How to Nail Down Solid Hardwood Over a Wood Subfloor

Why this matters

A nail-down solid floor that goes wrong does not fail on install day. It fails the first heating season as gaps that open in a band across the room, the first humid summer as cupping, or the first week as a board that clicks underfoot by the bathroom door. Almost every one of those traces back to something skipped in the first hour: the moisture match not taken, the boards not racked from several bundles, the nailer set so it crushed tongues, or a row fastened tight to a wall. This card follows one bedroom from felt to finish test so each step carries its own numbers.

The job this card follows

A 12 ft by 14 ft bedroom on the second floor. The subfloor is 23/32 inch OSB on joists at 16 inches on center, and the joists span the 12 ft direction. The floor is 3/4 inch solid red oak strip, 2-1/4 inch face, unfinished, to be sanded and finished on site. The maker's sheet calls for 1/2 inch expansion at the walls and a fastener schedule we will read in Step 6.

Before any of this starts, the subfloor has been checked and screwed down where it moved, and flatness is within the maker's tolerance (see related: Floor Flatness Tolerances by Floor Type, and How to Stop Subfloor Squeaks Before New Flooring). Base and shoe are off, door casings are undercut (see related: How to Undercut Door Jambs and Casing).

Step 1: Confirm the moisture match, in writing

Read the flooring and the subfloor with a meter on the correct species and material settings, enough readings to describe each, and compare them against the match the maker and the NWFA guidelines call for. The rule and the numbers are owned by Wood Moisture Content and the Match to the Subfloor; the procedure is How to Take and Log Wood Moisture Readings Before Hardwood. Do not restate a rule of thumb from memory; read the sheet.

In the bedroom: 20 readings across the delivered oak averaged 7.6 percent, 20 readings across the OSB averaged 8.9 percent on the meter's OSB setting, a 1.3 point difference, inside the match the maker's sheet states for strip flooring. The HVAC had been running at normal settings since delivery.

What breaks if skipped: a floor nailed dry over a wet subfloor takes on moisture from below and cups; a floor nailed wet shrinks into gaps. Neither is visible on day one, and the log is the only evidence that answers the callback.

Step 2: Lay the underlayment the maker calls for

Most solid-floor makers call for a vapor retarder sheet over a wood subfloor, typically asphalt-saturated felt or a rosin or kraft paper. It slows moisture moving from below and stops the boards squeaking against the subfloor. Lay it perpendicular to the run of the boards, overlap the seams by the amount the sheet states, and staple it flat.

In the bedroom: the sheet names asphalt-saturated felt, overlapped 4 inches. Snap pencil lines on the felt over each joist, measured off the joist screws in the subfloor, because the face nails in Step 4 and any extra fasteners near ends should land in framing where possible.

Step 3: Set the run direction and the starting line

Run solid strip perpendicular to the joists wherever the room allows. Strip running parallel to joists is spanning between them on subfloor alone, and the NWFA guidelines call for added thickness in that case; read them before you agree to it. In the bedroom the joists span the 12 ft direction, so the boards run the 14 ft direction, perpendicular.

Snap the starting line per How to Lay Out a Starting Line for a Plank Floor. In the bedroom: 144 inches wall to wall across the run, minus 1/2 inch at each wall leaves 143 inches; 143 divided by 2-1/4 is 63 full rows (141-3/4 inches) with 1-1/4 inches over. A 1-1/4 inch last strip face-nails poorly and splits, so the first row is ripped to balance: 2-1/4 plus 1-1/4 = 3-1/2 inches, split to 1-3/4 inches first and last. The line goes 1-3/4 plus 1/2 = 2-1/4 inches off the wall at both ends, with the groove edge toward the wall and the tongue facing into the room.

Step 4: Fasten the starter rows by face-nailing

The nailer needs room to swing, so the first two or three rows within about 8 inches of the wall cannot be blind-nailed with it.

1. Screw a straight board along the line as a fence, or set the first row's tongue edge exactly to the line.
2. Predrill and face-nail the first row along the groove edge, close enough to the wall that base or shoe covers the heads. Set the heads.
3. Blind-nail the tongue of the first rows by hand or with a top-nailer at the same spacing as the field.
4. Re-check the tongue edge against the line every 4 feet. A first row nailed 1/16 inch off the line becomes a visible taper across 64 rows.

Hazard: a predrill bit through oak binds and spins the drill; brace it with both hands. Eye protection for every nail driven, hand or pneumatic.

Step 5: Rack the floor from several bundles

Racking means laying out several rows dry, ahead of the nailing, so the lengths, colors and end joints are chosen rather than taken in carton order.

• Pull from at least three or four open bundles at once. Mills pack a bundle from one run of boards, and a floor laid bundle by bundle shows color bands.
• Stagger the end joints. Commonly at least 6 inches between end joints in adjacent rows for strip, more for wider plank; the maker's sheet governs.
• No H pattern. End joints in rows two apart must not line up. Two joints aligned across one row read as an H and look like a mistake from across the room.
• No stair-step. Joints that move the same distance every row march diagonally across the floor.
• Short pieces away from doorways. Save them for closets and under furniture.

