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
Match mortar type to the product: some membranes require unmodified thinset while porcelain on backer board wants modified. Mixing the wrong bag is a common DIY failure.
- 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.
Test moisture before you commit — a damp slab can blow thinset bonds months later. If readings are high, fix the moisture source or use a manufacturer-approved moisture mitigation system.
Wet-area requirements
Pair substrate choice with tile prep steps before ordering tile boxes.
- Shower floors: waterproof membrane + slope to drain — not beginner DIY
- Bathroom dry areas: uncoupling membrane optional but popular on plywood
- Kitchen: backer board minimum; membrane at dishwasher peninsula optional
- Always flood-test showers before tile if membrane is liquid-applied
Height build-up and transitions
Backer board plus thinset plus tile can raise the floor enough to need door undercuts and reducers into adjacent laminate. Measure finished height before you order transitions.
Uncoupling membranes are thinner than 1/2″ board — useful when you need to keep door clearances. Plan the stack-up in the doorway, not after the first row is set.