In a coastal environment, house foundations need to be water
tolerant as well as strong and stable. The classic poured-concrete
slab-on-grade — a simple monolithic pour with nothing more
than a thickened edge at the perimeter — is not enough where
floodwaters from storm surge or rising water levels pose a threat.
A coastal slab foundation often needs to be elevated above flood
levels using a raised perimeter stem wall, or, in colder climates,
it needs a deep frost wall. In either case, the construction is
In the flood-prone Gulf region, an elevated home is a no-brainer.
But damp conditions and termites are an everyday fact of life that
can makes for one ugly crawlspace. The solution? Fill the
crawlspace with drainable, compacted soils and cap it with a
Good Fit for
Ask building science consultant Joe Lstiburek why he likes slab
foundations, and he'll give you a simple answer: because they're
not crawlspaces. "The best crawlspace in the world," says
Lstiburek, "is one filled with concrete and called a slab."
For northern climates, Lstiburek says, a raised wood floor system
built on piers or a crawlspace foundation may be easier to
insulate, especially if you're after the affordable advanced
insulation and air-tightness levels desired by the U.S. Department
of Energy's Building America program (www.eere.energy.gov/buildings/building_america).
"Farther north, we go with the crawlspace, and we insulate
underneath the floor with rigid insulation," he says. But in South
Florida the elevated slab foundation comes into its own —
there, under-floor or perimeter insulation is inconsequential,
while the method's durability and flood tolerance are a major
"For an elevated slab," Lstiburek explains, "you do a block
foundation or a poured stem wall, you fill the whole thing with
fill material, and then I like to put a topping slab, a cap slab,
on the top — and that locks everything together." (See Figure
1.) It's typical in South Florida to build one-story houses and the
first story of two-story houses using concrete masonry units
(CMUs). "We put an inch-and-a-half seat in the perimeter of the cap
slab, as a seat for the block wall," says Lstiburek. "So we have
our stem wall, then a cap slab, and then another block wall on top
of the slab — and then we insulate on the inside of the block
wall. I think that's a real nice way of raising the slab."
Figure 1. In Florida, where the first-floor
living space is typically built with block, building scientist Joe
Lstiburek recommends a seat formed into the perimeter of a raised
slab. This detail functions as a kind of flashing that allows the
water that will store up in the concrete block to drain to the
exterior, rather than seep in over the first floor.
Not So Easy
Farther along the Gulf Coast, Louisiana building scientists are
trying to introduce the elevated slab technique to builders facing
the state's massive post-Katrina rebuilding problem. Even before
the storm, Louisiana State University's extension program used an
elevated slab as one of several foundation systems for the
"LaHouse" building demonstration project on the LSU campus (Figure
Figure 2. An L-block on the top course, which
serves as a form for the cap slab, is a common detail for stem-wall
slabs, but this specialty block may not be readily available in all
markets. The moisture-barrier membrane must extend all the way
across the stem wall — a detail that requires a liquid
waterproofing applied by hand under the sill plate.
Elevation works. The building's finish
floor sits 3 feet above the site's official Base Flood Elevation,
or BFE, notes project director Claudette Reichel, Ph.D. "We
recommend elevating every home in this area because it gives you
the lowest flood insurance premium," she explains. Base Flood
Elevations are only a statistical estimate of the flood risk, and
protected areas (which includes much of southeastern Louisiana),
flood projections rely on expected levee performance, not just on
topography, climate, or historic flood records.
"In all of southeast Louisiana, the whole New Orleans area and
beyond, once the levees were repaired and certified to supposedly
withstand a Category 3 hurricane — well, that's within the
one percent probability. But if you get a stronger storm, or
something goes wrong and a levee breaks or a pump system fails, you
are going to flood way above that," Reichel explains. While the
biggest premium break comes at 2 feet above the BFE, the LSU
program suggests a viable alternative is going one extra foot
— and using flood-tolerant materials and methods even above
that (see "Low Country Rx: Wet Floodproofing," July/August 2006;
Practical details. Although elevating the
slab has proved practical in Florida, says Reichel, translating any
new method to tradition-minded Louisiana can involve complications.
To begin with, labor in the masonry trade proves much more
expensive in Louisiana than in Florida, so a block stem wall
becomes a more costly option. Also, the design specified for
LaHouse by a local engineer called for L-shaped "header blocks" as
a top course for the stem wall. This course serves as a form for
the slab, but the details hit a snag when it turned out that local
masonry suppliers did not stock this specialty block. The builder
adapted by manually cutting standard block out into the required
shape (Figure 3), but that was a time-consuming work-around.
Figure 3. Lacking locally available L-blocks,
a builder in Louisiana resorted to cutting standard-size blocks to
create the slab form. While effective, this work-around increased
the cost of completion.
