FEMA 320- Taking Shelter From the Storm 


Section II: 
Planning Your Shelter


Now that you better understand your risk from a tornado or hurricane, you can work 
with your builder/contractor to build a shelter to protect yourself and your family from 
these extreme windstorms. This section describes how extreme winds can damage a 
building, explains the basis of the shelter designs presented in this booklet, and shows 
where you can build a shelter in your house.



Building Damage

Extreme winds can cause several kinds of damage to a building. Figure II.1 shows how 
extreme winds affect a building and helps explain why these winds cause buildings to 
fail.

To understand what happens when extreme winds strike, you must Þrst understand that 
tornado and hurricane winds are not constant. Wind speeds, even in these extreme 
wind events, rapidly increase and decrease. An obstruction, such as a house, in the path 
of the wind causes the wind to change direction. This change in wind direction 
increases pressure on parts of the house. The combination of increased pressures and 
ßuctuating wind speeds creates stress on the house that frequently causes connections 
between building components to fail. For example, the roof or siding can be pulled off 
or the windows can be pushed in. 

Buildings that fail under the effects of extreme winds often appear to have exploded, 
giving rise to the misconception that the damage is caused by unequal wind pressures 
inside and outside the building. This misconception has led to the myth that, during an 
extreme wind event, the windows and doors in a building should be opened to equalize 
the pressure. In fact, opening a window or door allows wind to enter a building and 
increases the risk of building failure.

Damage can also be caused by ßying debris (referred to as windborne missiles). If 
wind speeds are high enough, missiles can be thrown at a building with enough force to 
penetrate windows, walls, or the roof. For example, an object such as a 2" x 4" wood 
stud weighing 15 pounds, when carried by a 250-mph wind, can have a horizontal 
speed of 100 mph and enough force to penetrate most common building materials used 
in houses today. Even a reinforced masonry wall will be penetrated unless it has been 
designed and constructed to resist debris impact during extreme winds. Because 
missiles can severely damage and even penetrate walls and roofs, they threaten not 
only buildings but the occupants as well.


Basis of Shelter Design

The purpose of a shelter is to provide a space where you and your family can survive a 
tornado or hurricane with little or no injury. In hurricane-prone areas, the shelter cannot 
be built where it can be ßooded during a hurricane. Your shelter should be readily 
accessible from all parts of your house, and it should be free of clutter. To protect the 
occupants during extreme windstorms, the shelter must be adequately anchored to the 
house foundation to resist overturning and uplift. The connections between all parts of 
the shelter must be strong enough to resist failure, and the walls, roof, and door must 
resist penetration by windborne missiles. 

Extensive testing by Texas Tech University and other wind engineering research 
facilities has shown that walls, ceilings, and doors commonly used in house 
construction cannot withstand the impact of missiles carried by extreme winds. The 
shelter designs in this booklet account for these Þndings by specifying building 
materials and combinations of building materials that will resist penetration by missiles 
in extreme winds.

The shelter designs, including both materials and connections, are based on wind 
speeds that are rarely exceeded in the United States. Therefore, a shelter built 
according to these designs is expected to withstand the forces imposed on it by extreme 
winds without failing. Those forces may cause cracks or other signs of stress in the 
materials or connections used in the shelter, and they may cause materials or 
connections to yield. However, the intent of the designs is not to produce a shelter that 
will always remain completely undamaged, but rather a shelter that will enable its 
occupants to survive an extreme windstorm with little or no injury.
It is very important to note that predicting the exact strength of tornadoes and hurricanes 
is impossible. That is another reason why the shelter designs in this booklet are based 
on extreme wind speeds and why the primary consideration is life safety.
Designing a building to resist damage from more than one natural hazard requires 
different, sometimes competing, approaches. For example, building a structure on an 
elevated foundation to raise it above expected ßood levels can increase its vulnerability 
to wind and seismic damage. These design approaches need to be thoroughly 
considered. In ßoodprone areas, careful attention should be given to the warning time, 
velocity, depth, and duration of ßoodwaters. These ßooding characteristics can have a 
signiÞcant bearing on the design and possibly even the viability of a shelter. Your local 
building ofÞcial or licensed professional engineer or architect can provide you with 
information about other natural hazards that affect your area and can recommend 
appropriate designs.


Shelter Size

The amount of ßoor area per person that your shelter must provide depends partly on 
the type of windstorm the shelter is intended to protect you from. Tornadoes are not 
long-lasting storms, so if you are relying on your shelter only for tornado protection, you 
will not need to stay in the shelter for a long time. As a result, comfort is not of great 
concern, and a shelter that provides about 5 square feet of ßoor area per person will be 
big enough.

