A foundation is forever, so it makes sense to pay attention to the details that ensure it will remain dry and crack-free for as long as it has a house to hold up. When Tom Silva is building a house with foundation walls, he wants them plumb and level and free of the discolorations that are the signs of weak concrete. The requirements for a slab foundation are similar: a sturdy footing and a vapor-proofed, reinforced-concrete pad sitting on a bed of compacted crushed stone. The major difference is in the way the slab is insulated to protect against frost heaves. This illustration shows how Tom wants a slab foundation to look before he starts framing.