Should I use pressure-treated wood for the bottom plate of my garage in Riverview NB?

Yes, absolutely — pressure-treated lumber is mandatory for the bottom plate (sill plate) of any garage in Riverview and throughout New Brunswick wherever wood is in contact with concrete or within 6 inches of grade. This is not optional, not a matter of preference, and not something you can skip to save a few dollars. It is a requirement under the NB Building Code and a fundamental construction practice that every experienced NB builder follows.

The reason is straightforward: concrete absorbs and transmits moisture from the ground, even when a vapour barrier is installed beneath the slab. Untreated SPF lumber placed directly on a concrete foundation wall or slab absorbs that moisture through capillary action, and in Riverview's humid Maritime climate — with high relative humidity year-round, freeze-thaw cycling, and snowmelt moisture — untreated sill plates begin to rot within 5 to 10 years. Once the sill plate rots, the entire wall structure above it loses its bearing point, and the repair involves jacking up walls, removing siding, and replacing the plate — a job that can cost $3,000-$8,000 or more depending on how far the damage has progressed.

Pressure-treated lumber for sill plates is infused with preservative compounds (typically alkaline copper quaternary or ACQ) that resist rot, fungal decay, and insect damage. The treatment penetrates deep into the wood fibres, providing decades of protection even in constant contact with damp concrete. The cost premium is negligible — a pressure-treated 2x6x8 costs only $2-$4 more than an untreated one, and for a 24x24 garage perimeter you need about 20-24 linear metres of sill plate. The total extra cost for treated lumber is roughly $30-$60 — an insignificant amount compared to the cost of the garage or the cost of future repairs.

Between the sill plate and the concrete, you should also install a sill gasket — a thin foam strip that acts as a capillary break and air seal. This is standard practice in NB construction and costs about $15-$25 for enough material to do the entire perimeter of a two-car garage. The sill gasket prevents direct wood-to-concrete contact, reduces moisture transfer, and improves the air-tightness of the wall assembly.

Riverview's location along the Petitcodiac River means spring moisture levels can be particularly high, and garages built on lower-elevation lots near the river are especially susceptible to ground moisture issues. If your garage site has any history of dampness or is in a lower area, ensure that your foundation includes proper drainage — weeping tile around the footings, gravel backfill, and positive grading away from the foundation at a minimum 2% slope.

One important note about pressure-treated lumber and fasteners: ACQ-treated wood is corrosive to standard galvanized nails and hangers. Use hot-dipped galvanized (HDG) or stainless steel nails, screws, and connectors wherever they contact treated lumber. Standard bright nails will corrode within a few years, weakening the connection between the sill plate and the wall studs or the foundation anchor bolts.

This is one area where there is no DIY judgement call to make — treated sill plates are required, period. If a contractor suggests skipping treated lumber to save money, or if you notice untreated wood being placed on your concrete foundation during construction, stop the work immediately and insist on treated lumber. The cost difference is trivial and the consequences of using untreated wood are guaranteed rot and expensive structural repairs down the road. For your Riverview garage project, find experienced local builders through the New Brunswick Construction Network at newbrunswickconstructionnetwork.com.