There are three main foundation systems used on modern stick-built houses: slab, basements and crawl spaces. In western Washington, the slab foundation is very rare. You will see it more commonly in eastern Washington areas such as the Tri-cities, Moses Lake, and Yakima. Western Washington home builders usually build on a crawl space. If a lot is sloped, a home builder will either step the foundation or in some cases build on a basement.
Slab on grade
A slab on grade foundation is a flat concrete pad poured directly on the ground. It takes very little site preparation. This is most commonly used on level sites in warmer and drier climates. It is not a very good type of foundation for extremely wet areas, nor for areas where the ground freezes. Freezing can cause the slab to shift or crack, not to mention that they are unpleasantly cold to walk on. Radiant floor heating is a modern solution to this problem, however, this option is fairly pricey and so is not for everyone.
The perimeter of the slab on grade is deeper than the interior. This helps keep it from shifting. The interior is typically 4 or 6 inches thick. Underneath is a layer of gravel and a sheet of plastic which acts as a vapor barrier to keep moisture from developing under the slab. The concrete can be poured over wire meshing and/or steel reinforcing bars (or ‘rebar’) to strengthen it from cracking.
A crawl space has several advantages over basements and slabs: It elevates the house somewhat off of the ground which is especially important in damp climates like we have in the Pacific Northwest. It is a lot less expensive than a basement. Ductwork and plumbing can run in the crawl space, meaning that they are easy to service and move over the lifetime of the house. With a slab, the concrete is usually poured over the plumbing.
To create a crawl space, forms are set up in the dig out hole that the excavators made. A concrete truck pours concrete into these forms. As the concrete hardens the forms are removed, leaving the walls which form the foundation.
At various points between the walls, concrete footings are also poured. These will support ‘pony walls’ which in turn will support the joists that the floor of the house sits on.
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 walls, usually of concrete but sometimes of cinder-block walls form the outer walls.
On a sloped lot, both a stepped foundation or basement foundation will work and each has their advantages and disadvantages. Basements will be more expensive especially if they are finished. However, the extra costs involved can make the space either immediately or in the future, livable.
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 back-filled against the walls. At various points, you will find posts which are supporting the weight of the upper floors, though in finished basements, some of these posts may be incorporated into the walls.
Basements are considered either finished or unfinished. Finished means that it is a livable space. There may be plumbing for bathrooms or a kitchen, heating, and lighting. Often it is finished out for a recreation room, gym, home theater, or office, although you may find bedrooms in a basement as well.
Unfinished means that it is quite bare. The water heater may be kept in it, and there may be a few lights for safety. Space in unfinished basements are mainly used for storage, wine cellars, or sometimes for workshop space. Basements can also be partially finished, with a portion of the basement, usually where the stairs come down, being finished and part being bare for storage space.