Floor Levelling for SPC Vinyl Plank: The Step That Makes or Breaks Your Installation

SPC (stone polymer composite) luxury vinyl plank is one of the toughest floors you can buy — waterproof, dent-resistant, and beautiful. But its rigid core comes with one non-negotiable demand: the subfloor underneath has to be flat. A rigid plank cannot conform to humps and dips the way a flexible sheet vinyl can, so every imperfection below becomes stress on the plank above. Floor levelling is not an optional extra on an SPC job — it is the job.

The Industry Standard: How Flat Is Flat Enough?

Nearly every SPC manufacturer — and the governing standard for resilient flooring substrates, ASTM F710 — uses the same tolerance, and it applies to both plywood and concrete:

• Maximum 3/16″ (5 mm) deviation in any 10-foot (3 m) radius, and
• Maximum 1/8″ (3.2 mm) in any 6-foot (1.8 m) radius.

That’s it — about the thickness of three stacked quarters over a ten-foot span. Check it with a 10-foot straightedge (or the longest level you have) laid across the floor in several directions. Anywhere you can slide a coin underneath, mark it. High spots get ground or sanded; low spots get filled.

Substrate moisture matters too before you level or lay plank: wood subfloors should read 12% moisture content or less, and concrete should test below 90% RH (ASTM F2170) or 8 lbs MVER (ASTM F1869) — tighter limits apply over in-floor heat.

What Happens When You Skip Levelling: Deflection

When a rigid plank bridges a low spot, it’s essentially a tiny diving board. Every footstep flexes the plank down into the void and back up again — hundreds of times a day. That repeated vertical deflection is the number-one killer of floating SPC floors:

• Broken click locks. The tongue-and-groove locking profiles on SPC are precisely milled and only a few millimetres thick. Flexing over a hollow snaps them — and once a lock breaks, it never holds again.
• Seam separation and gapping. Planks rocking over high spots lever their joints apart. You’ll see end-gaps open up, usually in traffic lanes, letting in dirt and moisture.
• Vinyl layer separation. Constant flexing can delaminate the wear layer and decorative vinyl film from the stone core at plank edges — visible as whitening, peaking, or peeling seams.
• Crushed edges and corners. Point loads (furniture legs, heels) over an unsupported spot crack plank edges outright.
• Hollow sounds, movement, and squeaks that make a brand-new floor feel cheap.
• A voided warranty. Every manufacturer’s installation instructions state the flatness tolerance — install outside it and claims for locking or seam failures will be denied.

One related trap: thick, soft underlayment pads. They feel like they’d help, but too much cushion creates vertical deflection and damages plank edges and joints — most SPC makers cap attached or separate pads around 1.5 mm.

Getting It Flat: Plywood vs. Concrete

Plywood & Wood Subfloors

• Re-screw loose panels to the joists first — flat but bouncy is still a fail. The structure itself should meet the standard L/360 deflection requirement.
• Sand or plane high spots at panel seams.
• Fill lows with a cementitious patch or embossing leveller (we like Ardex Feather Finish and Mapei Planipatch) — not wood filler, which crumbles.

Concrete Slabs

• Grind high ridges and trowel marks.
• Fill lows with a polymer-modified patch, or pour a self-levelling underlayment over primer for larger areas — a properly primed pour gets a wavy basement slab dead-flat in one lift.
• Let patches and pours cure and dry per the bag before laying plank.

The Bottom Line

Levelling is the least glamorous hour of a vinyl plank job and the most important one. A floor installed over a properly flattened subfloor feels solid underfoot and lasts for decades; the same product over a wavy subfloor starts clicking, gapping, and breaking locks within months.

Sierra Flooring checks every subfloor against the manufacturer tolerance and carries the patches, levellers, and primers to fix what we find — for our own installs and for DIYers. Planning an SPC project? Visit our Penticton showroom at 1470 Clark Ave or book a free in-home estimate and we’ll make sure your floor starts flat and stays beautiful.