How to Stop Subfloor Squeaks Before New Flooring

Why this matters

A squeak under a new floor is a callback the customer hears every day, and once the floor is down it is the flooring crew's squeak no matter who built the house. The only time it is cheap to fix is with the old floor off and the subfloor bare, when every joist line can be screwed and every gap is reachable. Nailing hardwood over a squeaky subfloor does not quiet it; the new nails often add their own noise, and a floating floor over a moving panel lets the panel keep rubbing underneath.

This card covers silencing a wood subfloor from above and, where you can reach it, from below, before new flooring goes down. Judging whether the subfloor is sound enough to keep at all (rot, delamination, undersized panels, joist sag) belongs to 'Subfloor Assessment + Repair Reference'. Choosing between a fix from above and from below on a finished floor is 'Squeaky Floor From Above Vs Below Decision Tree'.

Where squeaks come from

Almost every subfloor squeak is two pieces of wood moving against each other, or a fastener moving in its hole:

• A nail in a loosened hole. Panels and joists shrink after construction, the nail loses its grip, and the panel rides up and down on the nail shank. This is the most common one, and it sits on a joist line.
• A gap between the joist top and the panel. A joist that shrank, twisted or was set low leaves the panel bridging air; it deflects onto the joist under load.
• A panel edge rubbing its neighbor. Tongue-and-groove edges between joists, with no support under them, move against each other.

Each has its own fix, so the walk has to tell them apart.

The procedure, run on one room

The room: a 13 ft by 15 ft bedroom over an unfinished basement, 3/4 inch tongue-and-groove OSB on 2x10 joists at 16 inches on center, joists running the 15 ft direction. Carpet has come off and solid hardwood is going down. Two people.

Step 1: Look from below before anything goes into the floor

Go to the basement before driving a single screw and look at what is in the joist bays under the room: supply and drain lines, gas pipe, ducts, cable runs, and above all radiant heat tubing stapled to the underside of the subfloor. A subfloor screw that misses its joist goes into the bay, and a 2-1/2 inch screw through a 3/4 inch panel reaches 1-3/4 inches below it, which is where staple-up tubing and many pipes sit.

If the bay holds radiant tubing, no screws go in anywhere except on confirmed joist lines, and those lines get confirmed from below as well as above. If a screw ever goes into a gas line, everyone leaves the house, nobody touches a switch or uses a phone inside, and the call to the gas utility is made from outside. If one hits a water line, shut the main and call the plumber the shop uses. Note where the cables run; cables bored through joists sit inside the joist, and a screw along the joist top does not reach them, but a cable stapled where it should not be might.

In this room: a copper supply line runs in the bay between the fourth and fifth joists from the east wall, 2 inches below the subfloor. No radiant, no gas in the room's bays. The pipe's bay gets marked on the plan.

Step 2: Snap the joist lines

Find the joists from above by the existing nail lines, confirm the spacing with a tape from a known joist, and check any doubtful one by drilling a 1/8 inch hole and feeling for wood. Snap a chalk line down the center of every joist. What breaks if you skip it: screws placed by eye miss joists, squeak themselves, and go into the bays you just inspected.

In this room: the joists measure 16 inches on center, ten lines across the 13 ft width, each running the 15 ft length.

Step 3: Walk the floor and mark the noise

One person walks a slow grid across the room, heel to toe, rocking weight onto each step. The other listens and marks each squeak with a crayon "S", and the person below, if there is one, watches for the panel lifting off a joist or a nail moving. Note for each squeak whether it sits on a joist line, at a panel edge between joists, or over a visible gap.

In this room: six squeaks. Four sit on joist lines, at old nails that have backed out slightly (nail-in-loose-hole). One sits on a panel edge midway between two joists. One sits on a joist line where, from below, daylight shows between the joist top and the panel over about 3 feet, a gap of about 1/8 inch.

Step 4: Screw the panels to every joist

Drive structural subfloor screws along every joist line, not just at the squeaks, because the nails that are quiet today are the next ones to loosen under the drag of a new floor going down. Commonly that is a #8 to #10 subfloor screw, 2 to 2-1/2 inches long, which gives at least about 1-1/4 inches of thread in the joist through a 3/4 inch panel. Set heads just below the surface, and set any proud nail heads with a nail set so nothing telegraphs or snags the new floor.

Default spacing: a screw every 6 to 8 inches along each joist line, staying about 3/8 inch in from panel ends where two panels share a joist, or tighter where the panel maker's fastening guidance calls for it.

