civil engineering

How House Construction Works - Foundations
by Marshall Brain

Basements, crawl spaces and slabs are the three main foundation systems used on houses. In wet and coastal areas, it is sometimes common to put houses up on posts as well.

Slab
The slab is probably the easiest foundation to build. It is a flat concrete pad poured directly on the ground. It takes very little site preparation, very little form work for the concrete and very little labor to create. It works well on level sites in warmer climates -- it has problems up north because the ground freezes in the winter and this freezing can shift the slab at worst and at least lead to cold floors in the winter. A cross-section of a typical slab looks like this:

Around the edge of the slab, the concrete forms a beam that is perhaps 2 feet deep. The rest of the slab is 4 or 6 inches thick. A 4- or 6-inch layer of gravel lies beneath the slab. A 4-millimeter sheet of plastic lies between the concrete and the gravel to keep moisture out. Embedded in the concrete is 6-inch by 6-inch wire mesh (shown by the dotted line in the slab) and steel reinforcing bars (shown by the white circles at the bottom of the beams). You will often hear this sort of foundation referred to as a "floating slab" -- it "floats" on the soil, with the deeper concrete around the edge holding it in place. In northern climates, the concrete around the edge has to extend deep enough to remain below the frost line in winter.

One thing about a slab is that the sewer pipe, and sometimes much of the electrical conduit, has to be put in place before the concrete is poured. The sewer pipes are actually embedded in the slab.

Basement
A house with a basement starts with a hole about 8 feet deep. At the bottom of the hole is a concrete slab, and then concrete or cinder-block walls form the outer walls of the basement. Actually, a basement is poured in three pieces in most cases: the "beams," then the walls, and then the slab inside the walls, like this:

This approach helps keep the basement waterproof. The L-shaped piece is a steel reinforcing bar to bind the beam and the wall together.

Crawl Space
A crawl space has several advantages over basements and slabs:

• It gets the house up off the ground (especially important in damp or termite-prone areas).
• It is a lot less expensive than a basement and comparable in price to a slab.
• Duct work and plumbing can run in the crawl space, meaning that they are easy to service and move over the lifetime of the house.

Most of the time, a crawl space is made of cinder block with a brick facing, as shown in the image on the right.
This is exactly how our sample house is put together. Here is how the finished foundation looks:

You might have noticed in the previous pictures that the concrete work for the crawl space was not done with much precision at all. One of the neat things that the mason (bricklayer) does is carefully adjust the height of the cinder blocks and bricks with mortar thickness so that the crawl-space walls end up exactly level all the way around.

One problem that arises in crawl spaces and basements is dampness. In order to keep water out, perforated pipe and gravel are used in a trench around the crawl space to route water away. The drainage system looks like this:

In a house with a basement, this same sort of drainage system is added along the bottom of the walls. The basement walls are then generally insulated with rigid foam board and then heavily waterproofed before dirt is backfilled against the walls.