Have you ever wondered what is underneath the flooring in your home? If you are going to be making repairs or improvements to your vinyl, wood, or tile floors, it is important to know your substructure. Here we look at typical floor construction.

The subfloor is usually constructed from 1-by-4 or 1-by-6 lumber or from plywood panels. In a lumber subfloor, boards are laid diagonally across joists. A plywood subfloor has panels that are laid in a staggered fashion, with the ends and edges butted together; panels are nailed (and sometimes also glued with construction adhesive) to the joists.

The thickness and stiffness of the subfloor determine the types of finished materials that can be laid on top of it. If your house is built on a concrete slab, the slab can serve as a base for almost any type of flooring. But if your home has a plywood or board subfloor, it's important to check out the type and thickness of the material used to determine what its limitations are. For instance, a floor that is slightly flexible or springy is not suitable for rigid materials such as ceramic tile and stone because the grout or materials will crack with movement.

If your home has a crawl space or basement, you can go underneath to check out the underside of the subfloor. If you can tell that it is plywood, look for a grade stamp that designates the thickness.

A few minor dips in floors are common. Some settling results from stresses and fatigue. But if floors sag, the house could have a serious structural problem. Also look for evidence of rot. Discolored areas on the joists or subfloor usually indicate a plumbing leak. If you are faced with these kinds of problems, call a contractor.