Negative pressure from hurricane-force winds pulled the top of the
gable end outward after it ripped off the sheathing at the edge of
the roof. The retrofit measures briefly described in this article
can dramatically reduce the risk of such an occurrence.
Observations made in the aftermath of Category 3 and stronger
hurricanes have clearly illustrated the vulnerability of gable
ends. Failure comes in three varieties: The most common gable-end
failure is one where the wall loses support along its top edge
— typically because the sheathing gets blown off. The second
most common failure is at the connection between the triangular
gable-end wall and the rectangular wall below it. In this scenario,
the wall gets pulled out by the negative pressure of the wind. The
third potential failure is collapse of the actual framing members
that make up the gable-end wall structure as a result of bending
stresses from positive wind pressures. In any case, the damage is
usually disastrous to the home and its contents. Not only is the
structural damage severe, but a collapse of the gable-end opens the
house to wind-driven rain, which can condemn a home to catastrophic
In Florida, the vulnerabilities of gable ends have been recognized
as so significant that the legislature mandated that the Florida
Building Commission adopt voluntary methods for retrofitting gable
ends of homes into its prescriptive building code. The code is
available online from the Florida Department of Community Affairs
c/default.aspx). But much more helpful than the language in the
code is a detailed, 78-page description of these prescriptive
gable-end retrofits offered in the state of Florida's online
Hurricane Retrofit Guide (see "Retrofit Resource,").
The Weakness of Gable Ends
A couple of decades ago, not much was typically done to secure
gable-end trusses. Hurricanes were infrequent, and the strength of
negative pressures and their prevalence were not commonly
recognized. At most, the gable end might have gotten a few
toe-nails, perhaps an occasional strap or diagonal brace. That
leaves some deadwood for the drywall, and maybe a few rat runs and
the wall sheathing for strength. But these measures prove
insufficient in a high-wind event, where negative pressures are the
most likely forces to pull apart such an underbuilt wall.
As a hurricane advances, the winds swirl around the eye of the
storm, and a home near the track of a storm will see strong winds
from a wide range of directions. If the eye passes over a home, it
will be subjected to winds approximately half the time from one
direction and half the time from the opposite direction. So at any
given time, the windward face of a structure will experience
positive pressures (pushing) while the leeward face will experience
negative pressures (pulling). The negative pressures are actually
about 20% higher, thanks to the aerodynamics of wind rising over
and moving around the building.
The parts of gable ends less than 3 feet high are generally short
enough to resist the bending stresses imposed by the push and pull
of the wind. But members 3 feet or longer are subject to bending,
and trusses are typically more vulnerable than stick-framed gable
ends. The studs in a conventionally framed gable end are oriented
perpendicular to the face of the wall, providing maximum strength
in bending. In essence, each stud acts as a 3 1/2-inch-deep beam to
the wind. The web members in a gable-end truss, however, are
oriented flat against the wall, providing a beam depth of only 1
1/2 inches. In fact, the 3 1/2-inch-deep orientation is about 13
times stronger than the 1 1/2, assuming the lumber used for both
have similar specific gravities. But while the studs of a
conventionally framed gable end may be stronger, the ends of those
studs still may not be well enough connected to the rest of the
structure. Consequently, the retrofit process for reinforcing a
gable end must not only brace the framing members but must also
strengthen the connections.
Once the gable end goes, it can unzip a house, not only
devastating the structure but also ruining the entire contents of a
home, which become saturated with water.
Strengthening and Bracing a Gable-End
Each stud in a gable-end truss gets this basic retrofit assembly:
Horizontal braces (each about 6 feet long) are first attached to
the top and bottom chords of the roof, and then a retrofit stud is
installed perpendicular to the gable-end truss. The L-straps must
be preinstalled at the end of each retrofit stud before the stud
gets secured to the horizontal braces.
Overview of the Retrofit Process
Retrofitting gable ends entails five distinct tasks. Three of these
tasks strengthen and brace the gable end. The fourth connects the
strengthened gable end to the rectangular wall below, while the
fifth task aims at a better connection between the roof decking and
the gable-end framing members and attempts to minimize the
vulnerability of the overhang.
Tasks 1-3: Strengthening and bracing the gable
end. There are a couple different methods that have
been worked out for securing retrofit studs. The drawing (above)
illustrates the basic procedures involved for the "L-strap method,"
and the photo (left) shows the finished product.
