How flat does a subfloor need to be for LVP installation in NB?

Your subfloor must be flat to within 3mm over a 2-metre span (roughly 1/8 inch over 6 feet) for luxury vinyl plank installation — and in New Brunswick's climate, hitting this tolerance matters even more than in drier provinces. LVP is a thin, semi-flexible material that conforms to whatever is beneath it, so every bump, dip, and ridge in your subfloor will eventually show through the finished floor.

To check your subfloor's flatness, lay a long straight edge (a 6-foot level or a straight 2x4) across the floor in multiple directions and look for gaps underneath. Slide a business card or a 3mm spacer under the straight edge — if it slides under freely at any point, that area needs attention. Check the entire room systematically, paying special attention to panel edges in plywood subfloors, areas around floor drains in basements, and transitions near walls and doorways.

High spots can be sanded down with a belt sander on plywood or ground with a concrete grinder on slab floors. This is straightforward work that experienced DIYers can handle. Low spots of 6mm or less can be filled with floor-patching compound or self-levelling compound, allowed to cure, and sanded smooth. Deeper depressions (more than 6mm) require a self-levelling pour ($2-$4/sq ft applied) that flows across the floor to create a uniformly flat surface. For plywood subfloors with severe waviness — common in older NB homes where the original boards beneath have cupped and shifted over decades — a plywood overlay ($1.50-$3.00/sq ft) may be the most practical solution.

Why flatness matters so much for LVP in New Brunswick comes down to our climate. NB's seasonal humidity swings cause subtle movement in wood-framed floors throughout the year. A subfloor bump that barely causes a problem in July may create a noticeable ridge by January as the surrounding wood contracts in the dry heated air. The reverse happens in summer — low spots can become more pronounced as the wood subfloor expands. LVP's click-lock connections are engineered to handle normal expansion and contraction, but they are not designed to absorb subfloor irregularities. Persistent high spots create stress points in the locking system that can cause clicks, pops, and eventual joint failure.

Beyond flatness, make sure your subfloor is also clean and smooth. Small debris — even a screw head protruding 1mm above the plywood surface, a dried drip of old adhesive, or grit left on the floor — will create a visible bump through LVP within weeks of installation. Sweep thoroughly, then check by running your hand across the surface and feeling for any irregularity. On concrete slabs, scrape off any adhesive residue from previous flooring and grind down rough patches or trowel marks.

Most NB homeowners can handle minor subfloor preparation for LVP — sanding a few high spots, filling small dips, and ensuring a clean surface. But if your subfloor needs extensive levelling work, particularly on concrete basements where moisture testing is also essential, a professional installer will ensure the substrate meets manufacturer specifications. This preparation work is typically included in professional LVP installation quotes ($5-$12/sq ft fully installed), though extensive levelling may be quoted separately.

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

• Forever Epoxy Inc
• Arctic Fox Construction Inc.
• Thirty Four Renovations
• M&L Commercial and residential services
• Modern Epoxy Inc