Surface Preparation and Substrate Assessment
This domain tests whether you can decide if a substrate is fit to receive tile. The exam expects you to assess flatness, soundness, cleanliness, moisture, deflection and compatibility, then specify a repair or membrane rather than tiling over a defect.
Core rules
- Substrates must be structurally sound, clean, dry and free of curing compounds, oil, paint, loose material and dust.
- Flatness tolerance for typical thin-set tile work is 3 mm in 3 m (1/8 in. in 10 ft); correct high spots by grinding and low spots with patching compound.
- Floor deflection limits are commonly L/360 for ceramic and L/720 for natural stone; verify joist size, span and subfloor thickness.
- Concrete must be cured; test moisture and pH as required by the adhesive or membrane manufacturer. Do not assume a slab is dry because it looks dry.
- Use waterproofing in wet areas, crack-isolation membranes over suitable cracks and movement joints at transitions and perimeters.
- Prime or prepare surfaces only with products compatible with the selected adhesive.
Worked example
A 3 m straightedge placed on a concrete floor shows a 6 mm gap at mid-span. The specified tolerance is 3 mm in 3 m, so the floor fails. Tiling over it would force the thin-set to vary in thickness, increasing lippage and reducing bond. The fix is to grind down high spots and fill low areas with a polymer-modified patching compound, then recheck with the straightedge before layout. If the same slab also gives a moisture reading above the adhesive manufacturer’s limit, install a moisture-control membrane or select a suitable system; do not rely on a surface sealer.
Traps
Skipping moisture testing; using mastic in wet areas; tiling over cracked or contaminated concrete; confusing “flat” with “level”; forgetting expansion joints; applying patch too thick or too thin; not priming a porous substrate.
Key points
- Check flatness with a 3 m straightedge; 3 mm in 3 m is the common tolerance.
- Verify deflection: L/360 ceramic, L/720 stone.
- Test concrete moisture and pH; follow the adhesive system limits.
- Waterproof wet areas and isolate suitable cracks.
- Patch, grind or prime only with compatible products.