After excavating, build the foundation and backfill the remaining excavated space with earth. Residential building floor levels are higher than the natural ground level. Fill the space with soil until it reaches the floor level, then compress it. The earthwork for the residential structures is now complete. For commercial buildings, fill the excavated area with dirt and tamp it down carefully. Residential foundations usually consist of several layers. The bottom layer is called the footer. It should be at least 3 inches thick and made of compacted gravel or crushed rock. This provides a stable base upon which to build. Next, place 2 to 4 feet of coarse material such as sand or clay. This forms the bulk of the foundation and is called the sub-base. Over this you can place smaller stones or brick for extra weight bearing capacity. The final layer is called the finish grade. This is smooth soil that covers everything else. It is used as an exterior surface to allow water to drain and not puddle on the floor of the structure. For commercial buildings, the footings are the same size as those for residences but they must be deeper. This is because commercial buildings require more stability than homes do. The bottom layer of the footing is also called the base course. It is made of small rocks or concrete placed about 6 inches apart. This helps to distribute load across a larger area of the bedrock below. Above this comes another course of larger rocks or concrete.