Preparing the Subfloor -- before Installing the Hardwood Floor



Whichever of the three types of wood flooring you plan to use, preparing a reliable base re quires the same basic steps. Wood floors are typically laid over a concrete slab, over a wood subfloor supported by joists and beams, or in some cases over an existing floor—depending on the old floor’s composition and condition, and on the kind of wood flooring you plan to install.

Block flooring, of course, re quires a solid, continuous subfloor, whether of boards or plywood panels. Laminated wood tile can be laid directly in mastic on a thoroughly dry concrete slab that has a waterproofing membrane below or above to keep it dry. But blocks of solid wood pieces are best installed over a plywood base built over the slab.

This section includes the in formation you’ll need to prepare a concrete slab, a wood subfloor, or an existing floor for new wood flooring.

Preparing a concrete slab

Before installing any kind of wood floor over a concrete slab, you must make sure the slab is dry, level, and clean. Even if a below-grade or on-grade slab appears dry, applying a vapor barrier under the floor will safeguard against future moisture problems. In addition, pay close attention to the recommendations that accompany the flooring you’re installing. Each supplier will have specific guidelines for slab preparation and adhesives.

Checking for moisture. It’s essential that the slab be free from moisture for an entire year be fore you install a wood floor over it. You’ll need to observe it and check for moisture periodically, particularly during wet times of the year. Watch for moisture not only from below but also from other sources—seepage through a wall or condensation dripping from a pipe—that could ruin your new floor.

If you’re thinking of installing a wood floor over a recently poured concrete floor, this year of observation should allow ample time for the concrete to cure. Just make sure the area is well ventilated, and turn the heat on during the cool season.

A simple test will tell you if a slab, new or old, is still releasing moisture. Cut a 16 to 20-inch square of clear plastic, tape the square to the concrete floor (sealing the edges with tape), and leave it in place for 2 or 3 days. Then lift the plastic and see if any moisture has condensed on the underside. If it has, give the floor more time to dry.

Fig. 46.jpg

If a slab has had ample time to dry but is still damp, it’s time to check the downspouts (consider extending them away from the house), assess the general drainage around the house, and look for leaky pipes. If improving drainage or fixing leaky plumbing doesn’t correct a moisture problem and you can not be sure that the floor can be kept dry, you should consider a type of flooring other than wood—one that will be less affected by moisture.

Cleaning a slab. Once you’re satisfied the slab is dry, make sure it’s level and clean. Small blobs of mortar or other spills of construction materials should be chipped away with a cold chisel. Fill minor dips or other irregularities with a patching compound.

Sweep the slab clean to re move as much dirt and dust as possible. Don’t clean the floor with water. Most auto supply stores sell a chemical cleaner that will remove grease and oil.

Finally, to provide rudimentary protection from moisture, brush or trowel on a coat of asphalt primer directly over the slab, and allow it to dry thoroughly.

Provide a moisture barrier. Your next step will depend on the type of moisture barrier you intend to use. A single layer of polyethylene film laid just under new wood flooring is usually considered adequate moisture protection for a dry slab floor (see instructions in the next section, “How to prepare a base for wood flooring over a slab”). But if you suspect excess moisture might be a problem, or if the slab is on or below grade, you’ll want extra moisture protection. A “two-membrane” vapor barrier laid directly over the primed slab can give you this protection.

To install such a two-membrane barrier, use a notched trowel to spread a coat of asphalt mastic over the primer. Al low the mastic to dry. Then roll out a layer of 15-pound asphalt- saturated felt, butting the edges and ends of each course.

Trowel on a second coat of mastic, then roll out a second layer of asphalt paper so that the seams run parallel to, but fall between, the seams of the first layer.

Fig. 47-0.jpg

How to prepare a base for wood flooring over a slab

Except for some kinds of laminated tile, wood flooring cannot be secured directly to a concrete slab. That means you’ll have to lay either strips of wood (called screeds or sleepers) or a base of ¾-inch exterior plywood over the slab so the flooring can be attached to the wood. Screeds will work only for strip or plank floors; wood block flooring requires a solid, continuous base of plywood.

Using screeds. The best material to use for screeds is 2 by 4 lumber, pressure treated with chemicals for pest and moisture resistance. If you’ve installed a two-membrane moisture barrier, you can set screeds in mastic directly on the asphalt paper. If not, set them in a coat of hot asphalt mastic applied directly over the primer, or attach with lag bolts, depending on what type of screeds you choose.

Fig. 47-1

Two ways to lay screeds. There are two common ways of laying screeds—in continuous strips or in staggered, short lengths (see illustration below). Continuous strips are the best choice in smaller rooms and over uneven concrete slabs. You can attach them with lag bolts se cured to lead anchors set in the concrete slab, and adjust them with shims to provide a level base for your new flooring.

But staggered screeds are simpler to install, as the pieces—only 18 to 48 inches long—are easy to handle and provide air circulation under the floor. These shorter screeds are imbedded in mastic on 12 to 16-inch centers. They’re called “staggered” be cause they’re overlapped at least 4 inches where they meet.

