by Tim Reinhold and
During a storm, the eaves of a home can expose the structure to the
ravages of wind-driven rain on two fronts. The most obvious danger
comes when soffits are blown out during a storm, but testing shows
water intrusion also occurs when winds of 90 mph or greater drive
water through vented soffits.
Following Hurricane Charley in 2004, some 75% of houses assessed
for damage were found to have lost soffit materials. This level of
damage prompted actions to address the problems in the Florida
Building Code, which previously had no soffit requirements. A
supplement to the FBC, effective December 2006, now requires that
soffit systems be able to withstand the same design pressures that
are applied to windows. This change came about in part because of a
recommendation by the Institute for Business & Home Safety's
consultant Eric Stafford.
To learn whether soffit systems can be installed well enough to
survive strong winds, IBHS worked with Bradenton, Fla., contractor
Richard Reynolds on pilot tests of soffit performance. During the
course of the testing, the team examined several homes under
construction to determine what installation methods were typically
being used. Almost none of the inspected soffits were being
installed according to the manufacturer's instructions. And in
nearly every case, the installation methods had shortcomings that
would likely result in the soffit becoming detached during a strong
This report provides an overview of what questions were asked
during the pilot testing, what the tests revealed, and what
builders can do to avoid a repeat of the damage under real-world
Real-World Techniques vs. Manufacturers'
The first round of testing focused on the soffit components as well
as on the anchoring methods and fasteners. One of the first
questions we tackled was whether the installation methods
recommended by manufacturers would stand up to the expected
pressures. We began with wind-tunnel tests that exerted the same
wind pressures on soffits as those used to test windows.
These static pressure tests showed promising results when the
manufacturers' installation recommendations were followed. However,
because the manufacturers' recommended procedures often are
overlooked in favor of the "usual way" of doing things, the team
decided to do further testing under real-world conditions.
Tests conducted with the Institute for Business & Home
Safety used a novel test rig developed at Florida International
University to replicate roofing and soffit damage, which was common
during the 2004 hurricane season (above). The apparatus (top),
known as the "Wall of Wind," relies on two air boat engines to
deliver a torrent of wind and water.
Pneumatic staplers. Testing showed that aluminum soffits
tend to tear when using pneumatic staplers without a depth
adjustment to regulate the depth-of-drive at the nose of the
stapler. When only the air pressure adjustments at the compressor
were relied on to change the throw of the stapler's drive pin, the
results proved highly inconsistent. The tearing was not obvious to
the naked eye, but the soffit material later tended to break loose.
Upon closer inspection, when the connection was carefully
disassembled and the aluminum examined, the tearing was
These tests were performed using several types of woods with
varying specific gravities, from 0.43 SPF to 0.53 SYP. Using a
stapler with a drive-pin adjustment produced a consistent depth of
penetration regardless of specific gravity or grain or staple
orientation. Once the drive pin was adjusted, testing showed that
the ratio of overdriven staples improved from 19-in-20 to 1-in-20
when fastening aluminum components.
This same success did not hold true for fastening vinyl soffit
components. In almost every case when vinyl soffit components were
used, the staples caused the vinyl to split. This problem was not
prevented with a depth adjustment, and it worsened in colder
temperatures. Based on these findings, the team recommended that
staples be used as a means of fastening aluminum soffits but not be
used at all for vinyl soffit materials. When installing aluminum
soffit materials, they noted that it is also critical to follow the
Although T nails were not tested, there was evidence from the
hurricane damage that T nails as well as staples tended to pull
through the bottom of F- or J-channel when loaded by wind. Using
channels with thicker bottoms in conjunction with staples is
recommended for a more secure connection for soffit
Tabbing. One method of installing channel, known as
tabbing, involves cutting a section of the J-channel and bending it
up to allow nailing to the wall. Many contractors feel this is an
unreliable means of attaching a soffit to the wall, but testing
showed that it can work for some field installations.
A simple comparison of J-channel installations showed that tab
spacing at 24-inch intervals was actually stronger than two other
installation techniques. Tabs spaced at 12 inches weakened the
soffit material because of the number of cuts required (Figure 1),
and nails driven at 12-inch intervals through the bottom of the
J-channel, without using tabs, resulted in weaker connections
because the nails often pulled through the material as the
Figure 1. Typical soffit attachment details
that resulted in blown-out soffits (shown) included securing the
soffit with fascia cover or coil stock at the front and using
tabbed J-channel against the wall. Tests found that tabbing can
work if the tabs are cut 24 inches apart, but that tabs spaced more
closely together tend to fail. Even though more fasteners are
included with the closer spacing, the integrity of the J-channel is
compromised by the repeated cuts.
Best practice. The most important finding to come out of
the testing was that following the manufacturers' recommendations
for installation produced the most reliable results. For example,
instead of J-channel, tabbed or otherwise, many manufacturers
recommend securing soffit panel with F-channel, which is designed
to attach to a vertical surface. In reality, the manufacturers'
recommendations may not always be the least expensive way, but it
could mean the difference between a soffit that stands up to high
winds versus one that fails and allows wind and water to blow
freely into the attic.
