Wood subfloor moisture

What the test measures

The test reads the moisture content of a plywood, OSB or plank subfloor with a wood moisture meter. NWFA’s installation guidelines put the rule for testing this way:

It is compulsory practice to always test for moisture regardless of conditions so that any unknown conditions can become known conditions, which then can be handled appropriately.

NWFA Installation Guidelines, Revised © 2025, p. 41 (PDF p. 45)¹

For a wood floor the subfloor reading is half of the check. The documents read also compare the subfloor with the flooring: the subfloor’s moisture content may be only so much above the flooring’s. That needs a reading of the flooring too, and the difference between the two. Both sets of limits are in the tables under Red flags.

A reading means something only against a limit, and the test does not set one. The limit is in the installation document of the floor product.

What a valid report looks like

A complete report names, at minimum:

• the meter used, and the setting it was read on for the material tested;
• the date of each reading;
• where in each room the readings were taken;
• the air temperature and relative humidity in the room;
• each subfloor reading, and their average;
• for a wood floor, the flooring’s readings, their average, and the difference from the subfloor;
• the product document the readings were compared with.

NWFA’s installation guidelines also set out how the readings are taken. Where a report says how they were taken, these are the points to hold it against, quoted from NWFA:

Subfloor readings

“After calibrating your meter to the subfloor material being tested, take MC readings in a minimum of 20 test locations for up to the first 1,000 square feet, and an additional 4 readings per 100 square feet thereafter, and average the results.”¹

Readings in each room

“Testing locations should be representative of the entire project and include a minimum of three tests per room receiving wood.”¹

Where to look

“Pay special attention to exterior walls and plumbing.”¹

Flooring readings

“Take MC readings of the wood flooring on a minimum of 40 boards for up to the first 1,000 square feet, and an additional 4 readings per 100 square feet thereafter, and average the results.”¹

Subfloor at delivery

“Upon delivery of the flooring to the jobsite, recheck and record the MC of the subfloor.”¹

Flooring at delivery

“Upon delivery of the flooring to the jobsite, recheck and record the MC of multiple boards of flooring from a variety of bundles.”¹

Unusual boards

“Any unusually high or low moisture readings should be isolated and not installed in the floor.”¹

NWFA adds “When testing for moisture, both the wood flooring and the subfloor should be evaluated.”¹ This is NWFA’s procedure, not the floor document’s; where your product’s document describes the test, that document applies.

Red flags

NWFA’s installation guidelines are an industry guideline, not the document for your product. Where the two differ, both are shown above.

Rigid core vinyl: the one company’s limit, from Mohawk SolidTech Installation.

The moisture content of the wood subfloor must not exceed 12%.

Mohawk SolidTech Installation, Rev. October 2018, PDF p. 1¹⁶

The difference between the subfloor’s moisture content and the wood flooring’s:

NWFA’s installation guidelines are an industry guideline, not the document for your product. Where the two differ, both are shown in each floor type’s section above.

1. Wood flooring: the report has a subfloor reading only. With no reading of the flooring there is no difference to hold against the second table. NWFA’s rule of thumb for solid wood:

   Where the wood subfloor MC is not aligned with the required conditions in the facility receiving wood, the general rule of thumb is to ensure the MC of the wood subfloor is no more than 4% greater than the MC of solid strip (<3” widths) wood flooring, and no more than 2% greater than the MC of solid plank (≥3” widths) wood flooring being installed.

   NWFA Installation Guidelines, Revised © 2025, p. 34 (PDF p. 38)¹

2. Solid hardwood: the product’s document gives two figures for the same check. One of the solid hardwood documents read, Bruce’s, prints different limits in two of its sections. For the subfloor, in II. Preparation:

   The wood subflooring materials must not exceed 12% moisture content.

   Bruce Solid 3/4" Strip and Plank, LA-295726-120, © 2020, PDF p. 1¹³

   and in III. Subfloor/Underlayment Requirements:

   The wood subflooring materials must not exceed 13% moisture content.

   Bruce Solid 3/4" Strip and Plank, LA-295726-120, © 2020, PDF p. 2¹³

   For the difference, in II. Preparation:

   The difference between the moisture content of the wood subfloor and the wood flooring must not exceed 3% (2% for plank).

   Bruce Solid 3/4" Strip and Plank, LA-295726-120, © 2020, PDF p. 1¹³

   and in III. Subfloor/Underlayment Requirements:

   The difference between the moisture content of the wood subfloor and the hardwood flooring must not exceed 4% for strip and 3% for plank flooring.

   Bruce Solid 3/4" Strip and Plank, LA-295726-120, © 2020, PDF p. 2¹³

   The document conflicts with itself. A report on this floor that cites it should say which section its figure comes from.

3. Any floor type: the report does not name the product document its readings were compared with. In engineered hardwood alone, manufacturers publish subfloor limits from 12 to 14 % MC across 4 documents. The limit that binds you is in your product's installation document.

4. Hardwood: the reading is compared with NWFA’s figure alone. For engineered hardwood and solid hardwood, NWFA’s guideline and the manufacturers’ documents give different subfloor figures; both are at the foot of the first table. NWFA’s sentence:

   In hot and humid climates, and during the humid season, the subflooring should not exceed 13% moisture content (MC).

   NWFA Installation Guidelines, Revised © 2025, p. 41 (PDF p. 45)¹

Questions to ask the installer

1. Which product’s installation document did you compare the readings with, and which limit in it?
2. For a wood floor: what did the flooring read, and what is the difference from the subfloor?
3. Which meter did you use, and on which setting?
4. How many readings did you take for the floor area, and how many in each room?
5. Were the readings taken again when the flooring was delivered?
6. If the product’s document gives two figures, which section did you follow, and why?