Nevertheless, the LaHouse project serves to demonstrate some
important principles for the Gulf region. Protection against ground
moisture is key in wet, rainy Louisiana; for the LaHouse, a strong
poly membrane was placed between the slab and the subgrade, and the
top surfaces of the perimeter block wall were waterproofed by hand
with a liquid waterproofing compound. Traditional practice in
Louisiana, even for slabs placed directly on ground with no stem
wall, has been to place poly beneath the slab area, says Reichel.
But rarely does this moisture barrier extend beneath the thickened
slab edge. Because moisture can wick along slabs from the edge and
add to interior air moisture loads as well as stress flooring
materials, a capillary break must be provided under the entire slab
to keep the living space dry. Installing a capillary break and
moisture barrier may be simpler with Lstiburek's "cap slab" method
than with header blocks, Reichel notes.
Termites are another serious issue in Louisiana, and the LaHouse
program emphasizes a strategy of multiple lines of defense. All the
cores in the perimeter block stem wall were filled to block the
insects' travel path (Figure 4). Besides traditional soil
treatments, the LaHouse also uses mesh termite screens around pipe
penetrations, and the entire framed structure uses borate-treated
Figure 4. In termite-infested locations, every
core in any block wall should be fully grouted.
Will the elevated slab technique catch on in Louisiana? Despite
local supply and labor hurdles, some Louisiana builders are already
using the method, says Reichel. But even with the simpler
Florida-style cap slab process favored by Lstiburek, she observes,
the scarcity of masonry labor makes the stem-wall/slab combination
a relatively high-cost option. "The higher-end homes favor that way
to elevate," she reports, but the more conventional, more
economical pier-and-beam foundation system is more common. Very few
stem-wall crawlspaces are being built, she reports, because with
masonry labor, that is also more costly.
For now, at least, the elevated slab is one more chapter in the
Gulf Coast's long saga of recovery — a story in which there
are many good ideas but no easy answers.
In the Northeast, slab foundations, in general, are much less
common. But Massachusetts-based design/builder Andrew DiGiammo has
been building slab-on-grade in this basement market for many years,
when it fits. On a recent multifamily project of 12 one-story
duplex homes, for example, he used slab foundations because it was
"Even when you build a basement, you still have to pour a slab," he
notes. A slab-on-grade does double duty as foundation and first
floor. Plus, the design brings the living space closer to the
outside grade. "As soon as you go to a wood-framed floor deck on
top of a foundation wall, you raise your floor at least another
foot above grade," DiGiammo points out. "Now you have to build
stoops and stairs, and some of our older customers don't like that
— they don't want to negotiate three steps to get inside.
With a slab-on-grade, it's one step and in — just enough to
keep the snow out."
Slab foundations also allow DiGiammo to avoid the risk of wet
basements where water tables are high. "It's one thing if we're
building someone's $2 million dream home on a wet site; we'll
engineer a solution to keep the water out of the basement. The
customer's willing to pay for that," says DiGiammo. "But when we do
a whole set of duplexes or town homes, we don't want to create 60
units with water in the basement — or come up with a $12,000
solution for each unit to keep the water out."
Elevating the concept. Suitably sited,
the slab-on-grade solution can also work well for homes in the
Coastal A zone, where builders are required to place the
first-floor living area above the base flood elevation for the
site. "In the A zone, I often do a slab-on-grade only because you
can't have a living area or mechanical area below the base flood
elevation," says DiGiammo. "A full basement doesn't have much value
if the homeowners can't use it."
Figure 5. In cold climates where the risk of
flood is not as great as the risk of frost heave, slab foundations
require stem walls that extend below frost depth.
Massachusetts-based design/builder Andy DiGiammo uses this detail
where the budget and scale call for an affordable foundation. When
the flood risk is high, the stem-wall height can be increased. But
if the water table and flood risk are low, DiGiammo prefers to keep
the slab near grade for accessibility.
In the relatively cold climate of Massachusetts and Rhode Island,
foundations must rest below the frost line for stability, and
insulating the slab perimeter is the trickiest part of the
foundation design. "I've seen it with infrared photography," says
DiGiammo. "If you don't create a thermal break at the edge of the
slab, all your heat just screams for that spot." DiGiammo's
preferred method is to create a base of compacted structural fill
inside the poured-concrete frost wall, and install a mudsill made
of four treated-wood 2x6 members directly on top of the wall. Then
he installs 2 inches of rigid foam insulation on top of the gravel,
and tacks 1 inch of the foam to the treated-wood sill to form the
edge of the slab (Figure 5). "The foam serves as a thermal break,
and it's also our expansion joint," he explains.Contributing editor Ted Cushman has been
covering construction business and technology since