Staggered or continuous, screeds should be installed at a right angle to the direction you plan to lay your strip or plank flooring. No screeds should be placed closer than half an inch to any wall, to allow for air circulation. You’ll need to vent the space between the screeds to the room. You can do this with at least two openings cut through the floor close to the two walls at the ends of the screeds. Cover the openings with warm-air registers or ventilating louvers.

Check the level of screeds. When the screeds are in place, take a long straightedge (the uncut edge of a sheet of ¾-inch plywood works well) and check to see that the screeds are level. Plane down high points. If the screeds are attached with lag bolts, you can build up low areas with wood shims (wood shingles work well) placed under the screeds.

Insulation is optional. If you want additional moisture protection, a quieter floor, and some extra insulation, now’s the time to pour vermiculite or perlite insulation between the screeds and level it with the top of the screeds.

Cover screeds with film. After leveling the screeds, lay 4 or 6- mil polyethylene film over the top of the screeds for moisture protection. Overlap the edges of adjacent sheets of film at least 6 inches; it doesn’t have to be attached. Take care not to puncture it.

Fig. 48-0

You now have a good base f or your strip or plank flooring. If installing a wood block floor, nail ¾-inch exterior grade plywood to the screeds. Use 6-penny cement-coated or ring-shank nails and stagger the joints. Leave gaps between the plywood and the walls and doorways, as de scribed below.

Using plywood. To prepare a solid plywood nailing base over a concrete slab, first make sure the slab is dry and clean, as out lined in the preceding discussion on testing for moisture and cleaning a concrete slab. If you haven’t already done so, seal the dry slab with a coat of asphalt primer. Then cover it with a layer of 4 or 6-mil polyethylene film (no mastic is required); overlap adjacent sheets of film 4 to 6 inches and extend the film under the baseboards on all sides of the room (for tips on removing molding and baseboards, see next section).

Lay out panels of ¾-inch exterior grade plywood over the entire floor, cutting the first sheet of every other run so that end joints will be staggered (see illustration). Leave a space of ¼ to ½ inch between panels and the walls. Around doors and other obstructions, where gaps won’t be hidden with molding, cut the plywood to fit, leaving a gap of only about 1/8-inch; the flooring will hide these spaces.

Starting at the center of each panel, use concrete nails to se cure the plywood to the slab. Use at least nine nails per panel, in the pattern shown in the illustration above. You’ll attach your new wood flooring directly to this plywood base.

Fig. 48-1

If you’re building a new subfloor

Either plywood or common pine or fir 1 by 4s or 1 by 6s provide a suitable base for wood flooring, and if you’re putting wood flooring in a new home or addition— that is, if you can choose the sort of subfloor you’ll be working with—those are your choices. Because it won’t warp, ¾-inch plywood is generally considered the best subfloor material; it’s easier to work with, too.

Lay plywood sheets with the grain running at a right angle to the joists, and nail them to the joists about every 6 inches. Butt panel ends together over joists—this should be no problem if the joists are on standard 16 or 24-inch centers—and stagger the joints in adjacent runs (see drawing).

Fig. 48-2

Lay boards diagonally across the joists and nail them in place (use two nails where each board crosses a joist), with 1/16 inch spacing between boards to allow for expansion.

Preparing an old floor for wood flooring

Though it’s possible to lay wood flooring over an old floor that’s in good condition, you may need to remove the old flooring to get down to the basic subfloor and make any necessary repairs or install underlayment. In the long run, this usually provides the most reliable base for your new floor.

Check the exposed subfloor for loose boards or loose ply wood panels. If planks are too badly bowed and cannot be flattened by nailing, give the floor a rough sanding with a commercial floor sander or cover it with 3/8 or ½-inch plywood or particle board.

Should you have an irregular subfloor made of planks less than 4 inches wide, and you in tend to install wood block, cover the old planks with a ¼-inch layer of untempered hardboard, rough side up, or 3/8-inch plywood. For planks from 4 to 6 inches wide, use ¾-inch plywood; for planks over 6 inches wide, 5 ply wood is best.

Fasten down 1/4 or 3/4-inch material with 3-penny ring- shank or cement-coated nails; for 5/8-inch plywood, use 4-penny ring-shank or 5-penny cement- coated nails. Space the nails 6 inches apart across the surface of the panels.

However, an existing wood floor in good condition can, if you wish, serve as a base for new strip, plank, or block flooring; in fact, block flooring can even be installed over old resilient sheet or tile floor covering. The advantages of laying new flooring over old are that you bypass the messy job of removing the old flooring and you gain some soundproofing and insulation from the old floor.

One disadvantage in leaving old flooring in place is that you’ll be unable to inspect the subfloor and correct irregularities. Another disadvantage is that there may not be enough space above appliances (a dish washer, particularly) after the new floor is installed. And if you don’t put new flooring under the appliances, you may even find it impossible to remove them in the future.

