How Subfloors Affect Your Hardwood Flooring

Plywood SubfloorHealthy flooring is built upon a healthy subfloor installation. When installing hardwood floors, builders often cover wood joists with wood subfloors.

At other times, installers lay engineered wood over concrete subfloors. Add to this the rapid evolution of eco-friendly adhesives, and you get a sense of modern flooring’s installation complexity. Today’s flooring choices are so versatile that subfloor moisture issues can have a large impact on the success of the flooring installation [i].

Moisture content (MC) is the proportion of moisture held in a material and applies to both wood and concrete. Every wood cell contains moisture in its walls and nucleus, while every concrete mixture contains some amount of water.

When installing wood floors over concrete slabs with adhesives, we add moisture between the floor and subfloor. Flooring adhesive manufacturers are replacing toxic volatile organic chemicals (VOCs) with water. Between the concrete, the wood and the flooring adhesives, it is impossible to avoid the need for proper MC measurement and management.

Industry rule-of-thumb is: Make sure the subfloor is clean, dry and flat before applying any flooring [ii]. But how can you be sure it is dry enough without moisture testing?

Following are some general guidelines as they relate to specific combinations of building materials in floors and subfloors.

Plywood and Oriented Strand Board (OSB) Subfloors over Joists

Man Nailing Plywood Subfloor

Newer homes with basements and crawlspaces utilize plywood and OSB subfloors over joists. The greater the space between joists, the thicker the plywood and OSB must be. Fasteners must be properly placed (according to manufacturers’ specifications) so they do not protrude. Subfloors must be free of contaminants such as paint, sealer or drywall compounds. This wood-based flooring combination must be flat for successful floor covering. The subfloor can be flattened by sanding down the high areas or by installing shims. If there are any creaks, squeaks or loose panels, refasten the subfloor to the joists before applying the flooring.

A wood moisture meter can be an extremely helpful tool to pre-empt warped wood subfloors. The National Wood Flooring Association (NWFA) recommends that flooring installers measure the MC of the subfloor and the joists first. In fact, the NWFA recommends a minimum of 20 moisture meter readings for every 1,000 square feet [iii]. Using moisture meters for wood is vitally important to guaranteeing the wood subfloor’s integrity.

Particleboard over Plywood Subfloors

Carpet Over Subfloor

That’s right! Installers frequently find particleboard as a subfloor layer over plywood when carpeting or vinyl is ripped out. Since particleboard has no holding power for fasteners, the only suitable wood flooring that can be installed is a floating floor. Other wood floors can be installed if the particleboard is ripped out.

Particleboard is manufactured by mixing wood particles or flakes with a resin and forming the mix into a sheet. In panel production, other chemicals including wax, dyes, wetting agents and release agents, which can be added to make the final product water, fire and insect-proof or to give it some other special quality. The sheets are cold pressed to reduce their thickness and to make them easier to ship. Particleboard is very prone to expansion and discoloration due to moisture, especially if not covered with a paint or sealer.

Ironically, plywood’s construction can also pose an MC challenge. Plywood consists of several thin sheets of veneer, which are first either air dried or kiln dried and then glued together. The veneer sheets are applied with an adhesive and stacked. The glued sheets are then usually run through a hot press to dry the glue and are then trimmed, sanded if necessary and then graded for quality.

Wood flooring lovers need not fear this, but they certainly must utilize moisture meters for wood to ensure stable MC in these types of materials. Improper MC levels in the subfloor can produce all kinds of residual flooring problems when covered by wood, carpet or vinyl. Thus, moisture meter testing is even more imperative with wood subfloors made from multi-sourced wood products.

Concrete Subfloors

MC is part of a dynamic concrete slab formation process. There is always water content in concrete mixtures, and those mixes are becoming more engineered than ever. Homes without basements may still have concrete subfloor foundations below the ground (known as “grade”).

A concrete moisture test is equally crucial to concrete subfloors. The water content in concrete migrates from the bottom of a slab to the surface, where it evaporates according to changes in relative humidity (RH). The internal moisture must be allowed to dry appropriately, based on the flooring and adhesive being used on the job. The internal moisture is key because once the finished floor product is installed, the moisture equilibrates and the surface moisture will eventually change. If the internal moisture is not appropriately dry, it may cause problems with the adhesives and flooring products.

Many builders mistakenly believe that surface MC levels reflect those of the whole slab. It is essential to verify internal slab moisture levels. A concrete moisture meter works for surface-level MC assessment only, but to see further into the slab, RH testing is required.

Successful flooring is built on a dry, clean, flat subfloor, period. Keep moisture testing and assessment at the top of your list.

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Tony Morgan

Tony Morgan is a senior technician for Wagner Meters, where he serves on a team for product testing, development, and also customer service and training for moisture measurement products. Along with 19 years field experience for a number of electronics companies, Tony holds a B.A. in Management and his AAS in Electronics Technology.

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