When the shelter is intended to provide protection from storms such as hurricanes, 
which can last up to 12 hours, the comfort of the occupants should be considered. For 
this type of shelter, the recommended amount of ßoor area per person is about 10 
square feet. Necessities, such as water and toilet facilities, should be provided. The 
shelter designs in this booklet are based on a maximum ßoor area of 64 square feet and 
a maximum wall length of 8 feet. A shelter of that size used for hurricane protection 
can accommodate up to six people in reasonable comfort. If you plan to build a shelter 
with any wall longer than 8 feet, consult a licensed professional engineer or architect.



New vs. Existing Houses

The shelter designs in this booklet were developed primarily for use in new houses, but 
some can be used in existing houses. When a new house is being built, the 
builder/contractor can construct walls, foundations, and other parts of the house as 
required to accommodate the shelter. Modifying the walls or foundation of an existing 
house as necessary for the construction of a shelter is more difÞcult. As a result, some 
of the shelter designs in this booklet are not practical for existing houses. The following 
sections discuss this issue further.
In this booklet, the term "retroÞt" refers to the process of making changes to an existing 
house.



Foundation Types

Houses on the following types of foundations are suitable for the installation of a 
shelter:

¥ basement
¥ slab-on-grade
¥ crawlspace


A house on a basement foundation (see Figure II.2) is usually built on a foundation 
constructed of poured concrete or concrete masonry. Most concrete foundations are 
reinforced with steel bars or straps, but many concrete masonry foundation walls have 
no steel reinforcement. The framing for the ßoor above the basement is supported by 
the exterior foundation walls and sometimes by a center beam.
In a new or existing house with a basement, the shelter should be built in the basement. 
You can build the shelter as an entirely separate structure with its own walls, or you can 
use one or more of the basement walls as walls of the shelter. If you use the basement 
walls, they will have to be specially reinforced. Typical reinforcement techniques used 
in residential basement walls will not provide sufÞcient protection from missiles. In 
new construction, your builder/contractor can reinforce the walls near the shelter during 
the construction of your house. Reinforcing the basement walls of an existing house is 
not practical.

The likelihood of missiles entering the basement is lower than for above ground areas; 
however, there is a signiÞcant chance that missiles or falling debris will enter the 
basement through an opening left when a window, a door, or the Þrst ßoor above has 
been torn off by extreme wind. Therefore, your basement shelter must have its own 
reinforced ceiling; the basement ceiling (the Þrst ßoor above) cannot be used as the 
ceiling of the shelter.

The least expensive type of shelter that can be built in a basement is a lean-to shelter, 
which is built in the corner of the basement and uses two basement walls. The lean-to 
shelter uses the fewest materials, requires the least amount of labor, and can be built 
more quickly than other types of basement shelters.
In general, it is easier to add a basement shelter during the construction of a new house 
than to retroÞt the basement of an existing house. If you plan to add a basement shelter 
as a retroÞtting project, keep the following points in mind:

¥ You must be able to clear out an area of the basement large enough for the shelter.

¥ Unless the exterior basement walls contain steel reinforcement as shown on the 
design drawings provided with this booklet, these walls cannot be used as shelter walls 
since they are not reinforced to resist damage from missiles and uplift from extreme 
winds.

¥ Exterior basement walls that are used as shelter walls must not contain windows, 
doors, or other openings.

¥ The shelter must be built with its own ceiling, so that the occupants will be protected 
from missiles and falling debris.


A slab-on-grade house (see Figure II.3) is built on a concrete slab that is installed on 
compacted or natural soil. The concrete may be reinforced with steel that helps prevent 
cracking and bending. If you are building a new slab-on-grade house and want to install 
a concrete or concrete masonry shelter, your builder/contractor must make the slab 
thicker where the shelter will be built. The thickened slab will act as a footing beneath 
the walls of the shelter to provide structural support. It will also help anchor the shelter 
so that it will stay in place during an extreme wind event, even if the rest of the house is 
destroyed.

In an existing house, removing part of the slab and replacing it with a thickened section 
would involve extensive effort and disruption inside the house. Therefore, building a 
shelter with concrete or concrete masonry walls in an existing slab-on-grade house is 
generally not practical. You can, however, build a wood-frame shelter, because its 
walls are not as heavy and do not require the support of a thickened slab. A wood-
frame shelter can be created from an existing room, such as a bathroom or closet, or 
built as a new room in an open area in the house, such as a garage.

You can also build a shelter as an addition to the outside of a slab-on-grade house. This 
type of shelter must have not only proper footings, but also a watertight roof. Because a 
shelter built as an outside addition will be more susceptible to the impact of missiles, it 
should not be built of wood framing. Instead, it should be built of concrete or concrete 
masonry. Access to this type of shelter can be provided through an existing door or 
window in an exterior wall of the house.

