Subfloor Moisture


Always check moisture content of wood flooring on both sides before installing.

Ensure moisture content of subfloor/substrate meets the appropriate industry standard for the finish flooring material to be installed.

For wood subfloors there should be no more than 2% difference in moisture content between wood flooring and subflooring materials.

For concrete screeds moisture content should not exceed 2% CM Moisture content.

Moisture testing

It is imperative all sub floors are tested for moisture prior to bringing the flooring into the property. This should include recording of all measurements taken including; the date, relative humidity, ambient temperature, the type of meter along with a plan of test locations

Ensure jobsite conditions are met prior to conducting moisture testing

NOTE: All tests give a result at the time the test is done. In general they give you the ability to start or not start a job. These tests do not give a permanent condition of your substrate merely an “at the time the test was performed” indication.

The use of vapour retarders reduces the potential moisture related problems; Ted Todd recommends the use of vapour retarders with all installations.

Moisture testing in wood subfloors

Test for moisture at several locations in the room, a minimum of 20 points per 93 square meters averaging the results.

Test for moisture using an electrical resistance moisture meter, ensuring tests are taken with the moisture probes placed in line with the grain.

Target moisture content of wood sub floor should not exceed 12% or be greater than 2% of the moisture content of the floor

A high reading in one area indicates a problem that must be corrected. Pay special attention to exterior walls and plumbing.

Ted Todd recommend using Sisalkraft 728 Builders paper for all nail down installations and Ted Todd Universol or Ted Todd Floorfix Float Professional underlay with taped seams for all floated installations over wood sub floors.

The vapour retarders have some extra benefits in that it eliminates wood-on-wood contact, planks slide more easily when positioning, minimizes the impact of seasonal humidity change and may reduce dust and noise levels.

Over a wood subfloor, do not use an impermeable vapour retarder material such as 1000 gauge polyethylene film or other polymer materials, as it may trap moisture on or in the wood subfloor.

Moisture testing in concrete subfloors

There are several types of test for measuring the moisture content of a concrete screed

  • Concrete Encounter (MC) – Non-destructive, very quick, surface test
  • Calcium Carbide Test (CM) – Destructive, quick and very accurate
  • Relative Humidity (RH) – Non-destructive, slow but very accurate

Each test has its advantages and disadvantages. British Standards 8201:2011 recommends using relative humidity (RH) testing of concrete sub floors however to comply with requirements set by the installation products Ted Todd recommends testing using the calcium carbide method.

Before moisture testing begins, the concrete slab must be a MINIMUM of 30 days old. Select test locations to provide information about moisture distribution across the entire concrete floor slab. For slabs on ground and below ground, include a test location within three feet of each exterior wall.

Perform 10 tests for the first 93 sq m and one test for every additional 93 sq m thereafter. The actual test area shall be clean and free of all foreign substances. Use approved work practices for removal of all existing flooring materials and debris.

Target moisture content of concrete screed should not exceed 2% (CM)

A high reading in one area indicates a problem that must be corrected. Pay special attention to exterior walls and plumbing.

Always follow the meter manufacturer’s instructions.

If using Relative Humidity testing any readings should not exceed 75% RH.

Ted Todd recommends using Ted Todd Primerfast liquid damp proof membrane (LDPM) prior to ALL glue down installations and Ted Todd Universol or Ted Todd Floorfix Float Professional underlay for ALL floated installations.

If the calcium carbide Method test gave a reading of greater than 2% (CM) moisture reading but no more than 5%, with a 60mm thick screed or less, using Ted Todd Primerfast liquid damp proof membrane can allow you to continue with the installation. If measurements are out side of these parameters do not bring the floor onto site.

When using Ted Todd Universol or Ted Todd Floorfix Float Professional underlay all seams must be taped with Aluminum Tape.

If the installation is over water fed under floor heating system do not use Ted Todd Primerfast. One coat can be used if required to seal a loose screed.

For all floated installations over under floor heating use Ted Todd Universol Underlay with taped seams.