Resisting Wind-Driven Rain
IBHS site investigations and exploratory tests using a novel test
rig developed at Florida International University (photo below)
provided insight on issues related to wind-driven rain. Not
surprisingly, the testing and observations clearly showed that
complete loss of soffit material led to the most critical cases for
significant water intrusion. This issue can be addressed best by
properly installing soffit materials to make sure they stay in
place during a severe windstorm (Figure 2).
Better Installation (under 16" overhang)
Best Installation (over 16"
Best Installation (under 16"
Best Installation (over 16"
Figure 2. Four options for securing soffit.
The better options more closely resemble manufacturers'
recommendations, which tended to hold up better in static pressure
tests than many of the accepted field practices.
But the tests also revealed significant water intrusion at eaves
and vented soffits installed at gable ends. This problem was
greatest on gable ends that had a drop-chord truss. A drop-chord
gable-end truss allows outlookers to be installed on edge and
cantilevered back to the top chord of the second truss.
Dye test. The wind-driven rain experiment was conducted by
blowing winds up to 110 mph on a roof assembly with plumb-cut 2x6
rafter tails and a 24-inch overhang on a 6/12 roof. It produced a
"boiling effect" as water pooled on the top of the panels and also
worked its way up to the top of the 2x6 subfascia board.
Using a fluorescent tracer dye, it was possible to see water
droplets accumulate in an attic with winds speeds as low as 90 mph
(Figure 3). The same result could not be seen at 75 mph.
Figure 3. Even when soffits are not blown out
of place by storm winds, water intrusion can still damage
interiors. Here, researchers blew water treated with a fluorescent
tracer dye to aid visualization, revealing how water droplets blown
by sustained winds through a vented rake along the gable-end wall
can accumulate in an attic.
The power of wind. The wind-driven water
was blown into the space above the soffit panels through the vented
holes of all types of soffit materials tested. As the pooled water
continued to be agitated by the wind, it was aerosolized into
droplets that were transported several feet into the attic. In
addition, water was blown into the space between the bottom of the
subfascia and the fascia cover.
This water first climbed up 5 1/2 inches between the 2x6 fascia
board and the aluminum fascia cover and then into the nailed
connection space between the roof sheathing and the top of the 1
1/2-inch face of the fascia board. It was clearly visible as
puddles on the top of the subfascia board.
The amount of water injected into the attic would cause severe
damage within less than an hour by saturating the ceiling drywall.
It eventually would flow into the interior of a house, causing even
more extensive damage to interior finishes.
The test showed winds will carry water droplets uphill and in
through soffit panels (Figure 4). Slight differences were visible
in tests performed on various soffit-panel designs. Still, the
tests had a common finding: the amount of water entering the attic
space was significant. Once trapped in such a space, the water is
unlikely to drain out and will continue pooling as long as winds
are strong enough.
FIGURE 4. This table illustrates how far a
column of water can be blown uphill by wind. Testing revealed that
air penetrates the water through vented openings, resulting in a
"boiling effect," and then blows the water droplets into the attic
in a geyserlike display.
Possible solutions. One solution, which
needs additional testing, is the creation of valve or baffle
Valves would allow air to pass through under ordinary conditions
but would be designed to close under high-wind conditions. The
baffle system would block the direct path of water droplets being
ejected from the surface of the soffit panel or cracks around the
fascia boards. These systems could significantly reduce the amount
of water entering through a vented soffit during a severe
While the installation ultimately determined a soffit's ability to
resist high winds, some differences were found in the materials,
too. Vinyl panels tended to perform a bit better in static pressure
tests. The panels bowed dramatically before finally pulling out of
the J- or F-channel restraints, unlike the aluminum panels that
flexed slightly and then failed completely.
In Florida, all soffit materials must be approved, and the approval
includes installation instructions. This approval system, called
"Product Approval," is described in detail by the Florida
Department of Community Affairs, and products can be searched
online at http://www.floridabuilding.org . Not all states have
these requirements. Contact the local building department in your
area to find out more.
Some manufacturers of soffits have product-approval engineering
that is both practical and effective. Builders in Florida are not
limited to this published engineering, but a prudent builder should
not deviate without an engineering review. If the engineering does
not cover a particular installation method, the builder has several
options, including asking the manufacturer to provide it or getting
an independent review. ~
Tim Reinhold, Ph.D., a former professor of civil
engineering at Clemson University, now serves as vice president and
director of engineering at the Institute for Business & Home
Safety. He provides technical direction for the IBHS "Fortified ...
for safer living" new-home construction designation program and the
Property Loss Database, a new IBHS program established to identify
and analyze emerging property-claim trends. Richard Reynolds is a
contractor in Bradenton, Fla., and chairman of the Florida Home
Builders Association Codes and Standards Committee.