Following the 2004 hurricane season, Florida-based Mercedes Homes
set out to design and build a truly hurricane-resistant home. But
as researchers and builders have come to understand only too well,
a home that provides complete protection from an intense tropical
storm must offer not just superior structural strength but also
afford both greater resistance to wind-driven rain and improved
The prototype to this hurricane-resistant home began in 2000, when
Mercedes Homes started working with Building America research teams
to develop an advanced wall system to replace the concrete masonry
unit (CMU) construction typically used in Florida. After
experimenting with a variety of test systems, including two
different types of precast concrete wall systems, Mercedes Homes
selected a cast-in-place concrete wall system, which it dubbed the
Solid Wall System (SWS), and began providing it as a standard
offering on its homes in Central Florida.
Beginning in 2001, with funding provided by the Federal Emergency
Management Agency (FEMA), the Mercedes concrete wall system was
rigorously examined and engineered for hurricane resistance by
researchers from University of Florida's Program for Resource
Efficient Communities and Steven Winter Associates, Inc.
Engineering analysis of this initial wall system showed a potential
for marked improvements in structural integrity over a CMU wall.
However, the 2004 hurricane season provided a different lesson
about hurricane protection: after a record number of tropical
storms pounded Florida and the Southeast, the majority of damage to
buildings was not so much structural failure but damage from
wind-driven rain intrusion. In order to be truly hurricane
resistant, the new wall system would have to provide not only
enhanced wind-load protection but also significantly reduced
incidence of water intrusion.
The 2x4 used in the "large missile test," which has become the
standard measure for impact resistance, completely shatters upon
impact with solid concrete, causing no damage whatsoever to the
advanced wall system developed by Mercedes Homes and the Department
of Energy's Building America program.
In much of Florida, the typical single-family home is built with
CMU walls. Although stronger and more durable than standard frame
construction when subjected to a tropical storm, these homes are
still susceptible to lateral and uplift wind-load failures as well
as to penetration damage caused by wind-blown debris ("missiles").
The Mercedes SWS cast-in-place concrete wall developed with the
help of the Building America program performs better than a CMU
wall (and monumentally better than a wood-framed wall) at resisting
these forces (Figure 1).
Figure 1. Windows that meet the ASTM E 1886
and ASTM E 1996 impact standards shatter but do not present an
opening that will either pressurize the interior or allow undue
amounts of water into the home (above). However, most conventional
wall systems do not fare as well under the same test. A typical
vinyl-sided wood-frame wall is obliterated by the large missile
test (bottom left). A concrete block wall performs a bit better but
still allows a sizable hole for wind and water to penetrate (bottom
The science of SWS. The SWS system uses 3,500-psi concrete placed
in modular aluminum forms to create its 6-inch walls. The concrete
encloses a steel reinforcing cage of 6x6x1/8-inch "road mesh"
combined with vertical 5/8-inch reinforcing bars placed at 4-foot
intervals (Figure 2).
Figure 2. The heart of the Solid Wall System
(SWS) is a steel reinforcing cage of 6x6x1/8-inch "road mesh"
combined with vertical 5/8-inch reinforcing bars placed at 4-foot
intervals (above). The cage is specifically designed to evenly
distribute lateral wind loads and minimize point stresses imposed
by missile impacts, which can lead to failure. The steel cage also
connects to steel in the footings. Wet-set tiedowns (hurricane
straps) are embedded every 2 feet in the concrete as the walls are
poured, providing a continuous path to resist uplift.
This steel-reinforced concrete wall assembly acts as a monolithic
composite system with superior resistance to hurricane-induced
forces. When a point impact load or a sustained lateral load is
imposed against the wall surface, the composite system acts to
spread the forces through the system, thereby reducing the "stress"
— and the potential for failure. The concrete (strong in
compression, but not in tension) and the steel mesh (strong in
tension, but not in compression) work in tandem, creating a wall
assembly that is strong in both tension and compression.
