Surface Preparation and Substrates
Surface Preparation and Substrates
This domain tests whether you can judge a substrate before tile goes down: flatness, deflection, cleanliness, moisture, and bond. Exam questions often give a substrate condition and ask for the correct repair or reject the installation.
Core rules
- Floor flatness: TCNA tolerances are 1/4 in. in 10 ft for tile with all edges shorter than 15 in., and 1/8 in. in 10 ft for tile with any edge 15 in. or longer. Check with a 10-ft straightedge; high spots are ground, low spots are filled with patching compound or self-leveling underlayment-not with thinset.
- Deflection: ceramic tile generally needs floor framing at L/360 or stiffer; stone usually L/720. Verify joist size, spacing, span, and subfloor thickness.
- Concrete must be cured, clean, and free of curing compounds, paint, oil, and loose material. Moisture and pH tests may be required before installing moisture-sensitive finishes.
- Wood subfloors need exterior-glue plywood or OSB of the specified thickness, fastened and gapped per code; cement backer board is embedded in thinset and fastened through the panel, with joints taped and treated.
- Remove bond breakers. Dust, sealers, adhesive residue, and wax prevent mortar from bonding.
Worked example
A 10-ft straightedge shows a 3/8-in. gap under its center on a floor receiving 12x24 tile. Because the tile has an edge longer than 15 in., the tolerance is 1/8 in. in 10 ft; the floor fails. Grind the high area or apply a cementitious patch to bring it within tolerance, then recheck before layout.
Traps
Skim-coating deep low spots with thinset; tiling over paint or adhesive residue; assuming backer-board joints need no treatment; and checking flatness with a short level instead of a 10-ft straightedge.