The subfloor is the foundation of every flooring installation — and the most frequently neglected part of a flooring project. A new floor installed over a subfloor that’s unlevel, too wet, structurally inadequate, or incompatible with the product being installed will fail. It will squeak, gap, crack, or lift — and the failure will be blamed on the product when the real cause was the preparation.

Understanding what to ask about subfloor preparation, and insisting on honest answers before installation begins, is the single most protective thing a homeowner can do in a flooring project.

“Every warranty claim I’ve investigated on a failed hardwood floor came down to the subfloor. Not dry enough, not flat enough, or not stiff enough. The product was fine. The installation looked correct. But the subfloor wasn’t prepared properly, and the floor failed because of it. The subfloor conversation is the most important conversation in flooring — and the one most contractors rush past.”

— Danielle Okafor, certified flooring installer and flooring industry consultant with 19 years of residential and commercial flooring experience in the local area

Questions About Subfloor Assessment

Faceless male master laying wooden laminate flooring using mallet in apartment during renovation

Will you assess the subfloor before installation begins?

Ask directly: will the subfloor be inspected before any flooring is installed? A professional installer should assess:

  • Flatness: Is the subfloor flat within the tolerances required for the chosen product?
  • Moisture: Is the subfloor’s moisture content within acceptable limits?
  • Structural integrity: Is the subfloor solid, properly fastened, and free of soft spots?
  • Compatibility: Is the subfloor type appropriate for the installation method being used?

What a good answer looks like: A specific subfloor assessment included before installation, with results communicated to the homeowner.

What flatness tolerance does this product require?

Different flooring products have different subfloor flatness requirements:

  • Hardwood (nail-down): Typically 3/16″ variation over 10 feet, or 1/8″ over 6 feet
  • LVP/LVT: Typically 3/16″ over 10 feet — though thinner planks are less forgiving
  • Ceramic/porcelain tile: Typically 1/8″ over 10 feet (stricter than wood products)
  • Carpet: Most forgiving — minor variations are absorbed by the pad

Ask specifically what flatness tolerance the product requires and how the subfloor will be measured. A contractor who doesn’t know the flatness tolerance for the product they’re installing hasn’t installed it enough.

How will flatness issues be addressed?

When the subfloor isn’t flat enough, there are several approaches:

Self-leveling compound (SLC): Poured over concrete or wood subfloors to fill low spots and create a flat surface. Appropriate for significant variations on concrete.

Sanding high spots: On wood subfloors, high spots can sometimes be sanded down.

Additional subfloor layer: A second layer of plywood over an existing wood subfloor can provide a flatter, stiffer base.

Shimming or sistering joists: For floors with significant structural deflection, more involved structural work may be needed.

Ask what the plan is if the subfloor isn’t flat enough — and what the cost of remediation would be.

Questions About Moisture

Close-up of a wet wooden floor with visible grain and screws in black and white.

Will moisture testing be done on concrete subfloors?

Moisture is the primary enemy of hardwood and engineered hardwood over concrete. Even a concrete slab that feels dry to the touch may have moisture vapor transmission rates that will damage wood flooring. Before any wood product goes over concrete, moisture testing should be performed.

The two main methods:
– Calcium chloride test (ASTM F1869): Measures moisture vapor emission rate. Most hardwood manufacturers specify a maximum emission rate.
– In-situ relative humidity probe (ASTM F2170): Measures relative humidity within the concrete slab. Often the more accurate method.

Ask specifically whether moisture testing will be performed, which method will be used, and what the acceptable threshold is for the product being installed.

What a good answer looks like: Moisture testing confirmed as standard before any wood product installation over concrete, with the acceptable threshold named.

What is the moisture content of the wood subfloor?

For wood subfloors, the moisture content of the subfloor itself should be within a few percentage points of the moisture content of the flooring being installed. Ask whether the contractor will measure both and compare them.

What moisture mitigation will be used if levels are too high?

If moisture levels exceed acceptable thresholds, installation should not proceed until the issue is addressed. Ask what options exist — moisture barrier membranes, additional drying time, dehumidification — and what the plan is if levels don’t come down sufficiently.

Questions About Structural Adequacy

Is the subfloor thick enough for this installation?

Most flooring products require a minimum subfloor thickness:

  • Nail-down hardwood: Typically requires at least 3/4″ plywood or OSB
  • Glue-down hardwood: Can work over thinner substrates in some cases
  • Tile: Typically requires 1-1/8″ total thickness (subfloor + backer) to avoid flex-induced cracking

Ask specifically what subfloor thickness is required for the proposed installation method.

Are there any soft spots, squeaks, or deflection in the existing subfloor?

Walk the existing subfloor (if accessible) and listen for squeaks and feel for soft spots. Ask the contractor to do the same as part of their assessment. Soft spots indicate rotted sheathing; squeaks indicate inadequately fastened panels. Both must be addressed before new flooring is installed.

For tile: is additional backer board required?

Tile is rigid — it cracks if the substrate flexes. On wood subfloors, tile typically requires a layer of cement backer board or an uncoupling membrane (like Schluter DITRA) between the plywood and the tile to prevent flex-induced cracking. Ask whether backer board or uncoupling membrane is included in the scope for tile installations over wood subfloors.

What a good answer looks like: Backer board or uncoupling membrane specified as standard for tile over wood subfloors.

Questions About Existing Floor Removal

If removing existing flooring, what is the condition of the subfloor underneath expected to be?

Old adhesive from previous vinyl, damaged OSB from a water leak, or inadequate subfloor thickness under a removed ceramic tile — these are common discoveries when existing flooring is removed. Ask what the contractor expects to find and what the remediation plan is for common scenarios.

Is removal and disposal of existing flooring included?

Confirm whether the quote includes removal and disposal of the existing floor covering. This can add significant time and cost.

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