Installing Precast Garage Slabs - Continued

Watch your elevations. Speaking of garage doors, make sure you plan ahead if you decide to put the slab on top of the wall (which my rep says is the most common application). If you put the 8-inch slab and 4-inch topping on top of the wall, you may have problems with your garage door header. I always build 9-foot first-floor walls (which usually gives me 10-foot garage walls), and I usually install 8-foot-tall garage doors. With the slab on top, we can only get 9-foot-tall garage walls — and with an 8-foot door, that leaves only 12 inches for a header.

Depending on the engineering requirements, that can be a problem. Around here I rarely get a garage door header that’s smaller than double 11 7/8-inch micro lams. In fact, the last house I framed had a 5 1/8 x 19 1/2-inch glulam for the garage header. So you can see that some thought has to go into this rather than just throwing the slab in. If you recess the slab, it doesn’t change any of your wall or header elevations for the garage, but it lowers the ceiling in the basement. On the other hand, if you lay the slab on top of the wall, you gain headroom in the basement. But you have to think about whether to raise the garage wall height, change the stepdown of your garage foundation, or come up with some other solution.

So far I’ve found the best solution is to pour a tall basement wall with a shelf for the precast slab to sit into. That way, the basement has plenty of headroom, and the elevations for the garage and house are the same as they would have been with a regular slabon-grade garage floor.

Figure 3.After sweeping the ledge clean, the install crew lays expansion joint material on the shelf around the whole perimeter.

Figure 4.The precast supplier places a 4x4-inch piece of 1/2-inch angle iron over the door opening into the room below the garage. The angle iron will function as a header to carry the edge of the precast slab.

Setting the Slab

Installing the precast slab could hardly have been easier. The supplier brings his own crane and crew. The slabs come just slightly shorter than your intended span, so they will slip in easily. The ledge in the foundation was cleaned off with a broom, and a layer of concrete expansion joint material was laid on the shelf all around the perimeter (Figure 3). The precast company supplied a 4x4-inch piece of 1/2-inch angle iron to place over the doorway into the room, to serve as a header (Figure 4). Then they lowered the slabs into place with a huge crane (Figure 5).

The slabs come 8 feet wide by whatever length you need, up to 30 or 40 feet long. It takes two guys to lower them in, one guy on each end. After all three are in place, you have to slide them around until the spacing between them is pretty even (Figure 6), then fill the spaces with a foam material and seal up the joints with concrete grout. When the topping slab is poured, it fills any remaining cracks and crevices.

Figure 5.Slab sections are lowered into place by crane. The crane hooks attach to steel cable loops that are cast into the slabs and will be buried in concrete when the topping slab is poured.

Figure 6.The installer uses a large pry bar to create even spaces between the slab sections. Voids will be injected with foam sealant, then filled with concrete grout.

Framing the garage walls. The rebar that sticks up from the top of the foundation wall leaves about 4 or 5 inches of that wall for the garage wall’s bottom plate, or mudsill. The foundation wall has bolts set into it for fastening the sill down, just as in a regular job. The plate gets set right onto the foundation just as it ordinarily would; so you have to frame all the garage walls as usual, then block in between each stud and apply felt paper over the blocking (Figure 7). Then you pour the topping slab up against the felt (Figure 8).

Figure 7.Garage walls are framed on a sill bolted to the foundation, as in any garage, and blocking is nailed between the studs to form a dam for the edge of the topping slab. Next, asphalt felt paper is stapled to the blocking to protect it from the wet concrete. Note how the rebar has been bent down to tie into the topping slab.

Figure 8.The last step is to pour the concrete topping slab, using a standard 4,000-psi concrete mix.

Comparing the Costs

Here’s how some of the figures worked out for the first home where I installed a precast slab.

Because of the slope of the lot, I had to have 8-foot walls in the garage, and some sections of wall were 12 feet tall, which would have been the case with or without the precast slab. With a regular site-poured slab on grade, I would have needed to fill the entire area under the garage with pea gravel, at a cost of at least $2,000.

I paid $4,500 for the suspended slab, another $900 to pour the basement slab, and about $250 to have a doorway blocked out and a door hung. That’s $5,650 in all; back out the $2,000 I didn’t have to spend for fill and I gained 550 square feet of basement space for a net cost of $3,650. I figure the area could be finished as a media room or playroom, for example, for less than $5,000. So for around $8,000 or $9,000, I added 550 square feet — pretty cheap space.

You get the best value with this technique on a hillside lot, where you’d need fill to support the garage slab otherwise. The next house I built was on a flat lot. In that situation, we would normally need only a 4-foot wall for the garage, but we put in 10-foot walls to add basement space. Extra costs came from more excavation, more foundation labor, more concrete, more rebar, and so on. Even so, the buyer of that house spent just $18,000 to gain 1,380 square feet.