Tiles Cracking Or Working Loose On A Timber Floor

What's happening?

This is a background problem, not a tile problem. A timber floor does three things tiles cannot tolerate: it deflects when you walk on it, it moves seasonally as the timber takes up and gives off moisture, and it is made of separate boards that move independently of each other. Tiles and cement grout are rigid and brittle, so a floor that flexes a millimetre or two under foot will crack the grout first and then the tiles, and drum the adhesive off the board. That is why the trade answer is never to tile straight onto floorboards. The deck is stiffened and made continuous first (overboarded with a tile backer or suitable plywood, or isolated with an uncoupling membrane), and the Tile Association publishes separate guidance on tiling to timber for exactly this reason.

What usually causes it?

• Tiles fixed directly onto floorboards, so every board moves separately under a rigid finish
• A floor that deflects underfoot because the joists are undersized, overspanned or notched for pipes
• Chipboard or plywood too thin, or not screwed down at close enough centres, so the deck flexes between fixings
• Timber that was damp when it was covered and then shrank as the house dried out
• No uncoupling membrane or backer board, leaving nothing between the timber movement and the tiles
• A standard cement adhesive used where a flexible, deformable adhesive was needed for a timber background
• Tiles butted tight to the walls and door frames, with no perimeter joint to absorb the seasonal movement

Is it dangerous?

The tiling itself is a nuisance rather than a hazard, but a springy floor under a heavy tiled finish is worth understanding rather than covering up. Tiles, adhesive and a backer board add real weight to a floor that is already flexing, and where the bounce comes from undersized or notched joists that is a structural matter under Approved Document A rather than a finish one. Open grout joints in a kitchen or utility also let water into the timber deck, and persistently damp timber is where wet rot starts. Lifting an old tiled floor off timber is heavy, dusty work, and cutting tiles releases the silica dust HSE deals with under COSHH.

Can I fix it myself?

Test the floor before anything else, because it decides the whole job. Walk it slowly and watch a full glass of water on the floor, or kneel and rock your weight while a hand rests on the tiles: movement you can feel is movement the tiles cannot take. Look in the loft or from below where you can, and check whether joists have been notched for pipes or cables. Screwing down a loose chipboard deck and fixing squeaks is sensible DIY. What is not worth attempting is tiling onto floorboards and hoping, or filling repeatedly cracking grout: the deflection has to be dealt with before a rigid finish goes on it.

When should you call a tiler?

Call a tiler before laying tiles on any timber floor, so the deck, the overboarding and the membrane are decided together rather than discovered afterwards. Call where grout keeps opening along the same lines, where tiles are drummy across a timber floor, or where tiles have cracked more than once. If the floor bounces noticeably, get the joists looked at first, because no amount of adhesive corrects a floor that moves.

What will the tiler actually do?

• Check the deflection and the deck before quoting, including joist spans, notching and how the boards are fixed
• Screw the existing deck down tight and refix or replace boards that are loose, springy or damp
• Overboard with a tile backer board or suitable plywood, fixed at the centres the board maker specifies
• Install an uncoupling membrane where seasonal timber movement has to be isolated from the tiles
• Fix with a flexible, deformable adhesive suited to a timber background rather than a standard cement adhesive
• Leave a perimeter movement joint at every wall, threshold and upstand, filled with sealant rather than grout

Who fixes this

• Floor tiling

Questions people ask

Sources & References

1. The Tile Association: Guidelines for tile fixing
2. The Tile Association: Technical
3. GOV.UK: Approved Document A (Structure)
4. NHBC: Standards