On a closed foundation, concrete or masonry footings beneath the site's surface support the outer foundation walls. The foundation wall encircles the structure and continues to the first level, entirely enclosing the foundation. On open foundations, the base of the building is connected only by the soil itself. No footers are used, and no wall extends beyond the edge of the footing. Closed foundations are more stable than open ones because they transfer some of the building's weight to the ground. This makes it less likely that an earthquake will cause the building to collapse. Also, any underground water that finds its way into an open foundation will drain away from the structure because there's nothing to hold it back. The most common type of foundation is the poured-concrete foundation. It's called "poured" because before it is completely set, wet concrete can be added to increase its strength. The concrete foundation consists of two main parts: the footings, which contact the soil, and the walls, which circle the house and connect to the footings. The footings should be deep enough so that their tops are at least as deep as the finished floor level of the building. This allows for room to add additional floor joists if necessary.