How do I prepare an uneven subfloor before installing luxury vinyl plank or tile?

Short answer

Make the subfloor clean, dry, structurally sound, and flat to your flooring’s tolerance. Fasten any loose areas, repair damage, grind or sand high spots, and fill low spots with a compatible floor patch or self-leveling underlayment (after primer). Recheck flatness, then install your luxury vinyl plank/tile per the manufacturer’s instructions.

Typical flatness requirement for LVP/LVT:
- Max 3/16" (4.8 mm) variation over 10 ft (3 m)
- Or 1/8" (3.2 mm) over 6 ft (1.8 m)
Always confirm your brand’s spec.

Why flatness matters

LVP/LVT will telegraph subfloor irregularities, causing gaps, rocking planks, or premature wear. Many warranty claims are denied due to out-of-spec subfloors, so taking time here saves headaches.

Tools and materials

• Measuring/marking: 6–10 ft straightedge, laser level, tape measure, pencil, painter’s tape
• Prep/cleanup: floor scraper, HEPA shop vac, broom, degreaser
• Wood subfloors: drill/driver, deck screws, belt sander or planer, floor patch (fiber-reinforced for wood)
• Concrete subfloors: angle grinder with diamond cup wheel and shroud, concrete patch/SLU, crack filler
• Leveling: primer (manufacturer-matched), self-leveling underlayment (SLU) or trowel-grade patch, mixing paddle, 5-gal buckets, gauge rake, smoother, spiked roller, spiked shoes, foam weatherstrip for dams
• Safety: eye/ear protection, N95 or P100 respirator, gloves, knee pads
• Optional: moisture testing (RH test kits or calcium chloride), moisture vapor barrier epoxy (if required)

Step-by-step preparation

1) Assess and mark highs/lows (30–60 min per room)

• Sweep/vacuum thoroughly.
• Use a long straightedge or laser level to map the floor. Mark high spots (H) and low spots (L). Note depths like “L 3/16 in.”
• Identify causes: loose subfloor, seam lips, slab humps, prior patch failures.

2) Fix structure first

• Wood: Add screws to every joist line (about every 6–8 in. along seams) to stop squeaks and movement. Replace any water-damaged or delaminated sections. Plane or sand proud seams.
• Concrete: Chip out loose material. Address cracks—fill non-structural cracks with a compatible filler; if you see differential movement or wide structural cracks, pause and get an assessment.

3) Clean and decontaminate

• Remove staples, nails, old adhesive ridges, paint overspray, and contaminants (oil/grease will ruin bond). Use a floor scraper and degreaser as needed. Vacuum with a HEPA filter.

4) Manage moisture (concrete)

• Check the product’s moisture limits. Many LVPs are fine up to 85% RH (ASTM F2170) or 5 lbs MVER (ASTM F1869). If your slab is high, plan for a two-part epoxy moisture vapor barrier or fix moisture issues before proceeding.

5) Correct high spots

• Concrete: Grind with an angle grinder and diamond cup wheel connected to a dust shroud/vac. Feather transitions.
• Wood: Plane or sand high seams and ridges. Do not over-sand through top veneer on plywood.

6) Prime and fill low areas

• Choose material:
  • Small dips/feathering: polymer-modified patch (fast-setting options cure in 30–90 min).
  • Larger areas: self-leveling underlayment (SLU) rated for your substrate (wood vs concrete). Some SLUs require metal lath over wood—follow the bag’s instructions.
• Prime per manufacturer; skipping primer is a top reason for failures.
• For SLU:
  • Create dams at doorways and vents with foam weatherstrip.
  • Mix with a high-torque drill and paddle to the exact water ratio. Working time is short (10–20 min).
  • Pour, gauge-rake to depth, smooth, and de-air with a spiked roller. Wear spiked shoes to walk the pour.
• Let cure as directed (walkable often 2–4 hours; ready to cover 4–24+ hours depending on product and thickness).

7) Re-check flatness and finish

• Re-measure with your straightedge. Spot-fill remaining lows with patch and feather edges.
• Final vacuum. Ensure the surface is clean, solid, and dust-free before installing LVP/LVT.

Wood vs. concrete: special considerations

• Wood subfloors:
  • If the surface is wavy or patched extensively, consider installing 1/4 in. underlayment-grade plywood, fastened per spec, then patch/feather. Avoid lauan—many manufacturers don’t approve it.
  • Deflection matters: floors should meet at least L/360 for vinyl. Excess bounce will cause seam and locking issues.
• Concrete slabs:
  • Remove curing compounds, sealers, and cutback adhesive. If you suspect old black mastic or vinyl may contain asbestos, stop and test before disturbing.

Safety

• Grinding and sanding produce silica and wood dust—use a shrouded tool, HEPA vac, and a properly rated respirator (N95 minimum; P100 preferred for heavy grinding).
• Eye and ear protection are a must; knee pads help during troweling.
• Follow chemical safety data sheets for primers, SLUs, and epoxies.

Tips for best results

• Match systems: primer + patch/SLU from the same brand improves compatibility and warranty support.
• Control environment: most LVPs want 65–85°F and normal humidity before/during/after install. Acclimate the flooring only if your brand requires it.
• Don’t add extra foam underlayment unless the LVP allows it; many have attached pads and adding more can cause joint stress.
• Work in manageable areas. Mix with cold water to extend working time.
• Use a 10 ft straightedge for final verification—it mirrors most manufacturer specs.

Common mistakes

• Skipping primer before patch/SLU.
• Trying to “float out” big dips with the LVP itself—won’t work and may void the warranty.
• Mixing SLU too thick or too wet. Follow the bag precisely.
• Not damming doorways—leveler can flow into adjacent rooms and under walls.
• Installing over active moisture problems or contaminated surfaces.

Cost and time

• Patch compound: $20–$35 per bag; SLU: $30–$45 per bag (about 45–55 sq ft at 1/8 in.). Primer: $20–$40.
• Tool rental: 10 in. concrete grinder $60–$100/day; HEPA vac $40–$80/day.
• A typical room takes 1–2 half-days to assess, prep, and pour, plus cure time.

When to call a pro

• Severe slab heaving or wide/active structural cracks.
• Moisture readings above your product’s limits, or persistent slab dampness.
• Suspected asbestos in old vinyl or black cutback adhesive.
• Significant floor deflection or sag that may need joist/sistering work.

Prepare it right, and your LVP/LVT will install smoothly, lock tightly, and wear evenly for years.