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T&G Plywood Subfloor: Thickness, Spacing, and a Deck That Stays Quiet
A t&g plywood subfloor sits between the joists and the finish, and the choices you make there decide whether the floor squeaks, dips, or stays flat for years. The tongue and groove edges lock the sheets into one deck, and the thickness, the joist spacing, and the gluing each play a part. This guide covers how to choose and lay a T&G plywood subfloor that carries the finish above it without movement.
The Subfloor and What It Carries
The subfloor spans across the joists and carries every load the room puts on the floor, from furniture to foot traffic. A T&G deck spreads that load across the seams so no single sheet flexes alone, and the interlock keeps the surface level under the finish.
Why T&G Beats a Butt Joint Here
A subfloor takes load across a wide area and over a long life, so the seams have to stay in line as the panels move. A butt joint leans on the fasteners to hold two edges together, while a tongue and groove joint holds the edges in line on its own. The interlock then keeps the seam from stepping under a finish that expects a flat surface.
The tight seam also stops dust and drafts from passing between floors and rooms, which matters above a heated space. The edges serve the structure and the space below it.
A butt joint that opens under load leaves a gap a finish cannot bridge, while the interlock keeps the seam tight through the seasons. The profile therefore earns its cost on any deck that carries a finish and takes regular traffic.
Flatness Before Finish
A finish follows the deck beneath it, so any dip or hump in the subfloor shows through a thin covering. A T&G deck that sits flat on the joists gives the finish a level base, while a deck with a step at every seam telegraphs through. Check the flatness as you lay, because the fix is easier before the finish than after.
Store the sheets flat and dry before the lay, because a panel that takes a bow in storage keeps the bow in the floor. A dry, flat stack then delivers a deck that sits true. A sheet stored on edge takes a permanent curve that no amount of fixing will pull out.
Check the joists for level before the first sheet goes down, because a deck laid over a high joist rocks and a deck laid over a low one dips. A straightedge across the joists shows the spot to plane or pack, and the fix there is quicker than a floor that fights the framing.
Choosing Thickness and Spacing
The thickness and the joist spacing work together to set how rigid the deck feels, and the span rating ties the two. A thicker panel or a tighter spacing both reduce flex, and the rating tells you the widest spacing the panel allows.
| Joist spacing | Minimum thickness | Deck feel |
|---|---|---|
| 400 mm | 15-16 mm | Firm |
| 500 mm | 18 mm | Firm |
| 600 mm | 18-19 mm | Standard |
| 800 mm | 22-25 mm | Needs thick panel |
Reading the Span Rating
The span rating stamped on the panel states how far it bridges between supports, so read it against the joist spacing before you order. A panel that spans beyond its rating flexes underfoot and cracks a brittle finish above it. The rating then decides the thickness rather than the habit.
Where the rating and the thickness disagree, trust the rating, because it reflects the panel build rather than the raw measurement. A well-built thin panel can outspan a plain thicker one, so the stamp on the face tells you more than the caliper alone.
A tighter joist spacing than the rating needs wastes timber, because the panel could have crossed more. Use the widest spacing the rating allows, and the framing then carries the deck without extra cost.
Thickness for the Load
A thicker subfloor carries more load and stiffens the floor, which suits a heavy finish such as tile or a thick timber. A thinner panel suits a lightweight finish over a tight spacing. Match the panel to the finish and the traffic rather than to a single rule.
A tile finish needs the stiffest deck, because tile cracks over a floor that flexes even a little. Build the deck for the finish, and the finish then stays intact. A lightweight covering tolerates a thinner deck, so the finish above the panel decides the thickness as much as the load does.
Run the numbers before the order, because a small step up in thickness costs far less than a floor that fails under a heavy finish. The thicker deck then carries the finish and the traffic without a complaint.
Laying the T&G Subfloor
Lay the panels so the tongue faces the open side, fasten along every joist, and glue both the joists and the seams. Leave a small gap at the walls for movement, and stagger the end joints so no seam lines up with the next course.
Fastening and Gluing
Fasten at every joist the panel crosses and space the fixings evenly so the load spreads, then glue the seams and the joists with a construction adhesive. The glue turns the sheets into one stiff deck and quiets the floor, while the fasteners hold it down. The two methods together outperform either one alone.
Drive the fasteners flush rather than proud, because a proud head stands above the deck and telegraphs through a thin finish. A flush head lets the finish sit flat and the deck stay quiet.
Movement Gaps and Stagger
Leave a gap of a few millimetres at the perimeter and at fixed obstructions, because the deck expands as the air takes on moisture. A deck that meets a wall with no gap lifts at the center when it swells. The gap then protects the flatness of the whole floor.
Stagger the end joints so the seams fall in a running pattern rather than a straight line, because aligned seams form a hinge that lets the floor flex. The stagger spreads the joints and stiffens the deck across the room. A layout sketch before the lay shows where each seam falls and keeps the pattern running.
Layout and Protection
Plan the deck before the first cut, because the panel works best when each sheet lands so the seams fall over a joist and the offcuts stay useful. A short layout sketch shows where the sheets meet and how many the deck needs, and the plan then sets the order rather than the order setting the plan.
Keep the deck dry through the build, because a subfloor that takes rain before the finish goes down swells and cups at the seams. A temporary cover costs little and protects the flatness the finish depends on.
Check the joists for level before the first sheet, because a deck laid over a high joist rocks and one over a low joint dips. A straightedge across the frame shows the spot to plane or pack, and a level frame lets the T&G deck sit dead flat on the supports. A frame that runs true also holds the seams in line, so the deck stays quiet underfoot for the life of the floor.
A deck that goes down straight and dry gives the finish a level base, and a level base keeps the whole floor quiet and flat for years. The few minutes spent on layout and cover pay back across the whole life of the room, long after the finish is down and the deck is out of sight.
Frequently Asked Questions
Short answers to the questions that decide a T&G subfloor order.
What thickness T&G plywood for a subfloor?
Use 18 to 19 mm for most subfloors on standard joist spacing and step up to 22 to 25 mm where the spans run wide or the finish is heavy. Match the panel to the span rating and the joist spacing so the deck stays rigid.
Should I glue my T&G subfloor?
Glue the joists and the seams as well as fastening, because the adhesive turns the sheets into one stiff deck and stops the seams from squeaking. The glue adds little cost and much stiffness underfoot.
Do you nail or screw a plywood subfloor?
Either works where you set the fixings correctly, but screws grip harder and resist a pull-through better than nails. Space the fixings evenly along every joist and drive them flush so the finish above sits flat.
For subfloor panels with the thickness, span rating, and edge profile a deck needs, HYY supplies Commercial Plywood, Film Faced Plywood, and MDF Board, so the deck arrives ready to lay and finish.