Installing Hardwood Flooring

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Start by carefully removing any good baseboard trim with a putty knife and a flat pry bar.

Then peel or scrape up the old material. It may be glued to the underlayment, so it's often easier to pry up whole sections at once, using a square-edge spade and taking care to lift, not cut, the anchor nails. Your goal is to determine if the floor is built to accept the new covering. With wood joist construction, 2 X 10s spaced 16" on center are common; the subfloor on top should be exterior-grade plywood of at least 1/2" thickness, though 5/8" is preferable, and 3/4" is more desirable yet. If a board subfloor is used, it must be full 3/4" squareedge stock: 1 X 4s or 1 X 6s in the top two grades of structural lumber, laid diagonally across the joists.

The two important things to check for are spacing between sheets or boards (1/4" is enough to allow for expansion) and adequate nailing, especially at the joists where two pieces meet (Fig. 2). With plywood, there should be a nail about every 6"; boards require two 8d or l0d commons at each bearing point. Building over a concrete slab presents its own set of circumstances. First, the slab needs to be flat, level and abovegrade. The floor installation, which we'll get into later, involves putting down plywood or wooden screed strips as a base, so a sound, dry pad is a must. Chronic moisture or humidity of any kind is a killer for wooden floors. At best, it can cause loosening and squeaking; in the worst cases, it brings on cupping and crowning, bending individual strips along the grain to destroy the uniformity of the surface. In a building with a crawlspace and no basement, it's best to cover the soil with 6-mil polyethylene sheets, and seal the seams and edges (see "Formulas for a Warmer Floor," MOTHER No. 113).

Slabs should have a vapor barrier beneath them, on top of the buried gravel fill. Even slabs poured a year or so earlier can still be giving up water, so it pays to make a simple test by duct-taping a 12"-square section of polyethylene to the concrete, sealing all sides (Fig. 3).

If the sheet shows no clouding or condensation underneath after 24 hours, the slab is dry enough for a wooden floor. Now, what if there's a finished wood floor in place and it's all in one piece? Generally, if it has no major depressions and is still substantial enough to take a sanding, it can be resurfaced—a process I'll get into soon. Boards or small sections can even be replaced if they're damaged, and a 3/4" floor can stand up to three or four sandings before becoming questionably thin. Any doubts? Lift a heating register to expose the flooring end grain; wood less than 5/16" thick may not be worth fussing over, but it can be covered with new strip flooring if you're able to level it. The rules of this game are to lay the new strips at right angles to the old boards, with a layer of builder's felt between the two surfaces.

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