ONE 4 x 8 PANEL, 8 ft edge across joists 16 in o.c.
   0     16    32    48    64    80    96 in
 +--|-----|-----|-----|-----|-----|-----|--+ edge
 |  o     o     o     o     o     o     o  |  4 in
 |  o     o     o     o     o     o     o  | 12
 |  o     o     o     o     o     o     o  | 20
 |  o     o     o     o     o     o     o  | 28
 |  o     o     o     o     o     o     o  | 36
 |  o     o     o     o     o     o     o  | 44
 +--|-----|-----|-----|-----|-----|-----|--+ edge
  | = snapped joist line    o = screw, 8 in apart
  0 and 96 are shared with the next panels

In this room: at 8 inches along a 15 ft (180 inch) line, 180 / 8 = 22.5, so 23 screws per line, and 10 lines x 23 = 230 screws. Over the pipe bay, the crew double-checks each line in that bay from below before driving. This step silences the four nail squeaks.

Step 5: Close joist gaps from below without lifting the floor

Where a joist top sits below the panel, pulling the panel down with screws from above bends it into the low joist and puts a dip in the floor. Fill the gap instead. From below, run a bead of construction adhesive into the gap, then tap a thin wood shim in only until it is snug; driving a shim hard lifts the panel and makes a hump. A squeak bracket screwed to the joist and the panel from below does the same job, with screws short enough that they cannot come through the top.

Work below with a light and eye protection, mind the overhead cables, and keep the adhesive's vapor ventilated in a closed basement.

In this room: the 3 ft gap of about 1/8 inch gets adhesive and three shims spaced about a foot apart, tapped snug. Back upstairs, the 2 ft straightedge across that joist line shows no hump.

Step 6: Support loose panel edges between joists

A panel edge squeaking between joists needs something under it. From below, glue and screw a 2x4 block between the joists, tight to the underside of the panel and centered under the seam, and screw up through the block into the panel with screws shorter than the block plus the panel. A 2 inch screw through a 1-1/2 inch block leaves 1/2 inch in a 3/4 inch panel, 1/4 inch short of the top. Where there is no access below, the fix is a screw from above into the edge of the adjacent panel, which helps less, or replacing the panel.

In this room: one block under the edge squeak.

Finish test

Walk the same grid with the same two people. Every marked spot is silent and no new squeak has appeared; every screw head is below the surface by hand and by straightedge; and the shimmed joist line passes the flatness check against the flooring maker's target. The basement is checked once more for a screw point showing in any bay, especially the pipe bay.

In this room: six marks, six silent, none new, no screw points in any bay.

What changes on other framing and other ceilings

Engineered I-joists. The top flange, commonly about 1-3/4 to 3-1/2 inches wide, is the only wood a screw can hold in, with nothing but a thin web below it, so the joist lines have to be found precisely and every screw has to land in the flange, not beside it. Never notch or cut a flange to fit a shim or a block, and use the I-joist maker's blocking and squeak-repair details below rather than a 2x4 nailed into the web; the maker's installation guide governs what may be attached to its joist.

Open-web floor trusses. Spacing is often 19.2 or 24 inches rather than 16, so measure instead of assuming, and treat a gap at a truss top chord the same way as at a joist: fill it, do not pull the panel down into it.

A finished ceiling below. With drywall under the room there is no from-below fix without opening the ceiling. Screwing every joist line from above still works; for a known joist gap, decide with the customer between opening a section of ceiling to shim it and accepting the small dip a screw pulls into the panel, and check that dip against the flooring maker's flatness target before the new floor goes down.

Glued subfloors. A panel glued to the joists at construction that squeaks has usually broken the glue line along that joist. Screws restore the clamp; they do not restore the glue, which is why the screw spacing along that joist is kept to the tight end of the 6 to 8 inch range.

Floating floors. A squeak in the subfloor does not stay under a floating floor quietly. The floor rides on the panels, so a moving panel moves the floor and the click joints over it. Floating does not exempt a floor from this procedure.

When the squeak is a symptom

Stop and route the floor to 'Subfloor Assessment + Repair Reference' when the walk finds a panel that deflects visibly, a soft or delaminated area, a joist with a crack or a notch, or a gap that runs the length of several joists, which points at framing sag rather than shrinkage. Screws and shims quiet those floors for a season, and the new floor inherits the movement.

References

• See related: Subfloor Assessment + Repair Reference
• See related: Squeaky Floor From Above Vs Below Decision Tree
• See related: How to Check a Floor for Flatness and Map It
• Subfloor panel maker's fastening guidance for panel type and thickness, and the I-joist maker's installation guide where I-joists are present; flooring maker's installation instructions for subfloor requirements.