Introduction

If you’ve ever laid a beautiful floor and had it telegraph seams, cup, or crack grout, you already know: the subfloor is the job. This guide walks you through a practical, field-tested process to get wood subfloors ready for LVP and tile so you avoid callbacks. We’ll cover moisture checks, flatness tolerances, structural deflection, underlayment builds, and the small details—transitions and clearances—that make your finish look pro and stay that way.

Table of Contents

• Key Takeaways
• Assess Moisture The Right Way
• Check Flatness And Deal With Highs/Lows
• Verify Structure And Deflection
• Build The Underlayment System
• Plan Transitions, Clearances, And Details
• Frequently Asked Questions
• Conclusion

Key Takeaways

• Subfloor readiness prevents most flooring callbacks; in general, substrate moisture, flatness, and structure drive a large share of failures.
• Typical targets: wood moisture under roughly 12% and within about 3% of the new floor; flatness commonly 3/16 inch in 10 feet for LVP and 1/4 inch in 10 feet for small-format tile.
• Deflection matters: L/360 for ceramic/porcelain, L/720 for natural stone—miss this and grout cracks follow.
• Fast, repeatable checks (pin meter, straightedge, feeler gauges) cut guesswork and reduce rework.
• Document conditions with photos and notes; clear assumptions in your proposal reduce back-and-forth and protect margin.

Assess Moisture The Right Way

Many contractors find moisture is the silent culprit behind gapping, peaking, and loose bonds. Wood moves with seasons, and finish floors follow.

Targets and Tools

• In general, aim for wood subfloor moisture at or below about 12% and within roughly 2–3% of the finished flooring product (check manufacturer data).
• Ambient conditions: commonly 60–80°F with 30–60% RH for 48 hours before and after install.
• Tools: pin-type moisture meter for wood; thermo-hygrometer for ambient temp/RH.

Steps

1. Record ambient temp/RH in at least two locations (center of room and near exterior wall).
2. Take wood moisture readings every 100–150 sq ft and across suspect areas (near sliders, kitchens, baths).
3. If readings are high, run conditioned air or dehumidification until stable for 48 hours; retest.

Example

On a second-floor LVP job, the hallway read 8–9% MC, but the bath threshold hit 13–14% due to a past leak. We delayed install, ran a small dehumidifier and exhaust, and re-tested at 10–11% in two days. Result: zero peaking over the threshold months later.

Check Flatness And Deal With Highs/Lows

A common mistake is confusing “level” with “flat.” Floors can slope a bit and still meet flatness tolerances. Finish floors care about flat.

Typical Flatness Tolerances

• LVP/Glue-Down Vinyl: commonly 3/16 inch in 10 feet or 1/8 inch in 6 feet.
• Ceramic/Porcelain Tile (under 15 inch edge): commonly 1/4 inch in 10 feet; for large-format, many specs tighten to 1/8 inch in 10 feet.

Use a 10-foot straightedge or a laser with story pole and feeler gauges to map highs and lows.

Fixing the Plane

• High spots: plane or sand (collect dust; avoid cutting through face plies).
• Low spots: patch with polymer-modified patch for wood, following depth limits and prime as required.
• Seams/lips: screw off framing and sand flush; avoid feathering with soft compounds right at panel edges.

Example

Kitchen LVP, sunroom tie-in. We mapped 1/4 inch dips across old register cutouts. Two lifts of patch (each under manufacturer max thickness), cured per label, re-checked with a 10-foot straightedge. Final variance: under 1/8 inch in 6 feet—glue went down smooth, no telegraphing.

Verify Structure And Deflection

Flat and dry won’t save you if the floor bounces. Deflection is the killer for tile grout and stone.

Structural Benchmarks

• Ceramic/Porcelain: L/360 minimum total load deflection.
• Natural Stone: L/720 minimum; plus stiffer subfloor recommendations.
• Typical subfloor minimums often include 5/8 inch TC plywood for resilient (many pros prefer 3/4 inch) and thicker or layered systems for tile.

Quick Checks

• Joist data: species, grade, size, span. Many online span calculators help verify L/360 quickly.
• Squeaks: add screws through subfloor into joists 8–12 inches on center before underlayment.
• Bridging/blocking: add where bays feel lively.

