[Home](https://www.referralflooring.com/)/[Services](https://www.referralflooring.com/services)/Subfloor prep The unglamorous part # Subfloor repair and levelling: where a floor is actually won or lost. Every floor we lay sits on something, and that something decides how the floor ages. A hollow becomes a squeak, a hump becomes a broken locking edge, a damp slab becomes a cupped board. Subfloor prep is the day nobody enjoys and nobody photographs, which is exactly why it gets skipped on a cheap quote. It is also the part covered by the same 1 year warranty as the floor on top. There are published numbers for how flat is flat enough, and they are below with the source. That is what "prepped properly" means here: not a promise, a measurement. What we do ## Six ways to bring a subfloor to tolerance. ### Screw-down The squeak fix - Every loose panel screwed into the joists - The guidelines say subfloor panels should be glued and nailed to the framing to minimise squeaks[1] - A floor laid over a subfloor that was only nailed will often talk, and the new surface gets the blame ### Planing High spots, joists included - Subfloor planing where a seam or a swollen panel sits proud - Joist planing where the framing itself is the hump - Common in the century homes and the fifties bungalows around here ### Self-levelling cement Low spots - Poured to fill dips and out-of-level areas - Primed first so it bonds - What a floating floor over an old kitchen usually needs ### Concrete grinding, low dust Slabs - High spots and old adhesive ground off with dust extraction - Slab tested for moisture before anything goes over it, see below ### Double-layer or replacement When it cannot be saved - The guidelines are direct: where a subfloor cannot be brought to tolerance, replace it or add a second layer rather than proceed[1] - New subfloor installation is part of the job, not a referral ### Included Not a separate trade - Priced into the same quote as the floor - Covered by the same 1 year warranty - Done by the crew that lays the floor, so nobody is pointing at anybody The measurable part ## How flat, in the guidelines' own numbers. These are the national installation guideline figures, not ours. Bring a straightedge if you want to check the work. ### Flatness, by fixing method Nail-down with fasteners 1½" and longer: flat to within ¼" over 10 feet, or 3/16" over 6 feet. Glue-down, floating and shorter fasteners, which covers most vinyl plank, laminate and engineered work, tightens to 3/16" over 10 feet, or 1/8" over 6 feet.[1] Flatness is to be measured across the span of each room to get the overall topography, not spot checked.[1] ### What a second layer has to look like When a double-layer system is the fix, the guidelines specify that the second layer be laid on a diagonal or offset by a minimum of 4" in each direction so no seams align, with 1/16" to 1/8" gaps between panels, a minimum ¾" expansion space at all vertical obstructions, and fastening at a minimum of 6" O.C. along panel edges and 12" O.C. through the field, using ring- or screw-shanked nails, proprietary screws or equivalent.[1] ### Moisture in a wood subfloor During the humid season a wood subfloor should not exceed 13% moisture content, and in regions that sustain higher levels, extra precautions belong in the product selection and the installation method.[1] The guidelines also point at APA Technical Note C468, *Floor Squeaks: Causes, Solutions and Prevention*, for addressing noise.[1] ### Concrete gets tested, not eyeballed In-slab relative humidity probes to ASTM F2170, calcium chloride to ASTM F1869, or calcium carbide testing, with a minimum of three tests per room and results representative of the whole project.[1] Appropriate moisture testing is described as the only way to identify whether slab moisture is adequate for the system being used.[1] ### Where a permit enters it Levelling, screw-down and planing are preparation. Structural repair is different, and in Hamilton the City's list of projects that require a building permit includes "repairs to structural members (i.e. subfloor, roof sheathing, beams, lintels, etc.)".[2] If what we find under your floor is a rotted joist or a subfloor that needs structural replacement rather than a second layer, we say so, and in Hamilton, Waterdown, Dundas, Ancaster and Stoney Creek that is a call to the Building Division at 905-546-2720 before it is anything else. Sources 1. National Wood Flooring Association, [Wood Flooring Installation Guidelines](https://nwfa.org/wp-content/uploads/2026/02/NWFA-Installation-Guidelines.pdf), revised 2025. Subfloor flatness tolerances and how they are measured, glue-and-nail to minimise squeaks, the APA squeak reference, double-layer subfloor system requirements, wood subfloor moisture content, and concrete moisture testing. 2. City of Hamilton, [Building Permit Application Requirements](https://www.hamilton.ca/build-invest-grow/construction-renovation/residential-building-renovation/building-permit), "Projects that Require a Building Permit". Read 9 September 2026. Sources last read 9 September 2026 Related ## Read next. - [Why a new floor squeaks, cups or gapsAlmost all of it comes back to moisture and what is underneath.](https://www.referralflooring.com/why-new-floors-squeak-cup-and-gap) - [What do you do if the subfloor is not level under vinyl plankCorrect it first. The numbers, and how each kind of problem is fixed.](https://www.referralflooring.com/flooring-order-of-work) - [Removal and disposalThe only time fixing a subfloor is cheap is when the old floor is up.](https://www.referralflooring.com/flooring-removal-and-disposal) - [The five standards on the home pageFlatness, humidity, moisture content and slab testing, with sources.](https://www.referralflooring.com/#standards) Next step ## Let us look at what is actually under your floor. The estimate is free and it is the one part of a quote that cannot be done over the phone. You get a firm number before we leave. [Book a free estimate](https://www.referralflooring.com/#book) [Call 289-689-4242](tel:+12896894242) Phone [289-689-4242](tel:+12896894242) Email [referralflooring@gmail.com](mailto:referralflooring@gmail.com) Warranty 1 year on floor repair and installation, subfloor work included. 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