New wood flooring should be laid on a clean, smooth, level, structurally sound base. Depending on the particular flooring, this base may be a previous floor covering, an existing wood floor in good condition, a new plywood subfloor, or even a moisture-proofed concrete slab.

By installing wood over an existing floor, you bypass the messy job of removing the old flooring and you gain instant soundproofing and insulation from the old floor. A disadvantage to leaving old flooring in place is that you must correct any irregularities in it. Also, the new floor may raise the flooring level too much, making the transition to a hallway or an adjoining room awkward.

Whether or not you're installing over old flooring, begin by removing doors and base shoe molding (remove baseboards only if there is no shoe molding). Number the molding pieces so you can easily replace them when you're finished.

If you plan to remove the old flooring before laying the new, do so at this stage. Install subflooring over the exposed joists, centering the long edges of 3/4-inch exterior-grade plywood with a tongue-and-groove edge on the joists. With 2-inch cement-coated or ring-shank nails, nail every 6 inches along the long edges and every 12 inches along intermediate joists. Stagger panels so no four corners come together in one place. Leave 1/8 inch between panels and 1/2 inch next to walls (where molding will cover the gap) to allow for expansion.