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Tying all the parts together is an important part of strengthening your wood frame walls
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To complete the development of an effective load path that connects the
roof to the foundations, you need to make sure that the exterior walls
are strong enough to withstand the wind forces and are well anchored to
the foundation. Although this is not as critical as some of the other tasks
on your list, inadequate wall strength and poor connections between the
wall and foundation can weaken your entire home. These vulnerabilities
tend to show up when the highest winds of a major hurricane (Category 3
or higher) strikes your home. This is especially true if you live in a
one-story home with large roof truss or rafter spans and a low-sloped roof
or a two story home. The walls that are most important to helping to hold
the roof on are the exterior walls that the ends of rafters rest on. These
walls can have a lot of uplift applied to them. The dead weight of these
walls is only 5 to 10 pounds per square foot. So if a wall is 8' high
its dead weight can only restrain 40 to 80 pounds of the hundreds of pounds
of uplift that may be applied in a strong hurricane. This points to the
necessity of tying the bottoms of walls to the floor below and ultimately
to the foundation.
In addition to transmitting (connecting) uplift loads on the roof down
to the foundations, wall sheathing also is a major contributor in helping
keep your house from being blown over or collapsing in a major hurricane.
The important factors that affect the performance of the wall sheathing
are the type of sheathing, how much of the wall is covered by wood sheathing,
how well it is connected to the top plate, bottom plate and the wall
studs, and whether joints between the sheets (if any) are blocked (covered
by a 2x4 with the sheathing nailed to the 2x4). Another important factor
is how well the wall is anchored to the foundation.
Generally, it will be pretty difficult to determine how well your walls
are built or anchored to the foundation unless you open up part of an exterior
wall. If you end up having to repair drywall on an exterior wall or are
having your home re-sided, you have an opportunity to check the wall construction
details. You may also be able to gain some insight into the likely wall
construction by talking with the building official in your community or
older well established builders and asking what was typically done at the
time your home was built. Other indirect clues include the absence of
hurricane straps connecting the trusses or rafters to the top of the wall
or straps that are installed on every other truss or rafter connection.
If either of these cases exists, chances are that you don't have a particularly
strong wall structure and that it may not be very well anchored to the foundation.
More than one story? Two story houses have more wind stress applied
to them than one story houses because they block the wind more and they
stick up in the air to catch winds that are stronger than those on the
ground. Further, the winds are stronger higher up because there is less
around the upper floors to shield the wind. The following questions were
developed with a one story house in mind. If you have a two or three story
house you can still use the questions listed below, just check the roof
to wall connections from the attic and the wall to foundation connections
on the first floor. Generally, you will not have any way to assess the
floor to floor connections unless you are re-siding your house. If you
are re-siding your house or have it opened up because of some sort of repairs,
you should find that either straps or continuous wood sheathing have
been used to connect between wall studs on the floor above and below.
If you don't find either of these to be true and really in any case if
you have a two story house, you should get a design professional to help
you decide how best to connect the floors before you install the new siding.
Generally, if your roof structure is poorly connected to the walls of
the top floor and the first floor walls are poorly connected to the foundations,
you can be reasonably certain that the connections across the floors
are also weak and that you home is more vulnerable than a comparable one
story house to hurricane damage. If you don't have a lot of confidence
in the strength of your multi-story house, do what you can for your roof,
protect the window and doors, try to limit water intrusion and plan to
evacuate early.
Evaluating your existing wood frame walls:
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When damage has occurred and you are gutting a house, there are unique opportunities to strengthen the structure of the walls and the connections - hopefully you will never get that "opportunity" again.
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If you have a wood frame house with an un-finished garage or remove drywall
from an exterior wall near one of the corners of the house or in the garage,
you can get a pretty good idea about how well your walls are built and
how well load paths are developed. The
Wood Frame Wall Checklist
is designed to help you capture information about your wall construction
if you are able to get enough access to be able to determine what kind
of sheathing was used, if any, and how the parts are connected together.
The questions on that checklist assume slab on grade construction.
For homes with wood frame elevated floors, the exterior walls have to be
connected by straps or sheathing to the rim joists (a band of wood members
that frame around the edge of the wood frame floor system) and the rim
joists have to be connected to the sill plate (pressure treated wood member
lying flat on the top of the foundation wall and anchored to the foundation).
The checklist also provides some suggestion about the implications
of what you find in your walls.
Retrofitting Wood Frame Walls:
If you want to strengthen your walls, probably the only time it makes sense
to try and retrofit the walls is if and when you re-side your house or
when you rebuild after an event. At that time, you could remove the fiber
board or foam panels and replace them with plywood or OSB panels. Make
sure that they are nailed around the perimeter using the recommended nailing
pattern required by the current building code in your area or if you don't
have a local building code in force, get a copy of one of the high wind
construction guides such as the AF&PA Wood Frame Construction Manual or
the International Code Council's SS TD 10-99. The AF&PA (American Forest
and Products Association) is in the process of releasing new guides with
great illustrations of proper wood frame construction for high wind areas.
The SS TD 10 is available as a
free download
from the International Code Council.
Back to Wood Frame Walls
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