Why tile needs proper underlayment
Tile is rigid — subfloor movement causes cracked grout and loose tile. Backer board stiffens wood subfloors. Uncoupling membranes absorb minor horizontal movement and waterproof in wet areas.
Common products
- Cement backer board (1/4 or 1/2 inch): screw to subfloor, tape seams, thinset under board
- Fiber-cement / foam boards: lighter alternatives — follow manufacturer mortar
- Uncoupling membrane (e.g. DITRA): mortar to subfloor, tile to membrane — great on plywood
- Waterproof membrane: shower floors and wet rooms — liquid or sheet
Concrete slab installs
Direct bond possible with proper primer and thinset if slab is flat and cured. Crack isolation membrane over control joints prevents cracks telegraphing through tile.