Mold problems caused by improper installation of wood furring strips in the basement below slab level

In residential construction, installing wood furring strips directly against or under the concrete basement slab is unfortunately still a widespread practice—and one with serious consequences. Years after construction, signs of mold and decay are frequently observed at the bottom of walls, compromising the health of the indoor environment and the integrity of the materials.

A practice that does not comply with the requirements of the Code

The Quebec Construction Code, Division B, clearly identifies the requirements to protect wood components against moisture and rot in exposed areas:

Article 9.23.2.2.

The end of a beam, joist or other timber frame member which is embedded in masonry or concrete shall be treated to prevent rot if the underside of the member is at or below ground level or, failing that, a 12 mm air space shall be provided at the end and sides of the member.

Therefore, any wood furring extending below the slab level without preventive treatment or an air gap constitutes a clear non-compliance. Wood exposed to a constant source of moisture, such as concrete, acts as a capillary absorption bridge. In the long term, this prolonged exposure promotes the growth of mold, wood rot, and damage to the wall structure.

Article 9.23.2.3.

Untreated timber frame members resting on concrete in contact with the ground or fill must be isolated from the concrete by a polyethylene membrane of at least 0.5 mm or by Type S roll roofing material. This requirement does not apply if the member is 150 mm above the ground.

This second article highlights the importance of capillary insulation between the wood and the concrete. Too often, furring strips are placed directly on the slab without a membrane, or even—more seriously—are attached vertically to the foundation right down to the base of the slab, creating a zone of stagnant moisture at the base of the walls.

Long-term consequences

When these basic rules are ignored:

• Mold grows in the walls at the base, directly linked to the moisture absorbed by the wood.

• Nauseating odors invade the spaces.

• The health of occupants may be affected (respiratory problems, allergies, etc.).

• Real estate value is compromised by the presence of unsanitary conditions.

• Costly fixes will need to be made, including replacing drywall, insulation and furring strips, as well as anti-fungal treatment of the structure.

Professional recommendations

It is imperative, when carrying out finishing or basement renovation work:

• To comply with the requirements of the Code, in particular the minimum elevation of 150 mm of the wood above the ground;

• Use pressure-treated furring if contact with concrete is unavoidable;

• To install a continuous moisture barrier membrane between any wooden component and the foundation concrete or slab;

Conclusion

The improper use of wood furring strips below the concrete slab level is a practice that should be strictly prohibited. It violates the requirements of the Building Code and, over time, can lead to costly and harmful problems for occupants. Proper and preventative execution remains the best protection against moisture-related damage in residential basements.