Pier and Beam Construction
Pier and beam construction uses wood supports that have inherent flexibility, which means they can shift more easily in response to soil changes and settling houses. This design makes pier and beam supports more resistant to structural changes and is one reason why you cannot simply fill your crawl space up with concrete. House foundations are constructed to manage incoming pressures from soil, upper stories and water weight. The concrete would press against the beams, changing the stresses of the house and rendering the foundation ineffective.
Pier and beam foundations are also used to make wiring and plumbing easier. When you can run wiring and pipes underneath the floor, connecting different systems and rooms becomes much faster than running wires through the wall. Since contractors make use of this space when building and wiring a house, filling the crawl space in with slab becomes impossible because of all the system work that is already in place, not to mention the insulation and any storage uses.
Crawl spaces created by the pier and beam foundation do have some weaknesses. If moisture becomes trapped in the space, it can eat into the wood and cause rot, mold and pest problems. This is another key reason that these spaces cannot be filled in with concrete. The wood needs space to release moisture and proper venting to remove it entirely. Concrete slabs would only trap moisture and lead to serious foundation problems over time.
A pier and beam foundation is installed in a concrete slab -- it needs the concrete bed for support. This slab does not come in contact with the actual house, but it can certainly be repaired if cracks or flaws occur. It is not the same project as filling in your crawl space entirely, but rather repairing or adding new support to a particular section of your foundation. New mortar and epoxies can be used in these repairs.