I build about eight custom and spec homes a year, mostly in
new residential developments. It’s mountainous in Utah,
where I live, so we have a lot of sloped sites. This winter I
tried out a new product that works really well on a slope
— precast garage slabs.
We were building a spec home on a steep lot in a subdivision. I
had built the same plan four times before, and it always sold
well. This time was a little different because of the grade:
The earlier houses happened to be on flat lots, but this lot
dropped off 12 to 14 feet from the front to the back. The back
wall of the basement needed a 4-foot-high foundation wall just
to get up to floor level in the basement. We decided to
wood-frame the whole back wall of the basement, creating a
walkout onto a patio underneath a covered deck.
The problem was that the common foundation wall between the
house and the back of the garage needed to go 12 feet deep just
to get the footings down onto good ground. I’m sure you
can imagine the hassle and costs to bring in 8 to 12 feet of
gravel to fill in that hole so we could pour a 550-square-foot
garage slab. We’re talking about more than 200 cubic
yards of gravel to hold up a 4-inch slab — that
didn’t look very cost effective.
Spandeck Installation Details
Figure 1.There are two main options for setting
the precast deck components onto the basement wall: You can
either set the slab inside the foundation on a shelf (left) or
set it directly atop the sill (right). Mudsills for the garage
walls are attached to the foundation with anchor bolts as in
any garage, but blocking is nailed in the stud bays to form a
dam for the topping slab pour. Asphalt felt paper is applied to
the perimeter walls before the topping slab is
A local contractor is also a rep for Eagle Precast, a Utah
company that makes a precast 8-inch slab that can freespan up
to 30 feet. Once I thought about using their product, the
decision to go with a garage over a full basement was a
no-brainer. I added an extra 550 square feet of space in the
basement of the house — space that ended up costing about
$8,000 to $10,000, finished. That’s only $15 to $20 a
square foot, compared to an average price of $125 or more per
square foot for a typical home around here.
This basement is also better than the usual basement space,
because there’s no need for a center girder or posts. The
slabs come 8 feet wide by 24 feet long and make the whole span
with no support. In fact, they can handle as much as a 30-foot
span, easily carrying loads of 100 psf or more. The 10-inch
panels can go even farther.
The only drawback was ceiling height. The panels had to sit on
a 4-inch shelf inside the basement wall, which lowered the
ceiling by 8 inches. The downstairs room ended up with only 7
feet of headroom once the basement floor was poured. But it
will still make an excellent media room when it’s
finished, or a great storage room.
On the next house I started, I decided to pour 10-foot basement
walls in the garage basement area, then install a 28x45-foot
precast garage slab. It’s on a flat lot, so there’s
no walkout, but we managed to get 9-foot ceilings downstairs.
On that house, the cost worked out to roughly $18,000 to gain
1,380 square feet — still less than $15 a square foot.
That’s pretty tough to beat.
Figure 2.The author set his precast slabs into an
8-inch-deep shelf in the foundation wall. The shelf must
provide at least 3 inches of bearing for the precast slabs.
Half-inch rebar cast into the foundation wall at 2 inches o.c.
will be bent down over the precast sections before the topping
slab is poured, to lock the assembly together.
Considering the value added, the whole process is pretty easy.
The first thing you have to determine is whether you want to
set the slab on top of the foundation or recess it (see Figure
1). I wanted the slab recessed, so I had the foundation
contractor put a ledge in the foundation all the way around the
The ledge is 3 1/2 inches wide and 6 inches tall (the slabs
need only 3 inches of bearing minimum on each end). The
foundation contractor also has to put in rebar all around the
foundation to bend over the top of the slab after it gets
dropped into place (Figure 2). This ties wall, slab, and
topping slab into one tight piece.
The topping slab is necessary for a couple of reasons. For one
thing, the suspended slab is pretty ugly and has big hooks
sticking out of it that are cast in for the crane to use when
setting it in place. The other reason is to provide enough
slope to your garage floor to get water out. I poured the
garage floor at 6 inches deep against the house, sloping to
just 4 inches deep at the garage door.