CHAPTER 6 
Wet Floodproofing



Introduction 

This guide describes two types of floodproofing: wet and dry. As its 
name implies, wet floodproofing allows flood waters to enter the 
enclosed areas of a house. In contrast, dry floodproofing (Chapter 7) 
prevents the entry of flood waters. The benefit of wet floodproofing is that 
if flood waters are allowed to enter the enclosed areas of the house and to 
quickly reach the same level as the flood waters outside, the effects of 
hydrostatic pressure, including buoyancy, are greatly reduced. As a result, 
the loads imposed on the house during a flood, and therefore the 
likelihood of structural damage, may be greatly reduced. Wet 
floodproofing is generally used to limit damages to enclosures below 
elevated buildings, walkout-on-grade basements, below-grade 
basements, crawlspaces, or attached garages. It is not practical for areas 
that are to be used as living space. 

Successful wet floodproofing involves the following: 

 ensuring that flood waters enter and exit the house 
 ensuring that flood waters inside the house rise and fall at the 
same rate as flood waters outside 
 protecting the areas of the house that are below the flood level 
from damage caused by contact with flood waters 
 protecting service equipment inside and outside the house 
 relocating any materials stored below the Flood Protection 
Elevation (FPE) 
This chapter describes the modifications that must be made to a house as 
part of a wet floodproofing project, and it discusses the most important 
considerations regarding wet floodproofing. Protection of service 
equipment is discussed in Chapter 8. 

HOMEOWNERS GUIDE TO RETROFITTING 


WARNING 

If your house has been 
substantially damaged 
or is being substantially 
improved, your communitys floodplain 
management ordinance 
or law will restrict the 
use of wet floodproofing 
to attached garages 
and enclosed areas below the Base Flood 
Elevation (BFE) that are 
used solely for parking, 
storage, and access. 
For more information, 
refer to Federal Emergency Management 
Agency (FEMA) Technical Bulletin 7-93, Wet 
Flood proofing Requirements for Structures 
Located in the Special 
Flood Hazard Area. 


DEFINITION 
Service equipment includes utility systems, 
heating and cooling 
systems, and large appliances. 

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NOTE 

Flood-resistant materials are discussed later 
in this chapter. 


WARNING 

If you are retrofitting a 
house that has been 
substantially damaged 
or is being substantially 
improved, your communitys floodplain 
management ordinance 
or law will not allow you 
to have a basement, as 
defined under the NFIP. 
The NFIP regulations 
define a basement as 
any area of the building having its floor 
subgrade on all sides. 
If your house has such 
a basement, you will be 
required to fill it in as part 
of any wet floodproofing 
project. Note that the 
NFIP definition of basement does not include 
what is typically referred 
to as a walkout-ongrade basement, whose 
floor would be at or 
above grade on at least 
one side. 

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Considerations 
Flood Protection Elevation 

All construction and finishing materials in the areas of the house that will 
be allowed to flood must be resistant to damage caused by direct, and 
possibly prolonged, contact with flood waters. Areas used for living space 
contain floor and wall coverings and other finishing materials, furniture, 
appliances, and other items that are easily damaged by flood waters and 
expensive to clean, repair, or replace. Therefore, wet floodproofing is 
practical only for portions of a house that are not used for living space, such 
as a basement as defined by the National Flood Insurance Program 
(NFIP) regulations, walkout-on-grade basement, crawlspace, or attached 
garage. As shown in Figure 6-1, the FPE (including freeboard) should be no 
higher than your lowest finished floor. 


Figure 6-1 
A typical wet floodproofed house that is compliant with the minimum 
requirements of a communitys floodplain management ordinance or law. 

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WET FLOODPROOFING 

CHAPTER 6 
If your FPE is above the elevation of your lowest finished floor, you should 
consider one or more of the other retrofitting methods described in this guide, 
such as elevation (Chapter 5). If you read Chapter 5, you will note that most 
of the elevation methods incorporate the principles of wet floodproofing. 
They raise the living space above the flood level and allow flood waters to 
enter the areas of the house below the living space. 

Hazards 

Wet floodproofing protects a house from the effects of hydrostatic 
pressure but not from other flood hazards, such as the hydrodynamic 
force of flowing water, erosion and scour, the impact of ice and other 
floodborne debris, and damage from floodborne contaminants. If you 
have seen evidence of these hazards in past floods in your area, or if your 
community officials confirm that your house may be affected by these 
hazards, you should consider an alternative retrofitting method, such as 
relocation (see Chapter 7) or elevation on an open foundation (see 
Chapter 5). Wet floodproofing a house does not change its vulnerability to 
damage from high winds or earthquakes. 

Post-Flood Cleanup 

Remember that flood waters are rarely clean. They usually carry sediment, 
debris, and even corrosive or hazardous materials such as solvents, oil, 
sewage, pesticides, fertilizers, and other chemicals. Allowing areas of a 
house to flood exposes those areas to whatever is in the flood waters. 
Cleaning up a wet floodproofed house after a flood may therefore involve 
not only removing mud but also washing, disinfecting, and decontaminating 
walls, floors, and other surfaces. This is another good reason why wet 
floodproofing is inappropriate for areas used as living space and, in some 
circumstances, why it may be inappropriate for any part of a house. 

Modifications Required for Wet Floodproofing 

Wet floodproofing requires a variety of modifications to your house, including 
its walls, construction and finishing materials, and service equipment. 