In general, it is easier to add a shelter during the construction of a new slab-on-grade 
house than to retroÞt an existing slab-on-grade house. If you plan to add a shelter to a 
slab-on-grade house as a retroÞtting project, keep the following points in mind:

¥ The walls of the shelter must be completely separate from the structure of the house. 
Keeping the walls separate makes it possible for the shelter to remain standing even if 
portions of the house around it are destroyed by extreme winds.

¥ If you are creating your shelter by modifying a bathroom, closet, or other interior room 
with wood-frame walls, the existing walls, including sheathing on either the inside or 
outside of the walls, such as drywall or plaster, must be removed and replaced with 
walls and a ceiling resistant to the impact of windborne missiles and other effects of 
extreme winds.

¥ If you intend to build a shelter with concrete or concrete masonry walls, a section of 
your existing slab ßoor will have to be removed and replaced with a thicker slab. As 
noted above, this is usually not practical in an existing house.


A house built on a crawlspace (see Figure II.4) usually has a ßoor constructed of wood 
framing. Along its perimeter, the ßoor is supported by the exterior foundation walls. 
The interior part of the ßoor is supported by beams that rest on a foundation wall or 
individual piers. Crawlspace foundation walls may be concrete, but are usually 
constructed of concrete masonry. Crawlspace foundation walls are often unreinforced 
and therefore provide little resistance to the stresses caused by extreme winds.

Building a shelter inside a house on a crawlspace foundation is more difÞcult than 
building a shelter inside a house on a basement or slab-on-grade foundation. The main 
reason is that the entire shelter, including its ßoor, must be separate from the framing of 
the house. As shown in Figure II.4, a shelter built inside the house cannot use the ßoor 
of the house. The shelter must have a separate concrete slab ßoor installed on top of 
earth Þll and must be supported by concrete or concrete masonry foundation walls. An 
alternative approach, which may be more economical, is to build an exterior shelter on 
a slab-on-grade adjacent to an outside wall of the house and provide access through a 
door installed in that wall.

Ventilation in the area below the ßoor of the house is also an important issue. The 
wood-framed ßoor of a house on a crawlspace foundation is typically held 18 to 30 
inches above the ground by the foundation walls. The space below the ßoor is designed 
to allow air to ßow through so that the ßoor framing will not become too damp. It is 
important that the installation of the shelter not block this air ßow.
In general, it is much easier to build a shelter inside a new crawlspace house than in an 
existing crawlspace house. If you plan to add a shelter to an existing crawlspace house 
as a retroÞtting project, keep the following in mind:

¥ The shelter must have a separate foundation. Building the foundation inside the house 
would require cutting out a section of the existing ßoor and installing new foundation 
members, Þll dirt, and a new slab - a complicated and expensive operation that is often 
not practical.

¥ A more practical and more economical approach would be to build an exterior 
shelter, made of concrete or concrete masonry, on a slab-on-grade foundation adjacent 
to an outside wall of the house, as described above.




Shelter Location
There are several possible locations in your house for a shelter. Perhaps the most 
convenient and safest is below ground level, in your basement. If your house does not 
have a basement, you can install an in-ground shelter beneath a concrete slab-on-grade 
foundation or a concrete garage ßoor. Basement shelters and in-ground shelters provide 
the highest level of protection against missiles and falling debris.
Another alternative shelter location is an interior room on the Þrst ßoor of the house. 
Researchers, emergency response personnel, and people cleaning up after a tornado 
have often found an interior room of a house still standing when all other above ground 
parts of the house have been destroyed. Closets, bathrooms, and small storage rooms 
offer the advantage of having a function other than providing occasional storm 
protection. Typically, these rooms have only one door and no windows, which makes 
them well-suited for conversion to a shelter. Bathrooms have the added advantage of 
including a water supply and toilet.

Regardless of where in your house you build your shelter, the walls and ceiling of the 
shelter must be built so that they will protect you from missiles and falling debris, and 
so that they will remain standing if your house is severely damaged by extreme winds. 
If sections of your house walls are used as shelter walls, those sections must be 
separated from the structure of the house. This is true regardless of whether you use 
interior or exterior walls of the house.

Figures II.5 through II.7 are typical ßoor plans on which possible locations for shelters 
are shown with yellow highlighting. These are not ßoor plans developed speciÞcally for 
houses with shelters. They show how shelters can be added without changes to the 
layout of rooms.


