Can I install new flooring over old board subfloors in a pre-1970s NB home?

Yes, you can install new flooring over old board subfloors, but not directly — you will almost certainly need a plywood overlay first to create a smooth, stable surface. Pre-1970s New Brunswick homes commonly have diagonal or straight-laid board subfloors made of 1x6 or 1x8 spruce or fir boards, and while these are structurally sound in most cases, they are too uneven and gappy for modern flooring materials.

The gaps between old boards are the primary problem. Individual boards shrink and swell with NB's dramatic seasonal humidity cycles, and after 50-plus years, the gaps between planks can be 3-6mm wide. These gaps telegraph through thin flooring like laminate, LVP, and even hardwood over time, creating visible ridges and dips in your finished floor. The boards themselves may also be cupped, warped, or sitting at slightly different heights — none of which modern click-lock or nail-down flooring can accommodate without a flat substrate underneath.

The standard solution is a plywood overlay. Install 3/8-inch or 1/2-inch exterior-grade plywood directly over the existing board subfloor, screwed down on a 6-8 inch pattern into the boards and underlying joists. This creates the smooth, flat, continuous surface that modern flooring requires. Use exterior-grade plywood rather than interior-grade — NB's Maritime humidity means the adhesives in interior-grade plywood can weaken over time, especially in homes without consistent climate control. Budget $1.50-$3.00/sq ft for the plywood overlay including materials and installation labour.

Before installing the overlay, walk the entire floor carefully and address any structural issues. Re-screw any loose boards to the joists below to eliminate squeaks and movement. Check for soft spots by pressing firmly with your foot — soft areas indicate rot or insect damage in the boards or joists beneath, which must be repaired before covering with plywood. In older NB homes, particularly those near the coast or in areas prone to spring flooding, moisture damage to subfloor boards is common and may not be visible from above.

Inspect from below if possible. Access through the basement or crawl space lets you check joist condition, look for water stains or mould, and verify that the board subfloor is adequately supported. Older NB homes sometimes have undersized joists by modern standards, which can cause excessive bounce. If the floor noticeably bounces when you walk across it, sistering additional joists (adding new joists alongside the existing ones) may be needed before any new flooring — this is structural work that requires a professional assessment.

One important consideration: adding a plywood overlay raises your floor height by 10-12mm, which affects door clearance, appliance fit (especially dishwashers and refrigerators), and transitions to adjacent rooms. You will likely need to trim the bottoms of doors and plan for transition strips where the new floor meets existing flooring in other rooms.

While handy homeowners can manage a plywood overlay as a DIY project, the assessment of the existing board subfloor's structural condition is best done by a professional, especially in NB homes over 50 years old. Hidden moisture damage, inadequate joist sizing, and insect damage are common enough in our housing stock that a professional eye can save you from covering up problems that will only get worse under your new floor.

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

• First united design & construction inc.
• 3Tone Construction Ltd
• Gionetterenovations
• FRS Flooring Solutions
• Modern Epoxy Inc