Whatever kind of old flooring you’re faced with, begin your preparations by removing all doors, grates, and other obstructions in the room. Use a chisel to take up the shoe molding from around the baseboards, taking care not to split the wood (see Fig. 49-0). Remove the baseboards themselves, avoiding damage to the walls or door frames. As you take up molding or baseboards, number the pieces with chalk or a pencil so you can replace them in their original positions once your new flooring has been installed.

An old wood floor must be structurally sound and perfectly level before you put new wood flooring over it. Separated, bowed, or buckling boards or any evidence of moisture damage in the original floor should serve as warnings of problems that you can’t hide—for long—simply by covering them with new flooring. If you have any reason to suspect the old floor or its sup porting structure, examine it very carefully before installing a new floor over it. (See “Locating the cause,” later in this section, and you’ll find suggested remedies for the most common structural problems.)

Once you’re satisfied the old floor is sound, you can proceed with your preparations for the new. Tap down and countersink any protruding nails. Secure loose boards by nailing them—directly into joists, where possible—with annular ring nails. Warped boards that can’t be made level should be sanded flat or replaced.

If you’re planning to install new wood block flooring (which is laid in adhesive), you’re likely to have to remove all wax, varnish, paint, or other finish from the old floor. This may require a rough sanding with a commercial drum sander. (Floor sanders are available at most tool rental companies; see “Tools and Supplies”.) For some types of new self-adhesive wood block, though, you may not have to re move an old finish, provided it’s well bonded and hasn’t started to flake. Check the manufacturer’s recommendations for the particular kind of block you plan to install.

Old resilient flooring that is in reasonably good condition, flat, and securely fastened down can serve as a base for new wood block—but not for wood strips or planks. If the old floor covering is extremely worn or irregular, you’ll be better off removing it before installing a new wood block floor.

To remove old resilient tiles, use a floor scraper. If the tiles don’t come up easily, place a piece of paper over them and warm them with an old iron; heat will soften most adhesives.

To remove resilient sheet flooring, cut it into strips with a utility knife, taking care not to damage the subfloor. Peel or scrape away the strips, then scrape any excess backing off the subfloor with a wide putty knife or floor scraper. Fill low spots by troweling on a patching material.

Vinyl flooring laid with epoxy adhesives is extremely difficult to remove. It may be easier to cover the vinyl with ¼-inch underlayment grade ply wood or hardboard. Fasten down the underlayment with 3-penny ring-shank nails or 4-penny cement-coated nails spaced 3 inches apart along the joints and 6 inches apart throughout the sheet. Stagger the joints and leave gaps about the thickness of a dime between panels.

Final preparations

New or old, the subfloor (or old floor, if that is to serve as the base for new flooring) should be cleaned thoroughly. Drive protruding nails below the surface with a nailset and hammer, and correct any other irregularities to make a perfectly flat base.

If you haven’t already done so, remove the molding from around the baseboards, using a chisel and taking care not to split the wood (see illustration). As you remove molding and baseboards, number the pieces and the wall with chalk or a pencil so that you can replace them in their original positions.

If you’ve removed an old floor and plan to install block flooring that is thinner than the old flooring, lay an underlayment of untempered hardboard or plywood directly over the subfloor to raise the surface of the new flooring to the same height as the old. This will ensure a level transition between the new floor and adjacent rooms, and make it unnecessary to trim door casings, doors, or molding.

Fig. 50-0

Finally, unless you’re laying the flooring in mastic, cover the subfloor with a layer of 15-pound asphalt-saturated felt (butting seams) or soft resin paper (over lapping seams 4 inches). Trim the paper flush with the walls. As you put the paper in place, use a straightedge or a chalk line to mark the paper over the center of each joist. The markings will serve as reference points when you attach the new flooring. The paper will act as a moisture barrier, keep out dust, and help pre vent squeaks in the new floor.

Protect the floor from heat. Flooring installed over the heating plant of a home needs extra protection to keep it from drying and cracking.

A double layer of 15-pound asphalt-saturated felt or a single layer of 30-pound felt will be effective. Or standard ½-inch nonflammable insulation board, cut to fit between joists from below, can be used as an alternative to heavier building paper.

Crawl space needs a barrier. If wood flooring is to be installed in a frame home with a crawl space, check the crawl space to see if a moisture barrier has been placed over the soil. If there is none, or if it has been torn or has deteriorated, use roofing material or 4 or 6-mil polyethylene sheeting to cover the ground (overlap edges by at least 2 inches). Use heavy objects such as stones or bricks to secure the moisture barrier in place.

Also check to see that the crawl space is well ventilated. There should be at least four vents, one in each corner of the building, with a total vent area of about 1½ percent of the floor area—more if required by code.

In dry climates where damp soil is not a problem, a moisture barrier isn’t necessary, but ventilation is still important.

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Thursday, 2013-06-20 21:30