Floor prep guide

How to Prepare a Wood Subfloor for Flooring

A wood panel substrate is prepared in a particular order, because a floor that moves cannot be fixed by making its surface flat.

The short answer

Clean the exposed panels, confirm the substrate is sound and not moving, deal with loose panels and protruding fasteners, look closely at panel edges and seams, assess any damaged areas, then check flatness and correct what remains with products approved for wood substrates.

Movement first, surface second. Filling or leveling a floor that flexes produces a flat surface that is still attached to a moving substrate.

Movement comes before flatness

Squeaks are a symptom rather than a diagnosis. They can come from fasteners, panel movement, framing or the connection between them, and the right correction depends on which. Significant movement, deflection, damaged framing or anything suggesting a structural cause needs someone qualified to evaluate it on site — that is not something FloorGauge can determine remotely or prescribe a repair for.

What to evaluate on a wood substrate

Plywood, OSB and other wood panel substrates
Condition	What you are looking for
Cleanliness	Debris, dust, old adhesive or residue that affects bond or hides the surface.
Panel condition	Sound, intact panels rather than swollen, soft or deteriorated areas.
Loose panels	Panels that move relative to the framing beneath them.
Movement and deflection	Flex underfoot, which is a substrate question rather than a surface one.
Squeaks	A symptom worth tracing to its cause before covering the floor over it.
Fasteners	Protruding nails or screws, and areas where fastening appears inadequate.
Seams and panel edges	Raised, swollen or uneven edges where two panels meet.
High edges	Localized high points that distort the rest of your flatness read.
Low areas	Dips that may be patched, depending on the product and the substrate.
Flatness	Measured against the flooring system's tolerance, on the exposed substrate.
Moisture	Evaluated with a method appropriate to wood and to the flooring system.
Compatibility	Whether this substrate, as built, is acceptable to the flooring you plan to install.

Fasteners

Set or address protruding nails and screws — anything above the surface becomes a high point under the floor.
Identify loose panels and where they are loose, rather than adding fasteners everywhere.
Add or rework fastening only where it is appropriate for the substrate and its construction.
Know what is below before driving anything: plumbing, electrical, radiant tubing and other hidden systems exist under floors.
Follow the substrate and flooring requirements for how the deck is expected to be fastened.

FloorGauge does not publish a fastener spacing. Panel type, thickness, framing, span and the flooring system all affect it, and the requirement comes from the relevant construction and product documentation, not from a general rule.

Panel edges and seams

A raised panel edge at a seam creates a localized high spot. Judged from the seam, the surrounding floor can look lower than it really is.

A raised panel edge can create a high spot even when the rest of the floor is relatively flat — and it distorts the measurement around it, making the surrounding floor look lower than it really is. Evaluate the seam itself before deciding that the area around it needs filling.

What a correction should be depends on:

The condition of the panels at that seam, including whether the edge is swollen or damaged.
How the substrate is constructed and supported at that point.
What flooring is going over it, and how forgiving that system is.
What the flooring and preparation manufacturers permit on a wood substrate.

FloorGauge does not publish a sanding depth or a universal panel repair method; those vary with the substrate and the products involved.

Damaged panels

Signs that warrant closer inspection include swelling, delamination, softness, deterioration, evidence of water damage, excessive movement and broken or damaged panel areas.

Significant damage may need repair or replacement before finished flooring goes down. FloorGauge does not declare a floor unsafe or specify a structural repair from a distance — where damage appears extensive or its cause is unclear, an on-site evaluation is the right next step. If water was involved, the source matters as much as the panel: see moisture checks before flooring.

Flatness on a wood substrate

The same distinction applies here as anywhere else — an older house can be noticeably out of level while its substrate is flat enough for the flooring, and a perfectly level room can still fail a flatness check. Read flatness vs. level for how to check and what the result means. The tolerance itself comes from the flooring manufacturer.

Patching and leveling over wood

Some patching and leveling products are approved for wood substrates — but compatibility is not assumed. A product formulated with concrete in mind is not automatically suitable over plywood or OSB.

What varies between products on a wood substrate
Factor	Why it is product-specific
Approved substrates	Each product names the substrates it may be used over, and wood panels are called out separately.
Primer	Wood substrates often have their own priming requirement, distinct from concrete.
Reinforcement	Some systems require lath, mesh or another reinforcement over wood; others do not permit it at all.
Thickness	Minimum and maximum thickness over wood can differ from the same product over concrete.
Installation conditions	Substrate condition, fastening and moisture requirements are stated by the product.

The two correction routes in detail: floor patch and low spots for localized areas, and self-leveling compound for broader ones. The overall sequence lives in how to prepare a subfloor.