The first step is to install 2x4 braces, called horizontal braces,
which butt up against existing gable-end studs. These must extend
at least 6 feet into the attic and be placed at each gable-end stud
(typically every 24 inches). Horizontal braces distribute the wind
loads on the gable-end wall to the ceiling and the roof diaphragms.
Distributing the load over a large area like this avoids local
failures of roof decking and of ceiling drywall or plaster.
The next step is to install an additional stud, called a retrofit
stud, beside each existing gable-end stud that is more than 3 feet
long. This supplemental stud, oriented perpendicular to the
gable-end wall, provides the beam necessary to resist bending
The retrofit studs must be connected to the ceiling and roof
diaphragms via the horizontal braces. The transfer of force is
accomplished by the use of straps, and perhaps with compression
blocks, depending on the retrofit method selected. See table
(below) for specific sizing and spacing of all the elements needed
for the retrofit assembly, including strap sizes.
The goal of these tasks is to render the gable-end wall, attic
floor, and roof structure as one "unitized" assembly. Retrofitting
a gable-end like this will make it about as strong as one built to
today's building codes, and the cost of materials will be only
about $300 for a 30-foot-wide gable end. Wider gable ends will cost
disproportionately more but are still reasonable.
Task 4: Strengthening the wall-to-wall
connection. The fourth task connects the gable-end wall to the
rectangular wall below. Based on hurricane experience and
observation of gable ends of older homes, it is assumed that the
lower wall is not well enough connected to the gable-end wall, and
that the upper part of the lower wall is apt to get pushed in or
(more likely) pulled out by wind forces. This connection can be
strengthened by installing straps, brackets, or screws between the
walls. The particular connection method depends on the construction
of the two walls. Generally, making a good connection is relatively
easy once one decides on the appropriate method. A table (not
included here, but available on the Web site) indicates the spacing
of the connections that again are based on exposure and wind speed.
Connections cost less than $2 per linear foot of wall.
This table, which is adapted from Table 1204.2 of the Florida
Building Code and reproduced in the Hurricane Retrofit Guide,
specifies the number of fasteners and minimum size for many of the
elements in the retrofit framing assembly. As winds increase in
severity (measured as both exposure conditions and wind speed),
more fasteners and framing lumber are needed.
Task 5: Strengthening the sheathing
connection. While in the attic, you may need to take steps to make
sure the roof decking stays attached to the roof structure. If a
roof has been reroofed and had the roof decking renailed to the
latest code requirements, a major step has been taken toward
reducing the likelihood that roof decking at a gable end will
become detached. However, if a roof has not been nailed in
accordance with today's building code requirements, apply wood
strips secured with AFG-01-rated construction adhesive to the
joints between the roof decking and all the structural members to
which it is fastened, including rafters or truss top chords,
gable-end walls, and the outlookers of overhangs. Construction
adhesive in 32-oz. tubes is relatively inexpensive, goes a long
way, and provides a reasonably effective bond. It is an imperfect
connection, though, because the long-term effectiveness of
construction adhesive is not known. But however imperfect, it will
do a lot to hold decking in place and reduce the chance of the roof
decking over the attic from getting peeled up in a high-wind event.
The only surefire solution is to secure the roof decking when the
roof covering is replaced.
Assembling the pieces: After the horizontal braces get
installed, metal straps must be preinstalled on the retrofit studs.
These retrofit studs can then be installed by bedding them in a
generous bead of an AFG-01-rated structural wood adhesive along the
edge of each gable-end stud.
Even with roof decking well fastened there may still remain a
nontrivial vulnerability — the overhang. Often, overhangs are
like sore thumbs waiting to a make a building "ouch" because they
are so weak and exposed. Insuring good fastening of roof decking
near the gable can help, but every means to strengthen the
fastening of outlookers or otherwise reinforce the overhang must be
A well-secured, well-sealed gable end should give a homeowner much
more confidence in his or her home. Bear in mind, however, that
opening protection must also be addressed. Even the best-framed
roof structure will not be able to resist the extreme forces of
internal pressurization. ~
Richard Reynolds is a contractor in Bradenton,
Fla., and chairman of the Florida Home Builders Association Codes
and Standards Committee. Photos by IBHS.