PreConstruct AIScrews are the cheapest line on a drywall takeoff and among the easiest to get wrong, because the count depends on things the square metres don't show: wall or ceiling, how far apart the framing is, whether the wall is fire rated, and whether it's wood or steel. This guide turns the Ontario rules into counts per sheet and per square metre, then into boxes.
The spacing rules
Ontario Building Code 9.29.5.9 sets the maximum spacing for single-layer gypsum board on wood or steel framing (the National Building Code says the same):
| Where | Screws no more than |
|---|---|
| Ceilings | 300 mm apart |
| Walls, studs no more than 406 mm apart | 400 mm apart |
| Walls, studs more than 406 mm apart | 300 mm apart |
| Walls where the board braces the building or gives fire protection | 300 mm apart, on studs and on the top and bottom plates |
| Every screw | at least 10 mm from the edge of the board, heads not through the paper |
These are maximums. Most crews add a few at butt joints and corners, and a fire-rated design can ask for tighter spacing than the code (see below).
What that comes to, per sheet and per m²
Laid out on a 4 × 8 sheet (1219 × 2438 mm, 2.97 m²): one line of screws on every stud or support the sheet crosses, and screws along each line at the maximum spacing, starting 10 mm in from each end. Walls are hung vertically, and ceiling board runs across the supports.
| Case | Spacing | Lines × screws | Per 4 × 8 sheet | Per m² |
|---|---|---|---|---|
| Walls, studs 406 mm (16 in) apart | 400 mm | 4 × 8 | 32 | 10.8 |
| Walls, studs 610 mm (24 in) apart | 300 mm | 3 × 10 | 30 | 10.1 |
| Walls that brace, or give fire protection | 300 mm | 4 × 10 | 40 | 13.5 |
| Ceilings, supports 406 mm apart | 300 mm | 7 × 5 | 35 | 11.8 |
| Ceilings, supports 610 mm apart | 300 mm | 5 × 5 | 25 | 8.4 |
That's before waste. Screws get dropped, stripped and re-driven, so an allowance of around 10 per cent on top is common. The first row is the familiar “32 a sheet”, which is where our drywall per square metre guide gets its 10.8 per m².
Fire-rated walls and ceilings: the design governs
A rated partition or ceiling is built to a tested design, usually named on the partition schedule (a ULC or UL design number, or a code table assembly). That design sets the board, the layers, the screw type and length, and the spacing, often tighter at the edges than the code's general rule. Take the spacing for rated types from the listed design, not from the table above. With two layers the base layer and the face layer each have their own spacing, and the face-layer screws are longer.
Which screw
| Framing | Screw | Why |
|---|---|---|
| Wood studs and joists | Type W, coarse thread | The wide thread bites into wood fibre and pulls the board tight |
| Light steel studs (up to about 0.84 mm, roughly 20 gauge and lighter) | Type S, fine thread, sharp point | Taps cleanly into thin steel. Coarse thread strips it |
| Heavier steel (about 12 to 20 gauge) | Type S-12, drill point | Drills through thicker steel before it threads |
How long
The screw has to pass through the board and far enough into the framing. GA-216 asks for at least 10 mm (⅜ in) through steel and about 16 mm (⅝ in) into wood, and OBC Table 9.29.5.5 sets 15 mm into wood for unrated walls, and more for rated ceilings. So: board thickness plus penetration, rounded up to a length that's sold.
| Board | On steel (board + 10 mm) | On wood (board + 16 mm) |
|---|---|---|
| One layer 12.7 mm (½ in) | 22.7 mm → 1 in (25 mm) | 28.7 mm → 1¼ in (32 mm) |
| One layer 15.9 mm (⅝ in) | 25.9 mm → 1¼ in (32 mm) | 31.9 mm → 1¼ in (32 mm): meets the code's 15 mm, right at GA's ⅝ in |
| Two layers 12.7 mm | 35.4 mm → 1⅝ in (41 mm) | 41.4 mm → 1⅝ in (41 mm) or longer |
| Two layers 15.9 mm | 41.8 mm → 1⅞ in (48 mm) | 47.8 mm → 2 in (51 mm) |
Check the rated design for its own screw length, which can be longer than this arithmetic gives.
A worked order
A small office fit-out on steel studs 406 mm apart: 520 m² of ordinary single-layer wall board, 140 m² on fire-separation walls, and 210 m² of ceiling on supports 406 mm apart.
- Ordinary walls:
520 m² × 10.8 = 5,616 - Fire-separation walls:
140 m² × 13.5 = 1,890, unless the listed design says more - Ceiling:
210 m² × 11.8 = 2,478 - Total:
9,984, with 10 per cent for drops and re-drives:10,982 - In boxes of 1,000: 11 boxes of fine-thread Type S, split by length: 1¼ in for single ⅝ in board, longer for two layers
Most of the error in a screw order comes from lumping walls and ceilings together, and from taking rated walls at the ordinary rate. Keep them apart and the count holds.
Doing it on the drawings
In PreConstruct AI you add screws to a wall type once, as so many per square foot of board hung (32 a sheet is exactly 1 per square foot), with a different rate on your rated types. Every wall you draw with that type then carries its screws next to the board, studs, track, batts, tape and compound, layers counted, rounded up to whole boxes if you buy them by the box, with the arithmetic on each line. You set the rates, and the screws follow the board.