Single family residences may be built on any of four general types of foundation: a full basement foundation is one that is made of four or more exterior concrete walls that carry most of the load of the house; a surface foundation is similar except that the earth between walls in not excavated and the footings never need go deeper than just below the frost line; a slab foundation is a horizontal layer of concrete that extends over the whole area on which the building will be constructed; and pier foundation is a series of individual concrete posts or piers that extend down to the frost line or else rest on well drained gravel beds. Loads in residential construction are usually small enough so that the minimum footing dimensions required by building codes are adequate to spread the loads. Many codes require a footing to be at least twice as wide as the wall it carries. It must have a depth at lest equal to the width of its projection beyond the wall and never less than 6 inches. The wall can be firmly tied to the footing by embedding Number 4 rebars as steel dowels vertically in the footing concrete at 1 foot intervals. This is good practice even though a keyway has been provided in the top of the footing. The keyway itself if made by setting a tampered wooden form into the top of the form at 4 foot intervals