 PLAN, 168 in rows, about 4 in per character
 row 5  |====================|=============|=====|
 row 4  |====|=======|=============|=============|
 row 3  |==================|=============|=======|
 row 2  |========|=============|============|====|
 row 1  |==============|==============|==========|
 inner | = end joint, outer | = wall
 closest end joints: about 12 in in adjacent rows,
 about 8 in in rows two apart, so no H

 SECTION, one blind nail
          board face
     +--------------+
     |              +======  tongue, toward the room
     |             /+======
     +------------/-+
 felt ===========/=====
 OSB            /   cleat at about 45 deg, from the
               /    tongue top corner into the OSB

Step 6: Blind-nail the field on the maker's schedule

Blind-nailing drives the cleat or nail at about 45 degrees through the top of the tongue into the subfloor, where the next board's groove hides it.

1. Set the nailer on scrap first. Adjust air pressure until a test cleat seats flush in the tongue's top corner. Too much pressure crushes the tongue, so the next board will not engage it; too little leaves the cleat proud and the next board will not close.
2. Seat each board before you nail it. Strike the board's tongue edge with the mallet through a tapping block or offcut until the joint closes. A gap left open is nailed open.
3. Follow the fastener schedule. For 3/4 inch solid strip the NWFA guidelines commonly call for fasteners about every 8 to 10 inches and within a few inches of each end of every board. In the bedroom the maker's sheet reads 2 inch cleats every 8 to 10 inches with a fastener 1 to 3 inches from each end, and that is what governs over any figure in this card.
4. Keep the expansion space. Nothing on the wall side of the last few rows is pushed tight to the wall. Spacers come out before base goes on.

Hazard: the nailer's plunger is driven by a mallet; keep the off hand on the board's far end, never on the strike path. Disconnect the air before clearing a jammed cleat.

Count check for the bedroom: 64 rows (62 full plus the two ripped rows) of 168 inches. With boards averaging about 3 ft, a row carries about 4-1/2 to 5 boards; at a cleat about every 9 inches plus one near each end, that is roughly 22 to 24 cleats per row, so about 1,400 to 1,550 cleats in the room. Order to the top of that range plus jams and a test strip.

Step 7: Finish the last rows and the doorway

When the nailer will no longer fit against the far wall, return to hand blind-nailing and then face-nailing, the mirror of Step 4. Rip the last row to the scribe with the 1/2 inch gap held. Where the floor runs through a doorway into a room with a different floor, a reducer or threshold goes on now (see related: Transition Profiles and the Accessible Level Change). Never face-nail the last row into the wall or through the shoe into the floor.

Hazard: the rip cuts and crosscuts throw fine oak dust. Cut outside or on a saw with dust collection. Oak dust is a recognized respiratory and sensitizing hazard; where a respirator is worn, it is worn under a written program per 29 CFR 1910.134 (applied to construction by 29 CFR 1926.103).

How to verify you got this right

The finish test is done before the floor is sanded and finished, because sanding hides nothing a loose board does.

• Walk the whole floor in socks, row by row. A loose board moves or clicks underfoot. Mark it, then add a face nail into the subfloor, predrilled and set, or a fastener from below where the ceiling is open.
• Look at the joints standing, in normal light. Side joints closed; no end joint gaps. A side gap that runs along a whole board means it was nailed before it was seated.
• Check the stagger. No end joints closer than the maker's minimum between adjacent rows, no H anywhere. Spot-check with the tape at five random points.
• Measure the gap at the last row. It should read the expansion space, 1/2 inch in the bedroom, all along both walls.
• Re-read the line. The tongue edge of the last full row measured back to the main wall at both ends should agree within 1/8 inch; in the bedroom row 63 should sit at 1/2 + 1-3/4 + (62 x 2-1/4) = 141-3/4 inches, and it read 141-3/4 and 141-13/16 inches at the two ends.

Where this goes wrong a second way

Wide plank is not strip. At around 3 inches and wider, many makers add a glue assist or tighter fastener spacing, and the moisture match tightens. Read that section of the sheet even when the procedure looks the same.

Over radiant heat, on a slab, or below grade, a nailed solid floor is often not permitted at all. Those cases are covered in Nail-Down, Glue-Down or Floating: Choosing the Method, and On-Grade, Below-Grade and Above-Grade Floors.

References

• The flooring maker's installation instructions (underlayment, fastener schedule, expansion space, stagger)
• NWFA Installation Guidelines, current edition, as the industry practice the maker's sheet usually references
• 29 CFR 1910.134, respiratory protection, where respirators are worn
• See related: Wood Moisture Content and the Match to the Subfloor
• See related: How to Lay Out a Starting Line for a Plank Floor