Typical wind speeds for a Category 5 tropical storm range up to 155
mph, with wind-borne debris traveling well upward of 100 mph.
Engineering analysis shows that the cast-in-place system can
withstand missiles fired up to 200 mph. Uplift forces are also more
efficiently resolved with SWS walls. In typical CMU construction,
the upper course of block masonry is usually a grout-filled "bond
beam" into which the roof truss tiedowns are set. Failure of this
system can occur when the uplift wind forces exceed the limits of
the bond-beam structural connections, which are most often
solid-grouted CMU cores with rebars at every 4 feet on-center.
Because the uplift load is resistant on a CMU wall at concentrated
points, the potential for failure is greater than for the SWS
walls. As with the impact-load resistance, the SWS walls
effectively "spread" the applied forces, reducing opportunities for
Other performance improvements.
Structural strength is not the only benefit of the cast-in-place
system. Concrete walls have high thermal mass, reducing energy
transfer between interior and exterior surfaces and reducing
homeowners' energy bills. The continuously poured system minimizes
interior temperature fluctuations and drafts, and it also reduces
noise transmission. Concrete inherently resists mold, termites, and
rot because it contains no organic mater. The moisture that can
enter CMU walls at joints, especially when deformed by structural
loads, is eliminated in the monolithic system, as is the potential
for flood water storage within the hollow CMU cavities.
Additionally, the houses are safe from more than just hurricane
disasters: the SWS structure, with its 6-inch solid concrete walls,
offers up to a three-hour Class-A fire rating, making the home
safer from wildfires.
Wind-Driven Rain Protection
In addition to some structural vulnerability, the CMU walls
typically found in much of Florida are also susceptible to water
intrusion when subjected to a tropical storm. The Building America
team focused on improving the resistance to water intrusion through
the SWS in a number of vulnerable points.
Ground-level intrusions. In the aftermath
of the 2004 storm season, researchers found that high-wind forces
on in-swing entry doors can allow water to enter the home in
significant quantities because the weather-stripping gaskets rely
on pressure in the opposite direction. Complete blow-in failure of
an in-swing door during hurricane-force winds can result in
significant water damage and even changes in internal pressure that
contribute to roof uplift and structural damage. Following
Hurricane Wilma in October 2005, Mercedes found that owners who had
opted for in-swing doors (rather than the now-standard out-swing
configuration) suffered more instances of damage.
Ground-level water intrusion can be mitigated by creating shallow
indentations or recessed seats in the foundation slab to prevent
rain from being driven or sucked into the home under the exterior
doors and walls (Figure 3). The recessed seats work well in
conjunction with out-swing entry doors — a somewhat
unconventional but practical solution to a common source of damage.
For additional protection for openings, the builder also offers
removable hurricane shutters as an option to all home buyers.
Figure 3. During the 2004 hurricanes,
wind-driven rain and excessive ground-level runoff leaked under
exterior doors and through the cold joint at the base of CMU walls.
Mercedes Homes solved these issues during slab forming: A recess
formed in the slab, in conjunction with out-swing doors, limits
water intrusion beneath doors (top). At the perimeter of the slab,
a recessed step provides a key for the poured walls that
effectively cuts off a direct pathway for ground-level water
Sealing walls. Although concrete is
poured continuously across multiple forms, the change in texture at
vertical form joints and tie connections creates a vulnerability to
water infiltration. To eliminate this potential pathway for water
intrusion, the exterior wall surfaces of the Mercedes SWS are
protected with elastomeric sealant at form joints and at the
concrete form snap-tie locations (Figure 4).
Figure 4. Form joints and snap ties are
possible water entry points for wind-driven moisture. Applying
sealant to these potential pathways is a simple, effective way to
prevent water damage to the interior.
