A.Bryan Readling, P.E.,
responds: Studies have shown that walls containing garage
doors perform poorly when subjected to lateral wind and seismic
forces. This is especially true when the wall containing the
garage door is offset by more than a few feet from other braced
wall lines parallel to the garage door (see illustration,
In that case, the open-ended side of the garage enclosure is
not braced well by the remainder of the home, and the often
narrow walls on either side of the door are subjected to
relatively large lateral and uplift forces collected within the
garage portion of the structure.
Making matters worse, garage doors themselves are vulnerable to
failure from relatively minor windstorms due to wind pressure
and flying debris. When a garage door is breached, the
resulting pressure on the interior walls of the garage can add
dramatically to the lateral and uplift forces already present
in this vulnerable area. Since few structural redundancies
generally exist in the garage area, failures tend to be
catastrophic in nature.
The International Residential Code requires "braced
wall panels" at the corners and at regular intervals (typically
every 25 feet), as well as bracing of the wall line at a
certain minimum percentage. Most of the wall-bracing options
listed in the IRC are not possible at narrow garage return
walls since the required minimum braced-wall length is
typically 48 inches. Anything less than 48 inches is generally
too flexible and weak.
An exception in the code states that when "continuous
structural panel sheathing" is used, the width of wall segments
considered as "braced" can be reduced from 48 inches to as
little as 24 inches depending on the height of openings
adjacent to the segment (IRC, R602.10.5). For garage
return walls, a 24-inch width is allowed if there is no story
or bonus room above.
"Continuous structural panel sheathing" means that all exterior
wall surfaces (and in some cases interior braced wall lines)
are sheathed entirely with plywood or OSB wall sheathing,
including the portions of walls above and below window and door
openings. This change, introduced to the IRC by the National
Association of Home Builders, is based on extensive full-scale
testing showing this type of construction to be inherently
stronger and more redundant than walls with structural
sheathing only at the corners and at regular intervals.
But what about popular designs with a bonus room or second
story above the garage? For that scenario, a site-built
solution known as the Narrow Wall Bracing Method, developed by
APA The Engineered Wood Association, can be used to
provide a braced wall as narrow as 16 inches. This approach
builds additional strength into the framing around the garage
opening by creating a rigid connection that resists rotation
between the garage return walls and the garage door header.
This is achieved by extending the header beyond the rough
opening (almost to the corner) and lapping the wall sheathing
over both the wall studs and the header.
Besides more nails, additional details include more robust
2x2x3/16-inch plate washers on the anchor bolts, three-stud
corner framing, and a 1,000-pound strap connecting the back of
the wall studs to the header. With this relatively inexpensive
site-built method, outlined in APA publication D420, side walls
can be reduced to as little as 16 inches in width, even with a
bonus room above. The Narrow Wall Bracing Method is now being
proposed for inclusion in future versions of the IRC and has
already been adopted for use in some states and local
jurisdictions. It is available for free download at
www.apawood.org/bracing. For a description
of how to build and detail the narrow walls, see
the article in
this issue of JLC.
Bryan Readling, P.E.,
is a structural engineer with APA's Field
Services Division in Davidson, N.C., specializing in wind
damage and the use of engineered wood products and building
structures to resist hurricanes and tornados.