What screw spacing does the Code want for an Ottawa drywall ceiling?

Ontario's Building Code sets the ceiling screw spacing at not more than 300 mm on centre (clause 9.29.5.9). That applies to a single layer of gypsum board on wood framing or furring, fixed with screws. These are minimums for new work, and you should ask the City of Ottawa's Building Code Services how they apply to your specific renovation.

A few details from the same clause matter for ceilings. Where the supports are more than 400 mm on centre, screws on walls go no more than 300 mm on centre; where supports are 400 mm on centre or less, wall screws can go up to 400 mm on centre. The ceiling rule is a flat 300 mm regardless of support spacing. Every screw must sit at least 10 mm from the board edge, and the head must not puncture the paper face.

If nails are used instead of screws, clause 9.29.5.8 sets a different ceiling pattern: not more than 180 mm on centre, or in pairs about 50 mm apart placed every 300 mm. Ask your contractor which fastener they plan to use, because the spacing requirement follows from that choice.

The Code's Table 9.29.5.3 also sets the maximum spacing of supports, meaning the studs, joists or furring the board is fastened to. That table varies by board thickness, whether the surface is painted or textured, and whether it is a ceiling or a wall. The table is worth raising with your contractor, because the board thickness they choose has to suit the framing already in place.

One other Code point for ceilings: clause 9.29.5.10 requires that heat be provided to keep the space at or above 10 °C for 48 hours before taping and finishing begins, and maintained for at least 48 hours after. If your project runs into cold weather, ask the contractor how the space will be heated.

Ask both quotes to name the board thickness and type, the fastener and its spacing, the finish level by number, and whether primer and paint are included.