Example

Small bath over 2x8s at 16 inches on center spanning 12 feet—too bouncy for tile. We added a mid-span beam under 8 feet of span, reducing deflection to satisfy L/360, then installed 1/2 inch cement board over 3/4 inch plywood. Grout held, no hairline cracks after a heating season.

Build The Underlayment System

The right build depends on finish material and install method. Wrong combo equals telegraphing seams or cracked grout.

Common Assemblies

Fastening and Gapping (Wood Underlayment)

• Many plywood underlayment specs call for 1/8 inch gaps at sheets and 1/4 inch at walls to allow movement.
• Fastener schedule commonly 4 inches on edges and 6 inches in the field; set heads flush (not broken through face ply).

Movement and Joints

• Tile: honor movement joints at perimeters and transitions; soft joints as per tile layout.
• Resilient: keep required expansion space (often 1/4 inch) at all fixed verticals.

Example

Condo hallway, glue-down LVP over old T&G plank subfloor. We overlaid 1/4 inch plywood underlayment (gapped and fastened 4/6), skimmed with a high-build patch, sanded, and vacuumed. Flatness verified at under 3/16 inch in 10 feet. Adhesive ridges transferred cleanly, zero telegraphing at seams.

Plan Transitions, Clearances, And Details

Little misses cause big callbacks—doors that rub, dishwasher that won’t slide, or abrupt transitions.

Clearances and Heights

• Appliances: verify finished floor height vs. appliance feet and counter clearances; many installers target at least 1/4 inch glide clearance.
• Doors: check swing and undercut; common practice is leaving about 1/2 inch above finished floor for bath fans to breathe.
• Thresholds: plan for ADA-friendly transitions where possible; keep trip edges low (in general, 1/4 inch vertical or less is more comfortable).

Transitions and Edges

• Use reducers/T-molds that match height change; pre-drill fasteners into wood underlayment, not just finish.
• Wet areas: seal at tub, shower, and exterior doors; maintain soft joints where needed.

Example

Kitchen to dining tile-to-LVP. We laser-checked both rooms, calculated a 3/16 inch delta, and specified a low-profile reducer. The dishwasher cavity had only 1/8 inch spare, so we set tile height with a tighter thinset bed and left a service note in the handover. Everything cleared.

Frequently Asked Questions

Do I Need To Acclimate LVP And The Subfloor?

In general, yes—most manufacturers want LVP and the jobsite at service conditions (commonly 60–80°F, 30–60% RH) for about 48 hours before and after install. Wood subfloors also stabilize under those conditions, which reduces movement after install. Always follow the specific product instructions.

What Moisture Number Is “Safe” For Wood Subfloors?

Commonly, wood subfloors are considered ready when they’re near or below roughly 12% moisture content and within about 2–3% of the finish flooring. Some products are more forgiving, others stricter—defer to the manufacturer. Log readings in multiple locations and fix wet zones before installing.

How Flat Is Flat Enough?

For LVP, many specs call for 3/16 inch in 10 feet (or 1/8 inch in 6 feet). Tile typically allows 1/4 inch in 10 feet for small-format and tightens to 1/8 inch in 10 feet for large-format. Use a long straightedge and feeler gauges; map and fix highs/lows before you lay.

What About Deflection—How Do I Know If I’m Good For Tile?

Ceramic/porcelain needs L/360 minimum total load deflection; natural stone often needs L/720. Check joist size, species, spacing, and span against a span calculator. If you’re close, add framing (sistering, mid-span beam) or switch to a more forgiving finish.

Can I Go Over Existing Vinyl Or Plank?

Often yes, but only if the manufacturer allows it and the substrate meets flatness, bond, and deflection criteria. For glue-down LVP, ensure the existing layer is sound, well-bonded, and compatible with the adhesive. For tile, going over resilient isn’t typical—remove or build a proper underlayment.

Conclusion

Prep is production. A few disciplined checks—moisture, flatness, structure, and the right underlayment—save hours later and prevent the callbacks that eat profit. Capture what you see while you’re on site so nothing gets missed in the handoff. With Donizo, you can speak your notes, snap photos of moisture readings and flatness maps, and turn that into a professional proposal in minutes. Send it for e‑signature, and when the client accepts, convert it to an invoice in one click. Less typing, fewer surprises, better floors.