Gables are indispensable to many house designs, but wide
overhangs (desirable for both aesthetics and shading) are real wind
catchers. One answer is to support the overhang with struts —
a characteristic of the bungalow style. In high-wind zones, the
attachment of the strut to the wall and the roof edge should be
reinforced with steel hardware.
Although not exactly a page turner, the latest edition (2005) of
ASCE-7 is the definitive source for wind-load design. For designers
with an engineering bent, the best way to gain a deeper
understanding of the issues is to engineer a simple home using this
document. Get help from a structural engineer or even from a sales
engineer for a fastener manufacturer, as specifying the right
number and type of anchor clips is crucial.
Here, we'll concentrate on the consequences of various roof forms
and details. The roof features that increase the risk of wind
damage, in order of decreasing impact, are wind catchers,
undesirable geometry, and interruptions (see box below).
Unfortunately, these roof features are also desirable features for
a stylish house. As compromise seems inevitable, it helps to know
which features most compromise wind performance.
When positive wind pressure gets under an open structure, it gets
added to the negative uplift pressure imposed by the wind, more
than doubling the wind load. In a closed house, the interior
pressure is the intermediate between positive and negative,
minimizing the loading on each envelope plane. But removing the
garage door or several windows (as the case might be if these were
blown out in a major storm) would cause the interior to become
positively pressurized, greatly increasing the load on all planes
except the one facing the wind. This is why it is essential to
reinforce the garage door and install windows that are firmly
attached and capable of resisting impact loads.
To make matters worse, open roofs, projecting canopies, and roof
overhangs are typically thought of as lightweight, inexpensive, and
primarily decorative elements, and as such are often poorly
detailed to resist uplift. Just where you need twice the wind
protection, you typically get half.
In designing a carport or patio roof, every element must be
• Hold down each layer of roofing against
• Size and secure the roof deck for
uplift. It pays to close up the deck with tape or
sealant to eliminate positive pressure on the roofing and
• Size each horizontal structural member for
uplift. Note that ordinary joist hangers don't resist
• Anchor columns top and bottom. Use column
bases that resist the design uplift.
• Provide enough weight or friction in the
foundations to resist total uplift. Small pier
footings may not do the job, and a slab may need to be thickened to
hold the column anchors.
Projecting elements without columns, such as canopies and wide roof
overhangs, need similar treatment for the roofing, deck, and
framing. The load can be carried back to the building in two ways:
with struts or as a cantilever. Struts anchored top and bottom are
always the best solution. Make struts as long as possible to
minimize loads, and engineer both top and bottom connections.
Without struts, the structural members must cantilever. Don't guess
at the cantilever design — use the expertise of a structural
engineer. Cantilevers are most highly stressed where they penetrate
the building envelope, so good flashing and waterproofing are
essential. Unfortunately, it is difficult to flash and seal a
cantilevered wood beam, because there is no vertical step to resist
horizontal water penetration, and the beam moves a lot when it is
loaded. Steel can be very useful in cantilever design.
An intermediate roof pitch conforms to the natural flow of air
around the obstacle (the home), reducing wind pressure on the roof.
A steep pitch blocks the flow, adding positive pressure on the
front plane and suction on the back. This added pressure occurs
high up, increasing the tendency for the home to tip over. A low
pitch creates suction over the entire roof. The current
"neo-modern" fad features both flat roofs and large overhangs,
neither of which are desirable features.
The height of the house impacts the wind load, too. A taller home
intercepts more wind, increasing the loads on the mudsills and
requiring more careful engineering and construction. A thin cross
section (as in a single-wide mobile home) increases the uplift on
the windward side and can be toppled if not securely tied down.
Both features, however, are important for creating good
neighborhoods with relatively high densities, which often requires
deviating from a wind-smart design. While each house in a dense
community to some extent protects the next from wind on the
vulnerable long sides, it's critical that you get the hold-down
Interruptions add wind load and compromise the roof's resistance.
Figure 1 shows an ideal wind-smart home: one story, almost square,
with a hipped roof at an intermediate pitch, no interruptions,
short overhangs, and no attached open roofs or canopies. Figure 2
shows a "gable-on-hip" design, adding interest and providing for a
window or attic vent. The projection adds turbulence, but the
intrinsic shell-like structure of the hipped roof is
Ideal Wind-Smart Roof
• Intermediate pitch
• Short Overhangs
• No attached open roofs or canopies
• Compact squarish form
• One-story design
Figure 1. A square hip is the most durable
roof design for high-wind zones.
Shaded band acts as a shell, providing added strength
Figure 2. A gable-on-hip design adds visual
interest. While the small gable will create more turbulence, the
basic design is still sound.
Gabled roofs are known to fail in hurricanes when compared with
hipped roofs, but they are indispensable for most designers. Figure
3 shows a "clipped gable" (also called a "snub-nosed gable"), where
the highly stressed gable peak is eased off with a small hip. Such
roofs (when built with desirable short rake overhangs) are found in
early 20th-century Tudor revival designs and in southern colonial
churches, neither of which are commonly found in high-wind zones.
Much more common are bungalow designs with clipped gables, but they
have undesirable long and thin roof overhangs. Struts under the
eaves are also authentic to the bungalow style (see photo, page 7),
and may be necessary in high-wind zones.
Clipped Gable (Hip-On-Gable) Roof
Reduces turbulence at peak uplift, where forces are greatest
Figure 3. A clipped gable reduces the wind
force where it is the greatest on a gable design — at the
The take-home message: Almost any design can be engineered to
resist almost any wind load, given careful engineering, closely
supervised construction, and a generous use of properly chosen
fasteners and tie-downs. But relative to commercial construction,
home building is usually weak in all these respects. Stay as much
as possible with wind-smart features, and if you need to deviate,
spend the extra engineering, supervision, and tie-downs on designs
that give you the most bang for the buck.
— Gordon Tully is an architect in Norwalk, Conn., and
teaches a summer executive education course at the Harvard Graduate
School of Design. Illustrations by the author.