Should I use pressure-treated lumber for the bottom plates when framing a basement in New Brunswick?

Yes, pressure-treated lumber is required for all bottom plates in basement framing in New Brunswick — this is a building code requirement, not a suggestion. The NB Building Code mandates that any wood framing in contact with or within close proximity to concrete must be preservative-treated to resist moisture absorption, rot, and fungal decay. In NB's Maritime climate, where below-grade concrete slabs and walls are constantly wicking moisture from the surrounding soil, untreated wood on concrete will absorb water, swell, and begin to rot — often within just a few years.

Concrete is not waterproof. It is porous, and moisture migrates through it continuously via capillary action. In New Brunswick, where spring thaw saturates the ground from March through May, summer humidity runs 70 to 85 percent, and water tables are seasonally high across most of the province, the concrete slab in your basement is always damp to some degree. An untreated spruce or pine bottom plate sitting on that slab will absorb moisture relentlessly. Within 2 to 3 years, the wood softens, mold colonizes the fibres, and the bottom plate loses its structural integrity — all hidden behind your finished drywall where you cannot see it until the damage is extensive.

Pressure-treated lumber rated for ground contact (look for the stamp "Ground Contact" or "UC4A" on the lumber) is the minimum standard. Standard above-grade treated lumber is not sufficient for bottom plates on concrete. The ground-contact rating means the wood has been treated with enough preservative to resist constant moisture exposure, which is exactly what a basement bottom plate endures in NB conditions.

Beyond using treated lumber, add a sill gasket (closed-cell foam strip) between the bottom plate and the concrete slab. This thin gasket, costing roughly $0.50 per linear foot, creates a capillary break that prevents moisture from wicking directly from the concrete into the wood. Some contractors use a strip of 6-mil polyethylene instead — both accomplish the same moisture break. This is standard practice for NB basements and costs almost nothing compared to the protection it provides.

A few additional details matter when working with pressure-treated bottom plates. Fasten them with concrete screws (Tapcon) or powder-actuated fasteners — regular nails will not hold in concrete and will corrode quickly in the treated wood's chemical environment. Use stainless steel or hot-dipped galvanized fasteners when attaching studs to the treated bottom plate, because standard bright nails corrode when in contact with the copper-based preservative in treated wood, loosening over time. Do not cut corners by using untreated wood and relying on a poly strip alone — the poly reduces moisture transfer but does not eliminate it, and any tear or gap in the poly exposes untreated wood to continuous moisture.

Pressure-treated lumber costs roughly 30 to 50 percent more than untreated SPF (spruce-pine-fir), but for bottom plates you are only buying a small quantity — typically $100 to $300 in treated lumber for an entire basement. Compared to the $20,000 to $35,000 cost of a basic basement finishing project, this is negligible insurance against rot and mold.

Any qualified basement contractor in NB will use pressure-treated bottom plates as standard practice. If a contractor proposes using untreated lumber on the slab, that is a clear warning sign that they lack experience with below-grade framing.

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Looking for experienced contractors? The New Brunswick Construction Network connects homeowners with qualified professionals:

• Gionetterenovations
• Thirty Four Renovations