How a good floor really gets built

For nail-down floors, the trade rule is that the wood should be within 4 percentage points of the wood subfloor for strips under 3 inches wide, and within 2 points for wider planks. Take about 20 readings per 1,000 square feet, average them and write them down. Over a concrete slab, installers probe inside the slab (ASTM F2170), and a common pass line is below 75 percent relative humidity unless the flooring maker says otherwise.

Before an engineered floor goes over concrete, the house should sit at normal living conditions, 60 to 80°F and 30 to 50 percent humidity, for at least five days. Some manufacturers still give a fixed time (72 hours for one brand), and the maker’s sheet governs.

The EPA’s lead rule (RRP) requires certification for anyone paid to disturb painted surfaces in homes built before 1978, one-person shops included. Older floor tile, sheet-flooring backing and the glue under them may contain asbestos, and sanding them can release fibers. Sampling is a job for professionals.

Old carpet sometimes hides hardwood, plus staples, tack strip and glue to scrape off. Pet urine can soak through carpet and pad into the subfloor, and then the stained subfloor has to come out and be replaced. On old floors, look for discolored or warped boards and point out every questionable spot to the owner.

An APA handout, citing the National Wood Flooring Association, puts the flatness limit for nailed wood floors at 3/16 inch in 6 feet or 1/4 inch in 10 feet. It also recommends ring-shank or screw-shank nails in the subfloor to cut down on noise and nail pops. To fix a squeak from below, tap a thin, glue-coated shim between joist and subfloor. From above, drill a pilot hole and drive a screw.

APA calls for a vapor retarder in crawlspace foundations, and a trade magazine puts a Class I vapor retarder on a slab beneath a wood subfloor. Over an unconditioned crawlspace, one trade guide says to record temperature, humidity and moisture both below and above the subfloor and keep the difference from getting too large.

One manufacturer’s sheet calls for about 1/2 to 3/4 inch between the planks and the walls, covered by baseboard, shoe molding and thresholds. A Santa Barbara installer quoted by a moisture-meter maker says the air there runs around 70 percent relative humidity, so he builds in extra expansion, such as a small strip of expanding material every three or four rows.

For nail-down floors, run the boards parallel to the longest wall and across the joists, plan the last row’s width ahead (it usually needs cutting), and pull from several boxes at once. Stagger the end joints. Sources put the minimum anywhere from 6 to 8 inches, so a good contractor follows the maker’s sheet. This Old House warns that cutting every patch board to the same length makes a repair look like a trapdoor.

Wood Floor Business calls herringbone unforgiving and says a slight veer can grow quickly. Its method starts from a center line, checks that the plywood backer is square, and keeps rechecking the distance from the corners to the next line while nailing. This Old House agrees: a fraction of an inch off sends the whole pattern off.

A Wood Floor Business guide to stair treads says the adhesive is much more important than the fasteners. Stringer tops are never level, so you work off the front edge of the riser, and stairs call for a trained carpenter. Specialty tools can template each tread and riser.

To undercut, you trim the bottom of the casing so the new board slides underneath. Score it first, then cut with a flush-cut saw or oscillating tool, using a scrap of the flooring as a height guide. Where two floors meet at different heights, a reducer ramps between them so no one trips.

Wood Floor Business and Hardwood Floors Magazine both say never to skip more than one grit, because the scratches you skip show up in the finished floor. A typical path runs 36, 60, 80, then 100 or 120. Going too fine can hurt too: it can burnish the wood so stain or finish won’t stick.

Trade writers say an edger can do more damage than almost any other machine. It’s supposed to leave circular marks; the job is blending them. Pros work with the grain and hand-sand the ends to bridge the edger’s scratch and the big drum’s. Dirt left between cuts causes swirl marks, so clean between grits.

Wood Floor Business lists the causes: a drum out of balance or out of round, hard spots, badly installed paper, compressed rubber, even bad electrical supply. Cutting the first pass at a slight angle, about 7 to 15 degrees, helps prevent waves and flattens the floor. Another trade piece admits every wood floor will show some sanding marks.

Hardwood Floors Magazine says most solid floors can be sanded six to eight times in their life, and a typical sanding takes off 1/32 to 1/16 inch. Engineered wear layers run from 1/16 to 1/4 inch, and sanding isn’t recommended under 3/32 inch. The real count depends on thickness, equipment and the skill of whoever sands.

The last grit changes the color. A finer final sanding gives a lighter stain, so a floor sanded finer than its sample won’t match it. Wipe sample colors on the floor after the second sanding, or make a panel about 2 by 3 feet sanded the same way, and have the customer approve it. A heavily pigmented stain can be hard to sand back out of a sample made on the floor, so a panel may be safer. Species and light also shift color over time (cherry darkens, walnut lightens), and a chip of a different species won’t match.

Dark walnut on red oak and on white oak looks like two different floors. Red oak is warmer, with open grain; white oak is cooler and tighter. Older houses often have both. On the color matching page we describe matching 80-year-old red oak to new white oak.

Maple, birch, cherry, pine and alder blotch the most. A maple specialist says small sanding flaws that would disappear under clear finish show up under stain. Water-popping wets the wood to open the grain and deepen the color, but uneven water can leave blotches you can only fix by resanding, and staining before the wood is dry can blotch too. On white oak under water-based finish, fine dust and water can pull tannins, so one trade writer insists on getting every bit of dust off and laying down a very thin first coat.

Wood Floor Business says to cut in along the starting wall and a few feet along the walls beside it, with room and floor at about 65 to 70°F. Working the applicator back and forth too much causes bubbles. Stop marks come from the flattening agents in the finish, where the applicator reverses or lifts. Bona’s data sheet for one waterborne finish lists 65 to 80°F and 40 to 60 percent humidity, notes that high humidity or low temperature slows drying, and says not to spread it thinner than 350 to 400 square feet per gallon.

That Bona waterborne finish is dry in 2 to 3 hours and walkable after 24 hours, though it scuffs easily then. It’s about 80 percent cured after one day and fully cured after three. Another Bona-brand finish takes seven days to reach full hardness. Bona tells customers stocking feet after 3 to 4 hours, furniture after 48 hours, and area rugs after about a week, once fully cured. Oil-based finishes take longer; some sources say several days for furniture and weeks for rugs.

Hardwood Floors Magazine says new finish bonds by mechanical or chemical adhesion, so the old finish has to be lightly abraded the way the maker recommends. It also gives two tests for wax: a rag that picks up yellow or brown color, or white spots about 10 minutes after a drop of water. Contamination can make the finish crawl into craters called fish-eyes.

Hardwood Floors Magazine says dust must not get back into the air anywhere between the sander and the dustbin, and that HEPA certification matters in older buildings. Wood Floor Business reports systems claiming up to 99.8 percent capture, but says many “dustless” claims fall flat from loose hoses, poorly sealed shrouds or hasty work.

This Old House says to cut carefully around the damaged boards with an oscillating tool and match species, grain and color. Wood Floor Business tells of one shop that tore back existing rows, mixed old and new boards and sanded them together, and nobody could find the repair. Old stock can be tighter-grained than anything sold now.

A board cups when its bottom holds more moisture than its top. Sand a cupped floor too soon and the boards can crown once they dry. The rule of thumb: sand a floor that isn’t flat only when top and bottom moisture are within 1 percent. Write the readings down and share them with the homeowner; they decide how long to wait.