A continuous drainage plane covers roof trusses on gable ends to
prevent sheeting water from entering the building assembly at the
truss-to-wall transition. In addition to the typical housewrap
layer over the vertical wall sheathing, a separate
building-paper-backed wire mesh is installed over the housewrap
prior to the synthetic stucco finish. Without this added layer,
moisture penetrating the stucco can seep right through a single
layer of housewrap. The extra layer provides a foolproof drainage
plane and allows for proper stucco adhesion.
Finally, SWS wall surfaces are finished with a high-performance
acrylic coating capable of flexure and bridging small gaps to
prevent the exterior walls from absorbing water during sustained
Roof protection. At the roof level,
researchers recommended several strategies to protect against water
intrusion. In Mercedes houses, a peel-and-stick underlayment
product adhers directly to the roof decking beneath the shingles to
create a secondary roof drainage plane. This provides backup
protection in the case of lost or damaged shingles — a common
casualty of tropical storms.
In another frequent envelope failure, water sheeting off the roof
spills down the fascia board and is driven by wind or surface
tension into the soffit vent openings. To alleviate this mode of
water entry, a redesigned fascia extends an inch below the soffit
to form a drip edge, directing water down and away. A perforated
soffit panel by Alcoa (www.alcoa.com), with recessed rather than surface
openings, limits water intrusion while encouraging greater air
circulation and faster drying within the eaves assembly (Figure 5).
A simple change to baffled ridge and roof vents is also used to
mitigate a common, and often significant, water-entry
Figure 5. The perforations in Alcoa Hidden
Vent are recessed in the panel grooves. The manufacturer claims
this design provides plenty of ventilation airflow, but when
combined with a fascia that extends a full inch below the soffit,
the potential for water sheeting off the roof and getting driven by
wind or surface tension into the vent openings is greatly reduced
Most of the losses from the 2004 hurricane season resulted from
water intrusion, and mold played a significant role. Mold can grow
undetected within the home for long periods of time, compromising
indoor air quality and causing significant long-term damage to the
home. When power outages prevent homeowners from drying out their
homes quickly following a storm, water damage and mold growth
Backup power as a means of mold mitigation. To aid in post-storm
recovery, Mercedes Homes offers a natural gas-fueled generator as
an option to home buyers throughout the Florida market. The unit
allows homeowners to use shop-vacs, fans, and dryers during
post-storm recovery, when power outages are common, thereby greatly
reducing the risk of mold growth and long-term damage.
Mercedes Homes also offers a generator-ready electrical service
panel that can be easily connected to a portable generator. Even if
a homeowner is initially unable to afford the upfront cost of a
natural gas generator, each home will be prewired and equipped for
a generator to be installed at a later date, or as needed (see
"Blackout Power Solutions," September/October 2007; available at
Companion strategy. Removing damaged drywall, and the mold it
supports, was one of the most common storm-related repairs faced by
homeowners following the 2004 hurricanes. To help remove the risk
of mold, which grows particularly quickly on paper, adhesives, and
other organic matter, Mercedes Homes uses non-organic finish
materials. These storm-resistant homes use a new paperless drywall
product offered by Georgia Pacific that has a glass reinforcing mat
facing over a gypsum core, which resists mold growth better than
traditional paper-faced products.
The concrete SWS homes are now a standard product for several
Mercedes Homes divisions in Central Florida. Although incrementally
more expensive than the CMU construction it replaces, the benefits
of the concrete construction to homeowners and to the builder
(production time for the walls are reduced to two days from five)
easily outweigh the costs. The now-standard water-intrusion and
storm-recovery measures add about $2,200 to the cost of the homes,
and the upgrade options (generator and shutters) add another
$8,000. Of the 343 new homes built using the improved
specifications between June and October 2005, only 16 reported any
damage (all minor) following a near direct-hit from Hurricane Wilma
in October 2005 (the last significant hurricane to hit the region,
as of this writing). With ongoing improvements, Mercedes expects to
see even less damage, and many more happy homeowners, in the
William Zoelleris an architect and senior
associate with Steven Winter Associates, Inc., of Norwalk, Conn.
Photos courtesy of Steven Winter Associates.