Layout and Subfloor Preparation
Layout and subfloor preparation decide whether the finished floor looks straight and stays flat. The exam tests squaring methods, cut balancing, flatness tolerances, panel spacing and concrete preparation.
Establishing square and layout
- Use a 3-4-5 triangle or its multiples to check corners, and compare room diagonals; a room can be out of square even when opposite walls measure equal.
- Snap centrelines from the two most reliable walls, then shift them to balance cuts. Avoid slivers: perimeter cuts should be at least half a tile where possible.
- Carpet: place seams away from main traffic and out of direct sunlight; keep pile direction consistent. Sheet goods: run the length of the room and minimise seams.
- Set the layout from a focal point such as an entry or fireplace, not from a crooked wall.
Wood subfloors
- Use APA-rated plywood or OSB, sound and fastened; add 1/4 in. underlayment plywood for resilient where specified, leaving 1/32-1/16 in. gaps between panels and no tight fit to walls.
- Fasten flush: ring-shank nails or screws about 6 in. on edges and 8 in. in the field for underlayment.
- Flatness: 3/16 in. in 10 ft is the common resilient tolerance; tile generally wants 1/4 in. in 10 ft, or 1/8 in. for large-format. Tile deflection is L/360 minimum.
Concrete
- Remove curing compound, paint, adhesive residue and laitance by grinding or shot blasting, then vacuum with HEPA.
- Fill cracks and bugholes with suitable patching material; prime before self-levelling underlayment.
- Verify moisture and pH before any adhesive goes down.
Remediation
- Self-levelling underlayment over a primed substrate; patching compound feathered 6-10 in. beyond the repair.
- Never use gypsum patcher under a moisture-sensitive adhesive or in wet areas.
Worked example
Checking a corner: measure 4 ft along one wall and 3 ft along the other; the diagonal should be 5 ft. You measure 5 ft 1 in., so the corner is not square. Rather than starting from that wall, strike a centreline, verify it with a second 3-4-5 check, and lay out from the corrected line.
Traps
- Laying tile off an unsquare wall and ending with a 1 in. sliver at the far side.
- Butting underlayment panels tight, creating ridges at joints.
- Screwing through a hydronic heating tube, or failing to mark tube locations.
- Skipping primer under self-levelling compound, which then debonds.