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Ceiling Drywall: Board Thickness by Joist Spacing, Across the Framing, the Lift, Truss Uplift, and Why Ceilings Go Up First
Drywall, Taping & Painting
Ceiling Drywall: The Board, the Direction, the Lift, the Crack in the Corner Every Winter, and Why the Ceiling Goes Up Before the Walls
The ceiling is the surface in the house that shows everything, because every window and every pot light rakes across it, and it is the surface that is hardest to do well, because it is over your head and 77 lb a sheet. Three things decide whether a ceiling looks right in ten years: the board (regular 1/2 in. on 24-in. trusses sags), the fastening to the trusses near the walls (done wrong, you get a crack along the corner every January), and the finish level (Level 5 under pot lights or you will see every joint). Here is each one.
The board
Regular 1/2 in. drywall is rated by the Gypsum Association for ceiling framing at 16 in. on centre. At 24 in., which is where most Ontario roof trusses are, it sags between the trusses, slowly, helped along by blown insulation on top and the humidity of a new house drying out. You have seen it: a wave in the ceiling every two feet, obvious in the evening light. The cure is 5/8 board, or the 1/2 in. ceiling board the manufacturers make for exactly this (CGC Sheetrock Sag-Resistant, CertainTeed Easi-Lite Ceiling, and the like), which is stiffer than regular 1/2 and lighter than 5/8. It costs about $0.10 a square foot more than regular and the boarders prefer it to 5/8 because of the weight. Specify it in writing; the yard will send regular board if nobody says otherwise. 1/2 vs 5/8.
The hanging
- Ceilings first. The wall board then runs up under the ceiling edge and holds it, and the corner joint is supported from both sides.
- Sheets perpendicular to the framing. Across the trusses, not along them. Each sheet then spans every truss it crosses and the joints land on framing. Along the framing, the long edges fall between trusses with nothing behind them.
- Longest sheets you can get in. 12-ft board across a 12-ft room means no butt joints on the ceiling, which is where butt joints show most.
- Screws at 12 in. in the field, 8 in. at the edges, and a panel lift. Two men can hold a sheet up and screw it; a $60 rental lift holds it tight to the framing while they do, and the difference is a ceiling with no gaps behind it.
- Adhesive on the trusses where the specification or the boarder’s habit calls for it, except where truss uplift matters: see below.
Truss uplift and the crack in the corner
In winter the bottom chord of a roof truss is buried in warm, humid insulation and the top chords are in the cold, dry attic. The bottom chord expands and the top chords shrink, and the whole truss bows upward in the middle by as much as half an inch. If the ceiling board is screwed to the trusses right at the interior walls, the board lifts with the truss and a crack opens along the top of the wall every January and closes every July. The homeowner fills it; it comes back.
The fix is in the hanging: no screws in the ceiling board within about 16 in. of an interior partition. The wall board, or a drywall clip on the top plate, holds the ceiling edge down, and the ceiling floats over the truss movement. On the houses I build the boarder knows this; on a lot of production houses he does not, and the crack is on the Tarion list every year. If you already have the crack, the honest fix is a flexible compound or, better, a small crown moulding that hides a joint that will always move. Cracks and pops.
The finish
Level 4 on a ceiling is fine with flat paint and ordinary lighting. With pot lights, a window wall, a dark ceiling or anything above flat, go to Level 5: a skim coat over the whole surface so the compound and the paper take paint the same way. It is $1.00 a square foot on the ceiling only, and it is the single upgrade I push hardest on a custom house, because the ceiling is the one surface you cannot hang a picture over. Level 4 vs 5.
Ceiling costs, Ontario, October 2026, per square foot of ceiling: ceiling board supplied and hung $2.05; taped to Level 4 $1.50; Level 5 add $1.00; cathedral or over 9 ft 6 in. add $0.35 to $0.75; flat white paint, primer and two coats, $1.65. A 400 sq ft great-room ceiling at Level 5, painted, is about $2,550 before HST.
Questions people ask
What thickness of drywall for a ceiling?
5/8 in., or 1/2 in. sag-resistant ceiling board, on framing at 24 in. on centre. Regular 1/2 in. only at 16 in. with nothing heavy above it. Most Ontario trusses are at 24 in.
Should ceiling drywall be hung perpendicular to the joists?
Yes. Across the framing so the sheet spans every truss and the long edges land on framing. Along the framing is the commonest reason for a wavy ceiling after the sag rule.
Why does my ceiling crack at the wall every winter?
Truss uplift: the trusses bow up in cold weather and lift the ceiling board that is screwed to them near the walls. The fix is floating the ceiling edge (no screws within 16 in. of the partition) at hanging, or a crown moulding after the fact.
Do I need Level 5 on ceilings?
If there are pot lights, a big window, a dark colour or any sheen, yes. The ceiling is lit from the side by everything and a Level 4 ceiling under pot lights shows every joint as a band.
Price the ceilings in the house
The drywall installation calculator separates ceilings from walls, prices cathedral and tall ones, and the complete interior finishing calculator puts Level 5 ceilings in the mid grade by default.
Sources and further reading
- Gypsum Association construction standards: GA-216 application and finishing (the hanger’s bible: fastener spacing, ceiling framing limits, joint treatment) and GA-600 fire resistance design manual, with the free downloads.
- CGC Inc.: the maker of most of the board and compound in Ontario; the product sheets give thicknesses, span limits for ceilings and the right compound for each coat.
- Tarion warranty coverage: what a new-home warranty covers for drywall cracks, nail pops and paint in year one and year two, and what it does not.
