In 2000, I moved from Kentucky to Florida to manage Solid Wall
Systems, a firm undertaking the construction of thousands of
new residences for Mercedes Homes. Instead of conventional
framing, these homes would be built using a cast-in-place wall
system above a slab-on-grade foundation (see Figure 1). I'd
been running a successful foundation business back home, so
making the transition to above-grade work was fairly
The author installs cast-in-place walls
on the thickened edge of a slab-on-grade building platform. No.
5 rebar "up-rods" are wet-set into the slab and, once cured,
wire-tied to the wall reinforcement.
Concrete-block walls began to replace wood framing in
Florida's residential construction many years ago. Between
fluctuating lumber prices, voracious tropical termites, and
Hurricane Andrew in 1992, the shift was only logical. Concrete
prices are historically stable, block is entirely termite
proof, and concrete's mass provides superior wind resistance,
as well as an ideal surface for a low-maintenance stucco
The Case for Poured Walls
Even a skilled block mason is limited to a personal output of
between 75 and 150 blocks per day, depending on the number of
window and door openings and other details. A single
8x8x16-inch block is equal to about 9/10 square foot of wall
area, so it typically takes a team of experienced masons
several days to complete an average, single-story house. But in
Florida's expanding residential market, the available pool of
skilled masonry workers is stretched to its limits.
Cast-in-place walls resolve not only the labor issue, but also
the key issues of speed, quality, and economy. The average pay
on our jobs is $13 per hour, and it takes only a few weeks to
train a new laborer in our methods. Furthermore, poured wall
construction, at $4 per square foot (standard) to $6 per square
foot (custom), compares favorably with block, which runs around
$4 per square foot.
Our current crew of around 50 field personnel is on target to
complete a thousand 2,000-square-foot concrete homes in 2003.
Obviously, we have to have a rock-solid system to manage that
kind of workload.
Blockheads. Although poured
house walls are obviously nothing new — in the
Caribbean and South America, cast-in-place house walls have
been common for decades — the idea is just starting to
catch on in the U.S. One likely deterrent is the start-up cost:
Although a block mason can go into business with a few thousand
dollars per crew for equipment, it costs at least $500,000 and
up to $1,000,000 to get a cast-in-place company up and running
at our current level. (When we started about a year ago, we
were completing one job a day. Today, we've got eight crews and
are completing four homes a day.) Another issue is
unfamiliarity: Tract builders are concerned about losing sales
with a different building method. An 8-foot ceiling height is
standard in the residential development market, but we work
with 9-foot forms. Many builders have trouble swallowing the
added interior material costs introduced with 9-foot ceilings.
But that extra foot offers some of the drama of a vaulted
ceiling without adding framing, drywall, and finishing
We work hand in glove with local independent slab companies.
The mild climate, high water table, and flat land make deep
footings unnecessary and impractical. The local soil is
referred to as "sugar sand," a fine, smooth sand that resists
compacting. The site is first prepared by placing a trucked-in
layer of compacted soil over the sugar sand. The perimeter is
then formed and deepened to provide a 12-inch-thick monolithic
footing. Once the in-floor plumbing is roughed in and
inspected, the slab crew lays a 6-mil vapor barrier over the
compacted soil to prevent ground moisture from wicking through
the slab. The slab is a 4-inch-thick 3,000-psi fiber-mesh
concrete, pumped, spread, and finished inside the formwork.
Fiber mesh replaces more costly 6-inch wire mesh and the 300 to
400 plastic chairs in the average 3,000-square-foot home.
Pumping costs extra, about $500 to $600 per day, but that cost
is more than made up for in labor savings.
To tie the walls to the slab, we use 4-foot-long #5 rebar
"up-rods," bent at right angles and "wet-set" into the freshly
poured concrete on 6-foot centers around the slab's perimeter.
The rods are set about 3 inches in from the perimeter forms and
stand 3 feet high. The slab is left to cure overnight, for a
minimum of 8 to 12 hours.
The morning after the slab has cured, the steel crew is on it.
An 8-foot-10-inch-long #5 reinforcing bar is wire-tied to each
perimeter up-rod. The up-rods are connected at the top of the
wall to a continuous tie beam formed from single 20-foot
lengths of horizontal #5 rebar to outline the exterior wall.
Above all door and window openings, the crew ties a double
layer of specially designed header reinforcement (Figure
Figure 2.The typical concrete window header is 16
inches deep and is reinforced according to the particular
Next we stand and tie 8x10-foot sheets of 6-inch-square welded
wire mesh against the up-rods. We start the panels at the
building corners and work toward the middle, overlapping sheets
by 1 foot. To avoid conflict with the 1-foot-on-center form
ties, the worker sets the mesh by propping the sheet on the toe
of his boot, lifting it about 2 inches off the slab. This
offsets the wire sufficiently from the ties. Because the mesh
is square and stiff, it forces the corner up-rods to stand
plumb. The plumb corners prevent problems with centering the
mesh between the forms. No leveling is needed as long as the
same worker positions the mesh (and doesn't change his boots).
Rebar spacers — 5-inch-diameter spoked plastic disks
— clip onto the wire mesh in a 4-foot-square pattern
to keep it evenly spaced between forms (Figure 3).
Figure 3.Six-inch-square welded wire mesh is
centered between forms with the aid of plastic spacers placed
on 4-foot centers. Mesh sheets are started at the building
corners, and runs overlap 12 inches.
The crew can tie two to three houses a day by using mechanical
rebar tie guns (Max USA, 800/223-4293,
www.maxusacorp.com), which take less than a
second to throw on a tie. Even at $2,500 per gun, the tool
quickly pays for itself; we've got seven of them (Figure
Figure 4.A mechanical rebar tie gun can make a
secure connection in less than a second.
The Florida building boom keeps local building officials too
busy to schedule inspections, so we allocate a full day for
steel reinforcement inspection.