Backfill beneath concrete flatwork along foundation walls should be done using a uniformly sized granular material. Some fill materials, particularly spherical stones like pea gravel, self-compact. "I propose using open-graded granular material," Tull explains. "This means that there will be an upper layer of coarse material which has been graded to allow water to drain away from the foundation wall." The lower part of the hole will be filled with a finer grade of material. This creates an air space between the foundation and the ground below which allows for some movement in response to temperature changes without causing problems such as cracking or leaking. Concrete flatwork (the surface upon which a floor is laid) requires special attention when backfilling underneath it. If the hole for backfilling contains any sharp edges they will eventually show up as cracks in the finished floor. To prevent this, use a backhoe or other mechanical digger to excavate a large enough hole to completely cover the flatwork with gravel or other filler material. Then smooth out any rough edges on the flatwork with a power edger or other tool before pouring the concrete. It is important to keep track of how much concrete was used for the flatwork so you do not run out before finishing the rest of the house.