Before the walls and ceilings are sheeted. Blocking behind every fixing, and the plasterboard finish level.
Once the sheet is on, adding it means cutting the wall. Where livable housing rules apply, grab-rail walls need 12 mm ply or 25 mm noggings. source
Ask: Has every fixing point got blocking, photographed before sheeting?
Level 4 is standard. Gloss paint, dark colours and raking light need Level 5, which is set when the board is stopped, not painted. source
Ask: Are any walls going gloss, dark, or in raking light?
Wet areas take a water-resistant board under the membrane and tile.
Ask: Is wet-area board specified for every wet wall?
Prompts, not rules. Most choices can wait; these can't, because a later trade builds to them. Code items cite their source; states adopt parts of the NCC differently, so check yours. Your builder, trades, certifier and each product's install sheet decide. Download the list (JSON, CC BY 4.0).
