House Foundation Design Guide

Written by john landers
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House Foundation Design Guide
Construction of a concrete slab foundation. (fresh concrete image by jimcox40 from Fotolia.com)

The house foundation is designed to provide support for the weight of the entire house structure. A foundation prevents the house from slipping, settling or sliding from its initial position. Foundations may take one of three forms: basements, crawl spaces and concrete slabs. Depending on the foundation type, it may consist of footings, foundation walls, concrete slabs or a combination of all three.

Footings

Foundation footings, which are made of concrete, are bases that support columns or foundation walls. Minimum standards usually call for the depth of the footing to be the same thickness as the wall the footing supports. The width of the footing is usually twice the width of the wall. Most building codes require footings to be installed below the frost line, which, at minimum, ranges from 18 to 48 inches depending on the region of the country. Footings are generally built separately, but you may be able to pour the footing along with the concrete slabs.

Foundation Walls

Foundation walls connect footings and the first floor of the house structure. Walls can be made from concrete blocks or poured concrete. For poured concrete foundations, your contractor will need to build forms that are level and properly braced to hold the concrete in place until the concrete cures. Usually a large crew is required to perform the job in a timely fashion---once the pouring is started, it must be completed. Mistakes are difficult to correct and can be costly.

Concrete blocks are easier to use than poured concrete and don't require forms. You can work at your own pace and start and stop as necessary. Slip-ups can be easily corrected and at little additional cost.

Basements

Basements, or T-shaped foundations, are typically found in northern regions where freezing is a concern. You will need to build your footings separately before constructing the concrete walls. The concrete floor is then poured between the walls to complete the structure. The standard height for basements is about 7 feet 10 inches. If insulation is desired, plan to install a radiant barrier or rigid foam board before pouring the slab.

Crawl Spaces

This type of foundation is common in regions where the soil has a significant amount of clay content. The basic methods for building a basement or crawlspace are very similar, but you will typically leave the floor. Crawl spaces have very limited headroom (often as little as 24 or 36 inches of space), but unlike concrete slabs, you should have room for some building systems; consult your contractor to determine how much space is needed for wiring, plumbing and heating ducts if you wish to have them installed underneath the house.

Concrete Slabs

The slab-on-grade method is very popular and will allow you to simultaneously pour the footing and the concrete slab on top of a bed of gravel. Wire mesh should be inserted into the concrete to add reinforcement. The typical thickness of the concrete slab ranges from 8 to 10 inches.

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