Installing Openings 

The most important part of a wet floodproofing project is installing wall 
openings that will allow the entry and exit of flood waters. The openings 
must be installed in foundation walls and in garage walls as appropriate, 
below the expected flood level (see Figure 6-1). The goal is not simply to 
allow the entry and exit of flood waters but also to ensure that the water 

HOMEOWNERS GUIDE TO RETROFITTING 


NOTE 

For more information 
about openings requirements for wet flood-
proofing, refer to FEMA 
Technical Bulletin 1-93, 
Openings in Foundation 
Walls for Buildings Located in Special Flood 
Hazard Areas, and 
FEMA 259, Engineering Principles and 
Practices for Retrofitting 
Flood Prone Residential 
Buildings. 

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CHAPTER 6 
level inside the house rises and falls at roughly the same rate as the water 
level outside so that hydrostatic pressures inside and outside are 
continually equalized. As shown in Figure 6-2, large differences in the 
interior and exterior water levels allow unequalized hydrostatic pressures 
and therefore defeat the purpose of wet floodproofing. 

Figure 6-2 
Wall openings must 
allow flood waters not 
only to enter the house 
but also to rise and fall 
at the same rate as 
flood waters outside. 

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FEDERAL EMERGENCY MANAGEMENT AGENCY 


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CHAPTER 6 
For equal water levels to be maintained, both the size and number of 
openings must be adequate. Otherwise, when flood waters are rising and 
falling, water will not be able to flow into or out of the house fast enough. 
The number of openings required and their size will depend on the rate of 
rise and the rate of fall of the flood waters (see Chapter 2) and on the size 
of the area that is being allowed to flood. In general, the faster the rates of 
rise and fall and the larger the flooded area within the house, the larger the 
number and size of openings required. 

If you are wet floodproofing areas below the BFE in a substantially 
damaged or substantially improved house, your communitys floodplain 
management ordinance or law will require you to install openings in the 
exterior walls of all enclosed areas below the BFE. The minimum 
requirements are as follows: 

 You must provide at least two wall openings for each enclosed 
area -- one in each of two different walls. In other words, you 
cannot put both openings in the same wall. 
 If your house has more than one enclosed area, you must 
install openings in the exterior walls of each enclosed area so 
that flood waters can enter directly from the outside. 
 The total area (size) of all openings for each enclosed area must 
be equal to at least 1 square inch for every square foot of floor 
space in the enclosed area. For example, if the enclosed area is 
25 feet by 40 feet (1,000 square feet), the total area of the 
openings must be at least 1,000 square inches, or roughly 7 
square feet. In this example, you could meet the size 
requirement by providing two 3 1/2-square-foot openings or 
several smaller openings whose total area equals 7 square feet. 
 The bottom of each opening must be no higher than 1 foot 
above the ground directly below the opening. 
 Flood waters must be able to flow in and out of enclosed areas 
automatically. If you place louvers, screens, or other types of 
covers over the openings (which many homeowners do to 
prevent animals from entering the enclosed areas) they must 
not block the flow of water. Because the need for human 
intervention reduces the reliability of wet floodproofing, you may 
not install any type of electrically, mechanically, or manually 
operated cover. 
FEMA developed these requirements to provide homeowners with a 
straightforward means of determining where and how to install wall 
openings without the aid of an engineer or design professional. The 

HOMEOWNERS GUIDE TO RETROFITTING 


NOTE 

If you cover wall openings with louvers or 
screens, keep in mind 
that the more restrictive 
they are the more likely 
they are to become 
clogged with debris during floods and to 
prevent the flow of water. Make sure that any 
screens or louvers you 
use will allow the passage of water that 
contains suspended 
sediment and other 
small debris. After flood 
waters have receded, 
screens and louvers 
must be cleaned of any 
other debris that may 
have accumulated. 

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CHAPTER 6 
NOTE 

For more information 
about flood-resistant 
materials, refer to 
FEMA Technical Bulletin 2-93, Flood-Resistant 
Materials Requirements 
for Buildings Located in 
Special Flood Hazard 
Areas. This bulletin includes a detailed list of 
common floor, wall, and 
ceiling materials categorized according to 
their applicability for use 
in areas subject to inundation by flood waters. 

requirements provide a margin of safety for wet floodproofed houses 
subject to flooding with rates of rise and fall as high as 5 feet per hour. If 
you wish to install openings that do not meet one or more of the 
requirements listed above, your design must be certified by a registered 
engineer or other licensed design professional and approved by your local 
officials. See FEMAs Technical Bulletin 1-93, Openings in Foundation 
Walls for Buildings Located in Special Flood Hazard Areas, for more 
information about openings requirements. 

Using Flood-Resistant Materials 

In the areas below the FPE, any construction and finishing materials that 
could be damaged by flood waters must be either removed or replaced 
with flood-resistant materials as required by your communitys floodplain 
management ordinance or law. Vulnerable materials include drywall, 
blown-in and fiberglass batt insulation, carpeting, and non pressure-
treated wood and plywood. Flood-resistant materials are those that can 
be inundated by flood waters with little or no damage. They include such 
materials as concrete, stone, masonry block, ceramic and clay tile, 
pressure-treated and naturally decay-resistant lumber, epoxy paints, and 
metal. In addition to resisting damage from flood waters, these materials 
are relatively easy to clean after flood waters have receded. 

Protecting Service Equipment 

When you wet floodproof a house, you must also protect the service 
equipment below the FPE, both inside and outside the house as required 
by your communitys floodplain management ordinance or law. Service 
equipment includes utility lines, heating ventilation and cooling equipment, 
ductwork, hot water heaters, and large appliances. Chapter 8 describes a 
variety of methods you can use to protect interior and exterior service 
equipment. 

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FEDERAL EMERGENCY MANAGEMENT AGENCY