Floor Plan 1: basement

Possible shelter locations in a basement include the following:

¥ in a corner of the basement, preferably where the basement walls
 are below the level 
of the ground

¥ in a bathroom, closet, or other interior room in the basement

¥ in a freestanding addition to the basement


A space that is to be used for a shelter must be kept free of clutter so that the shelter 
can be quickly and easily entered and so that the shelter occupants will not be injured 
by falling objects. For this reason, a bathroom is often a better choice for a shelter than 
a closet or other space used for storage. Remember, if the basement is below the level 
of storm surge or the level of ßooding from any other source, it is not a suitable location 
for a shelter. In this situation, a possible alternative would be to build an exterior shelter, 
adjacent to your house, on a slab-on-grade above the ßood level.


Floor Plan 2:  house on a slab-on-grade or crawlspace foundation

Possible shelter locations in a house on a slab-on-grade or crawlspace foundation 
include the following spaces on the Þrst ßoor:

¥ bathroom
¥ closet
¥ storage room
¥ laundry room (provided the load-bearing wall between it and the garage, as shown in 
Figure II.6, can be properly separated from the structure of the house)

¥ corner of the garage
Regardless of where the shelter is built, it must be equipped with a door that will resist 
the impact of missiles. Remember, if the Þrst ßoor of the house is below the level of 
storm surge or the level of ßooding from any other source, it is not a suitable location 
for a shelter. In this situation, a possible alternative would be to build an 
exterior shelter on a slab-on-grade 
elevated on Þll above the ßood 
level.



Floor Plan 3: house on a slab-on-grade foundation

Possible locations for an in-ground shelter include the following:

¥ below the slab in a closet or storage room

¥ below the ßoor of the garage, in an area where cars will not be parked


Because of the difÞculty of installing an in-ground shelter in an existing house, this type 
of shelter is practical only for new construction. Remember, if the Þrst ßoor of the 
house is below the level of storm surge or the level of ßooding from any other source, it 
is not a suitable location for a shelter. In this situation, a possible alternative would be to 
build an exterior shelter on a slab-on-grade elevated on Þll above the ßood level.
Tables II.1 and II.2 will help you decide what type of shelter is appropriate for your 
circumstances. Table II.1 applies to the construction of shelters in new houses. Table 
II.2 applies to retroÞt situations, in which a shelter is being added to an existing house.



Construction Materials

The materials your builder/contractor will need to build your shelter should be available 
from building material suppliers in your community. These materials have been 
carefully selected for their strength, durability, and/or ability to be readily combined in 
ways that enable them to withstand the forces of extreme winds and the impact of 
windborne missiles. Your builder/contractor should not substitute any other material for 
those speciÞed in the designs.

One of the most vulnerable parts of your shelter is the door. The materials speciÞed for 
doors in the shelter designs in this booklet were tested by the Wind Engineering 
Research Center at Texas Tech University for their ability to carry wind loads and 
prevent penetration by missiles. The installation of the door is as important as the 
materials used in its construction. Please conÞrm with your builder/contractor that the 
door to your shelter can be installed the way it is shown in the designs included with this 
booklet.

A complete list of the shelter construction materials, with their expected strengths or 
properties, is included in the shelter designs provided in this booklet. Your 
builder/contractor should use it when buying the materials for your shelter.



Shelter Cost

The cost of your shelter will vary according to the following:

¥ the size of the shelter

¥ the location of the shelter

¥ the number of exterior house walls used in the construction of the shelter
¥ the type of door used

¥ the type of foundation on which your house is bui
lt
¥ your location within the United States (because of regional variations in labor and 
material costs) 

¥ whether you are building a shelter into a new house or retroÞtting an existing house


Table II.3 shows the average costs for building three types of shelters- lean-to, 
aboveground (AG), and in-ground - in new houses on basement, slab-on-grade, and 
crawlspace foundations according to the design plans in this booklet. These costs are 
for shelters with a ßoor area of 8 feet by 8 feet.
The cost of retroÞtting an existing house to add a shelter will vary with the size of the 
house and its construction type. In general, shelter costs for existing houses will be 
approximately 20 percent higher than those shown in Table II.3.






NOTE
The shelter designs in this booklet are applicable for any on-site construction. However, 
in a modular house, the shelter location would be limited to the basement or the below-
ground module. A modular house is a house constructed of modular units that have 
been built elsewhere, brought to the site, and installed on a permanent foundation



WARNING

You should not install a shelter in a house supported by piles, piers, or columns. With 
building connectors commercially available, there is no economical way to separate 
the shelter from the ßoor framing and ensure that the shelter will withstand the forces of 
extreme winds.
You may be tempted to build a shelter under a house on a pile, pier, or column 
foundation. However, if the house is in a storm surge area or other ßood hazard area, 
the area under the house would be below the ßood level. A shelter built in that area 
would trap its occupants in rising floodwaters. See the warning on page 5 for more 
information