Surface Preparation, Substrate Assessment, and Backframing
Before board goes up, the substrate decides whether the finished wall will be flat, quiet, fire-rated and stable. This domain tests inspection of framing, backframing, barriers and existing surfaces.
Framing and substrate checks
- Studs at 400 mm (16 in.) or 600 mm (24 in.) on centre, straight within roughly 3 mm in 2.4 m (1/8 in. in 8 ft). Check with a straightedge and string line before boarding, not after.
- Service rough-in complete and inspected; boxes set to the finished face; no pipes proud of the stud line.
- Air barrier and vapour retarder continuous, sealed at plates, laps and penetrations; insulation correct and not compressed into a gap.
- Concrete must be cured, clean and dry by an appropriate test method, and free of form release, laitance and dust. Existing coatings must be sound and tested for adhesion.
Backframing
Backing is needed wherever something will be attached or a joint needs support: cabinets, grab bars, handrails, TV mounts, closet rods, door hardware, ceiling fans. Backing must be structural framing or approved blocking, not scrap drywall and not a screw into the back of the board.
Firestop and sequencing
Penetrations through a fire separation need a listed firestop system. Do not close a wall over a hole that leaves the assembly non-compliant.
Worked example
A bathroom renovation calls for a grab bar in a tub surround. The wall is 92 mm steel studs at 400 mm with one layer of 15.9 mm board.
Reasoning: a grab bar can be loaded by a falling adult, and barrier-free design commonly assumes a concentrated load on the order of 1.3 kN, so a drywall anchor is unacceptable. Install solid blocking or a listed backing plate spanning at least two studs, at the exact height on the elevation, and confirm with the drawing and client before the wall is closed. Coordinate with the waterproofing trade, because fastener penetrations in a wet area must be sealed.
Traps
Assuming the framer checked straightness; not recording blocking positions on the drawing; sealing the vapour retarder with the wrong tape; boarding over an unsealed penetration; setting board tight to a wet slab; forgetting backing for a future TV or cabinet.