What it is called, by market
One concept, four markets. Where a name means something else locally, the card says so rather than leaving you to find out on site.
United Kingdom
Same word, different thingsubfloor
also floor base, deck, substrate, underfloor void
Most often the BASE a covering is laid on — a screed, a slab or a boarded deck — so it is defined by what sits on it rather than by where it is in the structure. SUBSTRATE is the less ambiguous word.
Australia
Same word, different thingsubfloor
also underfloor space, bearers and joists, subfloor ventilation
Names the SPACE and the structure beneath the floor — the void, the bearers, the joists, the stumps. Subfloor ventilation is a standard requirement and refers to that space, not to any sheet.
United Statesyours
Same word, different thingsubfloor
also subflooring, deck, underlayment, sheathing
The structural SHEET fixed to the joists — plywood or OSB. UNDERLAYMENT is then a further thin layer over it for a covering, which is a fourth meaning of a word already overloaded.
Canada
Same word, different thingsubfloor
also subflooring, underlay, T&G plywood
As the United States, with tongue-and-groove sheet the standard product and UNDERLAY used where American usage says underlayment.
The governing standard, by market
United Kingdom
BS 8203 / BS 8204-1
Installation of resilient floor coverings including base requirements, and concrete bases and screeds to receive flooring.
United States
IRC R503 / APA Engineered Wood Construction Guide
Floor sheathing span ratings and fastening, and panel selection for subfloor applications.
Calculators for this
Each works in either measurement system, and the terminology on the page follows whichever market you have selected.
Frequently asked questions
- If a drawing says subfloor, which layer is it?
- Ask what is above and below it in the same sentence, because the word alone cannot settle it. On an AMERICAN drawing, subfloor is the structural sheet screwed and glued to the joists, and anything between it and the covering is underlayment. On an AUSTRALIAN one, subfloor is the space under the floor, so subfloor ventilation and subfloor inspection are about a void, not a sheet. On a BRITISH one it is usually the surface a covering will bond to, which on a ground floor is a slab or a screed and upstairs is a boarded deck — so the same word means a concrete thing downstairs and a timber one upstairs in the same building. The safe move when writing across markets is to say what it is: the deck, the screed, the slab, or the void.
- Why is the subfloor deck glued as well as screwed?
- Because a nailed or screwed deck squeaks and a glued one does not. A fastener holds the sheet down at a point, and between points the sheet can lift a fraction as the joist shrinks or the floor is walked on — that tiny movement against the fastener shank is the classic floor squeak. A continuous bead of construction adhesive along the joist before the sheet goes down removes the gap entirely, and it also makes the deck act compositely with the joist, which stiffens the floor measurably beyond what either contributes alone. It is why modern practice specifies glued-and-screwed, and why a squeaky deck cannot usually be cured by adding more screws in the same places.
- What does the base actually have to do for a floor covering?
- Be flat enough, dry enough, and sound enough, and those are three separate specifications a covering manufacturer will state. FLATNESS is measured as a deviation over a stated length, and a covering that bridges a hollow will crack or telegraph the defect; that is what levelling compound exists for. DRYNESS is measured, not judged — a slab still releasing moisture will lift an adhesive and blow a timber floor months later. SOUNDNESS means the surface itself is not a bond breaker: a dusting slab, a laitance skin or old adhesive residue will let go with the covering attached to it. Almost every flooring failure traced back honestly turns out to be one of those three rather than the covering.