Selecting Appropriate Mitigation Measures for Floodprone Structures
FEMA 551
February 2007

CHAPTER 7  DRY FLOODPROOFING 

7.1 Introduction 

A dry floodproofed structure is made watertight below the level that needs flood 
protection to prevent floodwaters from entering. Making the structure 
watertight requires sealing the walls with waterproof coatings, impermeable 
membranes, or a supplemental layer of masonry or concrete (Figure 7-1). 

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CAUTION: Dry floodproofing may not be used to bring a substantially damaged or 
substantially improved residential structure into compliance with the 
communitys floodplain management ordinance or law. 
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Figure 7-1. A typical dry floodproofed structure 
(Source: FEMA 312) 

Table 7-1 includes a summary of advantages and disadvantages for using dry 
floodproofing as a mitigation measure. 

Table 7-1. Considerations for Using Dry Floodproofing 

Advantages 
--Dry floodproofing is less costly than other 
retrofitting methods. 
--Does not require the additional land that 
may be needed for levees and floodwalls. 
--May be fundable under FEMA mitigation 
grant programs. 

Disadvantages 
--May not be used to bring a substantially damaged 
or substantially improved residential structure into 
compliance with the communitys floodplain 
management ordinance or law. 
--Dry floodproofing requires human intervention and 
adequate warning to install protective measures. 
--Does not minimize the potential damage from 
high-velocity flood flow and wave action. 
--Ongoing maintenance is required. 
--Flood shields may not be aesthetically pleasing. 

Dry floodproofing a structure includes the following: 
--Using waterproof membranes or other sealants to prevent water from entering 
the structure through the walls 
--Installing watertight shields over windows and doors 
--Installing measures to prevent sewer backup 

7.1.1 Waterproof Membranes 

Installing heavy plastic sheeting or waterproof membrane along a walls exterior 
surface is an effective means of waterproofing (Figure 7-2). The waterproof 
membrane can be installed relatively quickly; however, it does require human 
intervention. The membrane is unsightly and cannot remain in place indefinitely. 
Furthermore, the plastic will deteriorate with continued exposure to solar 
radiation. 

Figure 7-2. Photograph of membrane providing flood protection 
(Source: U.S. Army Corps of Engineers) 

7.1.2 Closures 

In conjunction with a waterproof membrane, openings in the walls need to be 
closed, either with temporary closures or permanently sealed shut (Figure 7-3). 

Figure 7-3. Permanently sealed opening 
(Source: FEMA 259) 

Low window openings at ground level can either have a pre-sized closure fitted 
over their surface or have a low wall constructed around the opening to a height 
above the flood protection elevation (Figure 7-4). 

Figure 7-4. Low wall construction 

Various closure systems can be manufactured to fit the individual openings, 
providing a way for it to be quickly closed and have a watertight seal. These 
types of closures can either be stored in a readily accessible location or 
permanently remain in place. Many of these closures have rubberized seals and 
other components that will require periodic care and maintenance (Figure 7-5). 

Figure 7-5. Small patio gate 
(Source: W.A. Wilson Consulting Services) 

7.1.3 Sewer Backup Protection 

Backup of sanitary sewers into a structure is a major concern due to the health 
hazards. Even after floodwaters have receded, contents and belongings that have 
been exposed to sewage are severely contaminated and can be nearly impossible to 
clean. The five main approaches to protect a structure against sewer backup are 
floor drain plugs, floor drain standpipes, overhead sewers, backup valves, and 
grinder pumps. For a detailed discussion of sewer backup protection, see FEMA 
511, Reducing Damage from Localized Flooding, Chapter 10 (pages 10-9 to 10-11) 
and FEMA 259, Engineering Principles and Practices of Retrofitting Floodprone 
Residential Structures, Section VI-D. 

7.2 Technical Considerations 

The flood characteristics that affect the success of dry floodproofing are flood 
depth, flood duration, flow velocity, amount of warning time, and floodborne 
debris. 

