (SFCE) (RS Means 

2016b) for a hospital with the characteristics shown in Figure 3-3, the total cost of construction 

would be approximately $51.7 million based on national average cost data. The roofing costs are 

a fraction of a percent of the total costs of constructing the hospital, with the difference between 

the aggregate ballasted and mechanically attached membrane being nearly indistinguishable. The 

additional costs of the fully adhered membrane lead to an increase of 0.07 % in total costs. Since 

the roofing costs are mostly insignificant relative to the cost of constructing the building, an 

increase due to alternative roof systems does not have detrimental impacts on the total cost to the 

building owner. 

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Figure 3-3 Representations of 5-Story Hospital: DOE (left)15 and RS Means (right)16

The high school has 2-stories, 19 592 m2 (210 887 ft2) of conditioned floor area, and 9796 m2

(128 120 ft2

) of roof area. Assuming a concrete deck with a single-ply membrane roofing system, 

the fully adhered EPDM system increases costs by $103 777 relative to the aggregate ballasted 

system. However, based on the RS Means SFCE (RS Means 2016b) for a high school with the 

characteristics shown in Figure 3-4, the total cost of construction would be approximately $24.0 

million based on national average cost data. The roofing costs are a fraction of a percent of the 

total costs of constructing the high school, with the additional costs of the fully adhered 

membrane leading to an increase of 0.43 % in total costs. Since the roofing costs are mostly 

insignificant relative to the cost of constructing the building, an increase due to alternative roof 

systems does not have detrimental impacts on the total cost to the building owner. 

Figure 3-4 Representations of 2-Story High School: DOE (left)17 and RS Means (right)18

From a retrofitting perspective, other building maintenance, repair, and replacement costs not 

involving the roof are far more significant than those for re-roofing projects. For example, the 

cost of installing the windows in the hospital and high school is $415 000 and $1.16 million, 

respectively. Window installation costs are approximately 12 times and 11 times the additional 

marginal costs of the fully adhered system and the aggregate ballasted system, respectively.

Additionally, the fully adhered system will significantly reduce the probability of window 

damage from wind-borne aggregate due to high winds or tornadoes from the buildings own roof.

 

15 Source: DOE 2016 16 Source: RS Means 2016b

17 Source: DOE 2016 18 Source: RS Means 2016b

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4 Summary

This study analyzed the potential impacts from adoption of a proposed change to the IBC that 

would restrict use of roof aggregate surfacing for Risk Categories III and IV buildings in the 

tornado-prone region of the U.S., which includes 22 states (with significant portions of 17 

states). This study identified the fraction of the roof construction that could be impacted from the 

restriction on aggregate surfaced roofs, characterized alternatives to aggregate-surfacing, and 

estimated the costs associated with these alternative roofing systems. Overall, the fraction of roof 

construction impacted and the associated costs with meeting the proposed change to the IBC on 

roof aggregate in tornado-prone region appears to be minimal.

4.1 Potential Impacts of Code Change 

The elimination of potentially millions of windborne missiles from loose aggregate, gravel, and

stone surfacing and ballast on roofs of Risk Category III and IV buildings in the tornado-prone 

region of the U.S. will reduce window and interior damage to these and nearby buildings, and 

reduce injuries resulting from this damage and from windborne aggregate directly. 

The percentage of new roof construction and re-roofing potentially impacted by the proposed 

code change is identified by combining building stock data by occupancy type and roofing 

system construction market share data. Based on available data, it is estimated that the code 

change would potentially impact less than 0.1 % of roof construction in U.S., less than 1.0 % of 

all non-low-rise residential roof construction in the U.S., less than 0.3 % of roof construction in 

the tornado-prone region, and less than 3.0 % of all non-low-rise residential roof construction in 

the tornado-prone region. The previously adopted code change that similarly prohibited 

aggregate-surfaced roofs in the hurricane-prone region impacted more than four times as much 

roof construction. 

Five roofing systems are considered in the cost analysis of alternative roofing systems, including 

two built-up roof systems and three single-ply systems, with some systems considered for 

multiple deck types. For a built-up roof, the system with aggregate surfacing leads to higher 

installation costs than the alternative system that replaces the aggregate with a cap sheet. For the 

single-ply systems, the mechanically attached EPDM has the lowest installed cost (assuming an 

assembly with a steel deck) followed by the aggregate ballasted EPDM system. The fully 

adhered EPDM system is the most expensive of the three options. These results hold for all 33 

cities studied within the tornado-prone region, for which costs were compared using RS Means 

data

Based on these results, the only new roof construction or re-roofing that would be negatively

impacted from a construction cost perspective by the code change are those that would have 

otherwise installed a ballasted single-ply membrane on a concrete deck. Any installation of BUR 

systems would realize lower costs using a cap sheet instead of aggregate surfacing, lowering the 

fraction of Risk Category III/IV buildings that could realize higher costs from the code change 

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proposal to under 10 % for both new construction (9.8 %) and reroofing projects (9.7 %). This is 

a drop from 0.2 % for new construction and 0.3 % for reroofing to less than 0.2 % of the total 

building stock in the tornado-prone region and from 2.2 % to 3.0 % to 1.7 % of the non-low rise 

residential roof construction in the tornado-prone region. Additionally, any roof assemblies with 

steel decks would cost less by using a mechanically attached EPDM instead of EPDM with 

aggregate ballast, further lowering the number of roof construction negatively impacted by the 

change in the code to restrict roof aggregate.

Two example buildings with concrete roof decks were considered to compare the installed costs 

of the EPDM systems. Comparatively, the additional cost of installing a fully adhered EPDM 

compared to a ballasted system is $34 615 for the hospital, or 0.07 %, of the overall costs of 

constructing the hospital ($51.7 million) while the additional costs for the high school is 

$103 777, or 0.43 % of the cost of the high school ($24.0 million). In either case the additional 

costs are a small fraction of the total costs of construction. 

4.2 Limitations 

The analysis completed in this study is limited due to currently available data. Assumptions were 

made, as documented in the report, on interpretation of existing building stock data and roofing 

construction data. The estimates of potential impacts on new construction and re-roofing, as a 

percentage of total roofing construction, are upper bound values, since the analysis tended to 

overestimate the number of buildings that would meet each of the five requirements. The actual 

values would be smaller, perhaps substantially so. Additionally, cost data can vary widely on a 

case-by-case basis. Even though RS Means is a well-respected and relied upon resource, RS 

Means cost data has significant uncertainty because the reported data are averages that may be 

the combination of widely varying values. The analysis does not include any sensitivity or 

uncertainty analysis.

 

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