Slabs, basements, and crawl spaces are the three main foundation systems:
Slab:
The slab is the most simple foundation to build.
It is a flat concrete pad poured directly on the ground.
It takes minimal site preparation, minimal concrete form-work, and minimal labor to build.
Slabs work best on level sites in warmer climates.
Slabs can be troublesome in cold climates because the ground freezes in the winter.
Freezing can cause a shift in the slab, and can cause cold floors in winter.
The outer perimeter of the slab, typically consists of concrete poured to a depth of 2'.
The remainder of the slab is 4-6" thick.
A 4-6" layer of gravel lies beneath the slab.
A 4-mm sheet of plastic lies between the concrete and the gravel to keep moisture out.
Embedded in the concrete, is wire mesh, and steel reinforcing bars (re-bar).
A slab is often referred to as a "floating slab" because it "floats" on the soil, with the deeper concrete around the perimeter holding it in place.
In colder climates, the perimeter concrete needs to be deep enough to remain below the frost line in winter.
Sewer pipes and much of the electrical conduit, must be put in place before the concrete is poured.
Basement:
A house with a basement starts with a hole about 8' 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.
A basement, is typically poured in three phases, in order to maintain waterproofing. First the concrete beams, next the walls, and finally the slab inside the walls.
L-shaped pieces of re-bar join the concrete beams and walls together.
Crawl Space:
A crawl space has some advantages over slabs and basements:
1. It gets the house up off the ground (especially important in damp or termite prone areas).
2. They are much less expensive than basements, and similar in price to slabs.
3. Duct work, electrical, and plumbing can be run in the crawl space, which makes them easier to service.
Typically, a crawl space is made of cinder blocks, with a brick facing.
Mortar thickness, is applied creatively, to ensure that all the crawl-space walls are level.
Dampness can be an issue with crawl spaces. In order to keep water out, perforated pipe and gravel are used in a trench around the crawl space to route water away.
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.
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