7.2.1 Flood Depth 

The depth of the floodwaters affects the hydrostatic pressure that is exerted on 
walls and floors. Because water is prevented from entering a dry floodproofed 
structure, the exterior pressure on walls and floors is not counteracted from 
the opposite side as it is in a wet floodproofed structure. The ability of walls 
to withstand the hydrostatic pressures depends partly on how the walls are 
constructed: 
--Typical masonry and masonry veneer walls, without reinforcement, can usually 
withstand the pressure exerted by water up to about 3 feet deep. 
--In flood depths exceeding 3 feet, unreinforced masonry and masonry veneer 
walls are much more likely to crack or collapse. An advantage of masonry and 
masonry veneer walls is that their exterior surfaces are resistant to damage by 
moisture and can be made watertight relatively easily with sealants. 
--Typical frame walls are likely to fail at lower flood depths, are more 
difficult to make watertight, and are more vulnerable to damage from moisture. 

7.2.2 Structures with Basements 

If a structure has a basement, the walls and floors must be specifically 
designed to resist hydrostatic pressure. Otherwise the risks associated with dry 
floodproofing a basement are high. Figure 7-6 illustrates how hydrostatic 
pressure operates on a structure during a flood. Structure a has no basement 
and the forces acting upon the structure are relatively small. However, for 
structure b, the forces are significantly greater due to the presence of a 
basement. 

Figure 7-6. Hydrostatic pressures on a structure 
(Source: FEMA 312) 

7.2.3 Flood Duration 

The longer a structure is exposed to floodwaters, the more likely it is that 
structural systems and floodproofing measures will begin to leak or fail. Most 
sealing systems will begin to allow some amount of seepage after prolonged 
periods of exposure to water. If the structure is in an area where high 
floodwaters can remain for days, a different retrofitting method should be used. 

7.2.4 Flow Velocity 

A seemingly low flow velocity of 1 or 2 feet per second can exert tremendous 
forces on a structure with only a few feet of flooding. Structures can be easily 
moved off of their foundations, which results in the total loss of the 
structure. Walls are easily damaged and subject to collapse by moving water. 

7.2.5 Warning Time 

Since dry floodproofing is often best suited for flooding conditions that last 
for a relatively short period of time, warning time and the time it takes to 
deploy the various components of a dry floodproofing system must be considered. 
Streams that are at flood levels for a short period of time generally reach 
flood levels quickly, thus the warning and preparation time is limited. 

Incorporating any type of flood warning system into the project design will 
enhance its ability to perform properly. 

7.2.6 Floodborne Debris 

The impact forces from debris in the moving water, such as trees, can compound 
the hydrodynamic forces of the moving floodwaters. 

7.3 Relative Costs 

The relative cost ranking is based on the combination of the estimated costs for 
the dry floodproofing project and a determination of cost-effectiveness. 

7.3.1 Estimated Cost 

Dry floodproofing a structure is generally an inexpensive mitigation measure. 
The costs for dry floodproofing a structure will depend on the following 
factors: the size of the structure, the height of the Flood Protection Elevation 
(FPE), types of sealant and shield materials used, number of plumbing lines that 
have to be protected by check valves, and number of openings that have to be 
covered by shields. 

Examples of cost estimating items that may need to be considered include the 
following: 
--Analysis to determine the effective dry floodproofing method and design of the 
dry floodproofing method 
--Dry floodproofing method selected, including: 
----Watertight shields for doors and windows 
----Reinforcing walls to withstand floodwater pressures and impact forces 
generated by floating debris 
----Drainage collection systems and sump pumps to control the interior water 
level, collect seepage, and reduce hydrostatic pressure on slab and walls 
----Membranes and/or other sealants to reduce seepage of floodwater through 
walls and wall penetrations 
----Anchoring the structure to resist flotation, collapse, and lateral movement 
--Construction 

To estimate the relative cost of a dry floodproofing project, examples of 
general cost estimates have been provided below and are included in FEMA 312, 
Homeowners Guide to Retrofitting: 
Six Ways To Protect Your House From Flooding and FEMA 259, Engineering 
Principles and Practices of Retrofitting Floodprone Residential Structures. 

