Do I need to replace my plywood subfloor before new hardwood in my older NB home?

Not necessarily — many older NB homes have plywood subfloors that are perfectly serviceable for new hardwood, but they need careful inspection first. The answer depends on the plywood's thickness, structural condition, moisture content, and flatness, all of which are particularly important given New Brunswick's challenging humidity cycles.

Start by checking what you actually have underneath your existing flooring. Many NB homes built before the 1970s have diagonal board subfloors rather than plywood, which is a different situation entirely (those require a plywood overlay). For homes with existing plywood subfloors, nail-down solid hardwood requires a minimum 3/4-inch plywood base. If your plywood is only 1/2-inch or 5/8-inch, you will either need to add a layer of 3/8-inch or 1/2-inch plywood on top ($1.50-$3.00/sq ft for materials and labour) or switch to engineered hardwood, which can go over thinner substrates.

Inspect for these specific problems that are common in older New Brunswick homes. Walk the entire floor slowly, feeling for soft spots — these indicate delaminated or water-damaged plywood that must be cut out and replaced. Check for squeaks, which suggest loose panels that need re-screwing to the joists. Look at the plywood from the basement or crawl space below for water stains, mould, or swelling at the edges. NB's Maritime humidity and the prevalence of older homes with less-than-ideal vapour barriers means moisture damage to subfloor plywood is more common here than in drier provinces.

Use a pin moisture meter to check the plywood's moisture content — it should read between 6-9% for hardwood installation in NB. Readings above 12% indicate a moisture problem that must be solved before any hardwood goes down. In homes near the coast (Saint John, Shediac, Bathurst) or in river valley locations (Fredericton, Miramichi), elevated subfloor moisture is especially common due to higher ambient humidity and ground moisture.

Flatness is the other critical factor. Your subfloor must be flat to within 3mm over a 2-metre span for hardwood installation. Older plywood often develops humps at panel edges and dips between joists. High spots can be sanded or planed down, while low spots are filled with floor-levelling compound or shimmed. If the subfloor has extensive waviness across the entire floor, overlaying with a new layer of plywood is often faster and more reliable than trying to flatten the old surface.

For a typical 1,000-square-foot main floor in an older NB home, budget $1-$5/sq ft for subfloor preparation, depending on condition. Simple re-screwing and minor patching might cost $1,000-$2,000, while a full plywood overlay adds $1,500-$3,000. The hardwood installation itself ($8-$14/sq ft fully installed for solid domestic species) goes on top of that.

Have a professional flooring installer assess your subfloor before committing to hardwood. They can identify structural issues, check moisture levels, and recommend the most cost-effective preparation approach for your specific home. This assessment typically happens as part of the quoting process — most NB flooring contractors will inspect your subfloor during their free estimate visit.

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

• M&L Commercial and residential services
• The Garbage Guys Ltd
• Arctic Fox Construction Inc.
• Gionetterenovations
• Modern Epoxy Inc