Subfloor Prep & Moisture Check: the full procedure
Bring the subfloor to a clean, flat, dry, sound condition and document the moisture test before any flooring goes down.
- Applies to: Lead Installer, Crew
- Frequency: Per job (before installation)
- Scope: Defines the consistent sequence for assessing and preparing the subfloor and recording moisture results. Acceptable moisture limits, flatness tolerances, primers, and patch products defer to the manufacturer's installation instructions, the product label/SDS, and the applicable industry method (ASTM, NWFA, TCNA/ANSI, CRI).
What you need
- Moisture meter and/or concrete RH/calcium chloride test kit
- 6-10 ft straightedge
- Floor scraper
- Shop vacuum
- Self-leveler/patch and trowel
- Chalk line
The procedure, step by step
- Clear and clean the subfloor — Remove old flooring, fasteners, adhesive ridges, and debris. Vacuum so the surface is clean enough to read flatness and bond per the product spec.
- Confirm structural soundness — Check for movement, squeaks, loose panels, and rot. Re-secure or repair before proceeding; structural problems are corrected, not covered.
- Check flatness with a straightedge — Move the straightedge across the floor and mark high and low spots. Compare against the flatness tolerance in the manufacturer's instructions or applicable standard (for example TCNA flatness criteria for tile).
- Run the moisture test — Perform the moisture test method required by the manufacturer for that subfloor type. For concrete this is commonly ASTM F1869 (calcium chloride) or ASTM F2170 (in-situ RH); for wood, a moisture meter comparison. Follow the product's pass/fail numbers.
- Record the moisture results — Log the reading, method, locations, date, and ambient conditions on the job record. If the result exceeds the manufacturer's limit, STOP and escalate - do not install.
- Flatten and patch as needed — Grind highs and fill lows with the specified patch or self-leveler, priming per the product label/SDS. Let it cure to the manufacturer's time before testing flatness again.
- Install vapor retarder/underlayment if required — Where the product or subfloor calls for a vapor retarder or underlayment, install it to the manufacturer's instructions before flooring.
- Final pre-install sign-off — Confirm clean, flat, dry, sound. Photograph the prepped subfloor and the moisture log, then sign off before laying material.
Quality check before you finish
- Subfloor clean, structurally sound, and free of squeaks
- Flatness verified against the manufacturer/standard tolerance with a straightedge
- Moisture tested by the manufacturer-required method and within the stated limit
- Moisture reading, method, locations, and date recorded on the job
- Patch/self-leveler cured before flooring
- Vapor retarder/underlayment installed where required
- Prepped subfloor and moisture log photographed
This is a free, source-anchored standard operating procedure (SOP) you can print and hand to staff. It documents the work sequence for a Flooring business — not safety or regulatory rulings, which defer to the cited authorities, the applicable code, and your own health-and-safety plan. Open the tool above to print it, toggle ink-saver, or (with a free ToolFluency Business account) edit it to match your own workflow.
Sources
- NWFA (wood subfloor & moisture) (https://nwfa.org)
- TCNA/NTCA (tile substrate flatness) (https://tcnatile.com)
- Manufacturer installation instructions + product SDS
About Free Subfloor Prep & Moisture SOP (Printable)
Free printable flooring SOP for subfloor prep and moisture testing — flatness, cleaning, and documenting readings before install. Defers limits to manufacturer specs.
How to use
- Read the full procedure top to bottom before the work — the SOP runs in order and each step builds on the last.
- Toggle Ink-saver (black & white) for a cheaper mono print for the binder; leave it off for the full-color version.
- Click Print SOP to print or save as PDF. Print one per crew, laminate it for the binder, or attach it to the job in your scheduling system.
- Train new hires on it and have staff sign off. Found something out of date? Use the feedback link — flagged SOPs are re-researched against the source list.
Frequently asked questions
What moisture reading is too high to install flooring?
There is no universal number — the acceptable limit is set by the flooring manufacturer’s installation instructions for the specific product and subfloor. Many resilient and wood products reference ASTM F1869 (calcium chloride, often quoted around 3-5 lbs/1000 sq ft/24 hr) or ASTM F2170 in-situ RH thresholds, but you must use the limit printed in the product instructions. If the reading exceeds it, stop and remediate; installing over excess moisture voids most warranties.
How flat does the subfloor need to be?
Flatness tolerance is defined by the product and standard, not a single figure. Tile commonly references TCNA criteria (for example tighter tolerances for large-format tile), while resilient and wood products state their own. This SOP standardizes that you always check with a straightedge and correct highs and lows to the tolerance in the manufacturer’s instructions before installing.
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