The figures in Table 7-2 are example cost estimate numbers used in a study for 
the St. Louis Metropolitan Sewer District. These numbers were generated using 
the U.S. Army Corps of Engineers publication, Flood Proofing - How to Evaluate 
Your Options, and updated to 2002 and adjusted for the St. Louis area. It is 
important to note that the cost estimate numbers are location and time 
dependent. 

Table 7-2. General Estimates of the Unit Costs for Typical Dry Floodproofing 
Projects 

Dry Floodproofing 
Waterproofing a concrete block or brick-faced wall by applying a polyethylene 
sheet or other impervious material and covering with a facing material such as 
brick. $3.50/square foot 
Acrylic latex wall coating. $3.00/ square foot 
Caulking/sealant - a high performance electrometric urethane sealant is 
recommended. $2.50/linear foot 
Bentonite grout (below grade waterproofing, 6 feet deep) $20/linear foot 

Appendix C, Cost Estimating, provides guidance and references for conducting a 
more detailed cost estimate. Additional cost estimates can be obtained from R.S. 
Means Contractors Pricing Guide. A blank preliminary cost estimating worksheet 
(Worksheet D) is provided in Appendix B. 

7.3.2 Determination of Cost-Effectiveness 

A component of the relative cost scoring is to include a determination of cost-
effectiveness. Table D-1 in Appendix D, Determining Cost-Effectiveness, provides 
a quick screening for the cost-effectiveness of a project. The attributes 
included in the table are frequency of flood, level of damage, project cost, 
project benefits, and criticality (impact or loss of function). For 
example, if the frequency is the 10-year flood, the project will have a very 
high likelihood of cost-effectiveness. 

Based on the combination of the estimated cost of the project and the likelihood 
of cost-effectiveness, a relative cost ranking will be assigned on Worksheet B, 
Appropriate Mitigation Measures. If the likelihood of cost-effectiveness is low, 
the ranking of relative cost will be either moderate or high, based on the 
estimated cost of the project. However, if the estimated cost is low and the 
likelihood of cost-effectiveness is very high or high, the relative cost ranking 
will be low. 

7.4 Additional Considerations 

Most floodproofing projects use more than one technique; this is especially true 
in dry floodproofing. A good dry floodproofing project can be enhanced through 
the use of small flood barriers or modifications to the structures foundation 
drainage system. The small flood barriers will move the floodwaters away from 
the structure, thereby reducing the forces exerted on the subsurface portions of 
the structure. Additional considerations for using dry floodproofing are 
human intervention, annual maintenance, non-residential buildings, and 
structures without basements. 

7.4.1 Human Intervention 

Property owners must be able to install all flood shields and physically perform 
the activities required for the successful operation of the dry floodproofing 
system before floodwaters arrive. 

7.4.2 Annual Maintenance 

The components must be inspected and maintained on a regular basis. Because dry 
floodproofing has window and door closures as part of the system, closures must 
be available and in good condition. Some considerations to facilitate a 
successful maintenance schedule are as follows: 

--Develop an inventory and location list of all flood shields and closures. 
--Develop an inspection plan to ensure flood shields and closures fit properly. 
--Check walls, floors, and floodproof coatings for cracks and potential leaks. 

7.5 Available Resources 

FEMA 259. Engineering Principles and Practices of Retrofitting Floodprone 
Residential Structures. See Chapters VI-D, Dry Floodproofing. 

FEMA 312. Homeowners Guide to Retrofitting: Six Ways to Protect Your House from 
Flooding. See Chapter 3, An Overview of the Retrofitting Methods and Chapter 7, 
Other Methods  Dry Floodproofing. 

FEMA 511. Reducing Damage from Localized Flooding. See Chapter 10, Retrofitting. 

The Louisiana State University (LSU) Extension Center website 
(http://www.louisianafloods.org) lists many retrofitting publications, provides 
advice on floodproofing methods, and flood insurance and links to online 
shopping for retrofitting products and contractors. 

R.S. Means Contractors Pricing Guide. 

USACE. Flood Proofing - How to Evaluate Your Options. 

[End of Chapter 7]