Subfloor flatness

What the test measures

The check finds the high and low spots in a subfloor: how far the floor falls away under a straight line laid across it, over a stated length. NWFA’s installation guidelines describe it this way:

Division 9 specifications for concrete floors to receive a wood floor provide floor flatness requirements in terms of an allowable gap (1/8” in 6’ or 3/16” in 10’) under an unleveled straightedge.

NWFA Installation Guidelines, Revised © 2025, p. 73 (PDF p. 77)¹

and add:

There is no ASTM procedure for this measurement.

NWFA Installation Guidelines, Revised © 2025, p. 73 (PDF p. 77)¹

Flat is not the same as level. NWFA:

The floor does not need to be level in most situations, but should be flat.

NWFA Installation Guidelines, Revised © 2025, p. 58 (PDF p. 62)¹

A tolerance means something only for the floor it was written for. It is in the installation document of the floor product, and the table under Red flags shows how much the tolerances manufacturers publish differ.

What a valid report looks like

A complete record of the check names, at minimum:

• the tool used: straightedge, string line or laser, and the length of the straightedge;
• the date of the check;
• where in each room it was measured;
• each gap found, with the span it was measured over;
• what was done about each spot out of tolerance;
• the product document the gaps were compared with.

NWFA’s installation guidelines also set out how the check is made. Where a record says how it was made, these are the points to hold it against, quoted from NWFA:

Tools

“Assessing Wood Subfloor Flatness: Measure subfloor flatness using a laser-level, string-line, or straight-edge by taking measurements across the plane of the line to determine tolerances.”¹

Where to measure

“Subfloor flatness should be measured across the span of each room receiving wood flooring to get an overall perspective of the topography of the subfloor.”¹

Covering the floor

“Move the straight-edge to multiple locations, and rotate it 180° at each location to cover the entire floor space.”¹

Where rooms meet

“Where one room meets another, the subfloor flatness should remain in tolerance.”¹

Marking the floor

“Mark out any discrepancies on the subfloor itself, giving a good indication of what alterations will be necessary.”¹

NWFA closes with “Document and photograph the results.”¹ This is NWFA’s procedure, not the floor document’s; where your product’s document describes the check, that document applies.

Red flags

1. Any floor type: the report says “flat” with no gap and no span. Every tolerance in the table is a gap over a length; a word alone cannot be held against one.

2. Any floor type: the report does not name the product document its gaps were compared with. In laminate alone, the documents read print 6 differently worded tolerances across 6 documents. The limit that binds you is in your product's installation document.

3. Hardwood: the looser NWFA tolerance is applied to the wrong installation. NWFA gives one tolerance for floors nailed with longer fasteners:

   For installations using mechanical fasteners of 1½” and longer, the subfloor should be flat to within minimum tolerance of 3/16” in 6’, or ¼” in 10’, unless otherwise specified by the wood flooring manufacturer.

   NWFA Installation Guidelines, Revised © 2025, p. 58 (PDF p. 62)¹

   and a tighter one for the others:

   For glue down installations, floating installations, and installations using mechanical fasteners of less than 1½”, the subfloor should be flat to within minimum tolerance of 1/8” in 6’, or 3/16” in 10’, unless otherwise specified by the wood flooring manufacturer.

   NWFA Installation Guidelines, Revised © 2025, p. 58 (PDF p. 62)¹

4. Concrete: patch or self-leveler went down before the slab’s moisture was tested. NWFA:

   Prior to applying any patching compounds or self-levelers, the moisture conditions of the slab must be assessed through applicable moisture testing methods.

   NWFA Installation Guidelines, Revised © 2025, p. 73 (PDF p. 77)¹

Questions to ask the installer

1. Which product’s installation document did you check the floor against, and which tolerance in it?
2. What did you measure with, and over what length?
3. Where were the high and low spots, and how big was each gap?
4. How will the spots out of tolerance be fixed: grinding, patching, or a self-leveler?
5. For a nailed wood floor: how long are the fasteners, and which NWFA tolerance did you use?
6. For a concrete slab: was the moisture tested before any patch or leveler went down?