Trade secretsHow a great hardwood floor is really built, and the parts that are hardest to get right

One trade magazine’s rule for nail-down floors: 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. It also says to take about 20 readings per 1,000 square feet, then average and record them. Over a concrete slab, installers use a probe test inside the slab (ASTM F2170). A common pass line is below 75 percent relative humidity, unless the flooring maker says otherwise.

A flooring-trade writer puts it this way: acclimation is judged with a moisture meter, not with a calendar. Another trade article suggests the house should be at normal living conditions, 60 to 80°F and 30 to 50 percent humidity, for at least five days before an engineered floor goes over concrete. Some manufacturers still give a fixed time, such as 72 hours for one brand, so the maker’s sheet governs.

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

Hardwood sometimes hides under old carpet, along with staples, tack strips and glue to scrape off. Pet urine can soak through carpet and pad into the subfloor, and one repair writer’s blunt advice was to pull the stained subfloor and replace it. A trade writer on old floors says to look for discolored or warped boards and point out each questionable area to the owner.

An APA handout citing the National Wood Flooring Association gives the flatness limit for nailed wood floors as 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 noise and nail pops. A common squeak fix from below is a thin, glue-coated shim tapped between joist and subfloor. From above, the usual trick is a pilot hole and a screw.

APA calls for a vapor retarder in crawlspace foundations, and a trade magazine says a Class I vapor retarder goes on a slab beneath a wood subfloor. Over an unconditioned crawlspace, one trade guide advises recording temperature, humidity and moisture both below and above the subfloor so the difference is not too large.

One manufacturer’s installation sheet calls for a gap of about 1/2 to 3/4 inch between the planks and the walls. With no room to expand, boards can buckle. Baseboards, shoe molding and door thresholds cover the gap. In Santa Barbara, one installer quoted by a moisture-meter maker says the air runs around 70 percent relative humidity and that he builds in extra expansion, for example a small strip of expanding material every three or four rows.

Guides for nail-down floors say to lay boards parallel to the longest wall and across the joists, to plan the last row’s width ahead of time (it usually needs cutting), and to mix boards from several boxes. End joints are staggered. Sources differ on the minimum, 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 an unforgiving pattern and says a slight veer can grow quickly. The method it describes starts from a line down the center of the room, checks that the plywood backer is square, and keeps rechecking the distance from the corners to the next line while nailing. This Old House says the same: veering off by a fraction of an inch sends the whole pattern off.

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

Undercutting means trimming the casing so the new board slides underneath. The cut is made with a flush-cut saw or oscillating tool, scored first, with a scrap of the flooring as a height guide. A reducer joins two floors of different heights with a gentle slope so no one trips.

Wood Floor Business and Hardwood Floors Magazine both say not to skip more than one grit in the sequence, because skipped scratches show up in the final result. One typical path runs from 36 to 60 to 80 and then to 100 or 120. Sanding too fine can also hurt: it can burnish the wood so stain or finish does not stick.

Trade writers say an edger can do more damage than almost any other machine, and that it is supposed to leave circular marks, so the job is to blend them. Pros work with the grain and hand-sand the ends to bridge the difference between the edger’s scratches and the big drum’s. Dirt left on the floor between cuts can create swirl marks, so cleaning between grits matters.

Wood Floor Business lists the causes: a drum that is 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 flatten the floor. Another trade piece admits that every wood floor will show some sanding marks. The goal is marks too small to see standing up.

Hardwood Floors Magazine says most solid floors can be sanded six to eight times in their life, and a typical sanding removes 1/32 to 1/16 inch. Engineered floors have wear layers from 1/16 to 1/4 inch. If the wear layer is under 3/32 inch, sanding is not recommended. The same trade writer says the count depends on the thickness, the equipment and the skill of the person sanding.

Trade sources say the last grit changes the color: a finer final sanding gives a lighter stain, so a floor sanded finer than the sample will not match it. They also say to wipe sample colors on the actual floor after the second sanding, or to make a panel about 2 by 3 feet, sanded the way the floor will be, and to have the customer approve it. One caution: a very pigmented stain can be hard to sand out of a sample made on the floor, so a panel may be safer. Species and light also change color over time (cherry darkens, walnut lightens), and a chip of a different species will not match.

One retailer notes that dark walnut on red oak and on white oak looks like two different floors: red oak is warmer with an open grain, white oak is cooler and tighter. Mixed woods are common in older houses. One of La Cumbre’s Yelp reviews describes exactly that: There was an issue with my wood-some was white oak; some red oak. Tony was able to add a tint and make it all blend. Georgene V., on our home page.

Maple, birch, cherry, pine and alder are the species most likely to blotch. A maple specialist says even small sanding flaws that would not show under clear finish show up under stain. Water-popping wets the wood to open the grain and deepen color. But uneven water can leave blotches whose only fix is to resand, and staining before the wood is dry can also leave the color blotchy. On white oak under water-based finish, fine sanding dust and water can pull out tannins, which is why one trade writer stresses removing all the dust and applying a very thin first layer of finish.

A Wood Floor Business how-to says to cut in along the starting wall and a few feet along the adjoining walls, with room and floor at about 65 to 70°F. Working the applicator back and forth too much can cause bubbles, and 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, and says high humidity or low temperature slows drying. It also says not to spread too thin: 350 to 400 square feet per gallon.

For that Bona waterborne finish: dry in 2 to 3 hours, walkable after 24 hours but easy to scuff, about 80 percent cured after one day and fully cured after three. For another Bona-brand finish, full hardness takes seven days. Bona’s customer advice is stocking feet in 3 to 4 hours, furniture back in 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 a new finish bonds by mechanical or chemical adhesion, so the old finish must be lightly abraded to the maker’s recommendation. It also describes tests: a rag that picks up yellow or brown color may mean wax, and so may white spots about 10 minutes after a drop of water. Contamination can make 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 for older buildings. Wood Floor Business reports systems that claim to capture up to 99.8 percent of dust, but says many “dustless” claims fall flat because of loose hoses, poorly sealed shrouds or hasty work. That figure is the makers’ claim, not a test we could verify.

This Old House says to cut around the damaged boards carefully with an oscillating tool and to match species, grain and color. A Wood Floor Business story describes one shop tearing back existing rows, mixing old and new boards and sanding them together, after which the repair was undetectable. Old stock can be tighter-grained than anything sold now.

Cupping happens when the bottom of a board holds more moisture than the top. Sanding a cupped floor too soon can leave the boards crowned once they dry. One trade rule of thumb: sand a non-flat floor only when top and bottom moisture are within 1 percent. Another trade piece says a floor can feel dry on top and still hold moisture below, and that readings should be written down and shared with the homeowner. We found no published number for how long to wait. It depends on the readings.