Above Grade Concrete Slab

To a large extent, the area you live in and how nature affects it determines which is the best kind of foundation for your house. The four main types are crawlspace, concrete slab, stilts, and basement. If the water table in your locale is too high, a basement is not a good idea.

On the other hand, if the home is located on the 100 year flood plain, stilts are a great way to protect your investment; it also gives you a nice carport without sacrificing any real estate. This article will focus on the above grade concrete slab type of foundation.

In some respects, the concrete slab is the easiest to build. No extensive digging is required and that keeps labor to a minimum, thus freeing up construction budget funds for other purposes. This type of foundation is best suited to temperate climates where freezing is not an issue. Freezing can cause the slab to shift which in turn affects the rest of the house.

Preparation of the Above Grade Slab

The boundaries of the slab are determined using a survey and the perimeter is marked by batterboards at the corners with taut strings strung between them. This tells the workers where to dig.

The concrete slab is basically just a concrete pad poured on the the prepared ground's surface. But integral to that pad, and what gives the slab its stability, is a concrete beam about two to three feet deep that follows the perimeter of the slab.

One of the main things that gives the beam its strength is the lengths of rebar (long steel rods with vertical ridges that serve to give the concrete more surface area to bond to).

Wherever the lengths of rebar either intersect or lap over, they are tied together with wire. Of course the beam is not poured independently of the slab proper.

Excavation

The dirt between the beams (the main part of the slab) is excavated to a depth of about eight inches. A bed of gravel is spread on the graded dirt dirt surface and then a sheet of plastic, usually four mil, is rolled out over it. This is a moisture barrier. Then a hatchwork of rebar is installed a couple of inches over the plastic, supported by plastic supports.

Since we are building an above grade concrete slab, the carpenters must install wooden forms around the perimeter of the pour area. These will be “busted off” once the concrete is cured.

Other Preparations

Before any concrete is actually poured, other trades have things to install. For example, plumbers have to put in sewer lines. Also, the electricians may have a need to run some conduit that is below the grade of the finished concrete but stubbed up out of it. All these things must be done before pouring concrete.

At this point, forward thinking builders will stand up lengths of all-thread around the perimeter to bolt the framing bottom plate to. Otherwise, these can be added after the pour, although this will involve more labor.

Pouring and Finishing the Concrete

The concrete contractor will send the load to the site and either pour it directly from the back of the truck or use a boom if he can't get close enough. The concrete finishers will wade in with rubber boots and spread it out. Once it is set up enough to work they will use floats to smooth the surface.

Once the Slab Has Cured

As soon as the slab has cured, the carpenters will return to bust the forms, or, remove all the wooden forms that they installed to dam in the slab during the pour. Then the slab is turned over to the framers to begin building the house.