How do I tile onto a chipboard or floorboard floor?

Why timber and tile are a difficult marriage, the three things that have to be true before you start, and the build-ups that actually work.

Difficulty: advanced
Time: 4 hr
Read: 5 min
Safety: caution

Short answer

Three things must be true: the floor must be stiff enough that it barely deflects, the boards must be securely fixed and not moving, and there must be a layer between timber and tile that copes with the timber's movement — an overlay board, a decoupling membrane, or both. Then use a flexible adhesive and grout, and leave movement joints at the perimeter.

Timber moves with moisture and flexes under load; tile does neither and breaks when asked to. Every successful tiled timber floor is a way of managing that mismatch, and every failed one skipped a stage of it. Be honest early — some timber floors are simply not stiff enough, and that has to be dealt with structurally rather than with a better bag of adhesive.

Safety

Screwing an overlay down means screwing into a floor that may contain pipes and cables — lift a board and look, or use a detector and short screws, before putting hundreds of fixings in. If there is underfloor heating in the floor, do not screw into it at all. Cutting cement backer board makes silica dust: score and snap, or cut outside with extraction and an FFP3 mask.

What you'll need

overlay — plywood, cement backer board or a decoupling membrane
flexible (S1 or better) tile adhesive
flexible grout
screws suited to the overlay
flexible sealant for the perimeter
a long straight edge

Step by step

Assess deflection honestly first.Bounce on the middle of the floor. Any noticeable movement is too much for tile. Stiffening — noggins between joists, sistering joists, or a thicker structural deck — comes before anything else and cannot be done from above with adhesive.
Fix the existing deck down properly.Screw down every loose board or chipboard sheet, into the joists, everywhere. A single board that flexes independently will crack the tile above it.
Deal with any damp or rot before covering it.Once tiled you cannot see the timber again. Check for damp, especially near external walls, bathrooms and any historic leak.
Decide on the build-up.Common options: WBP plywood screwed down at close centres; cement backer board bedded and screwed; or a decoupling membrane bonded to a sound deck. Each has its own thickness, fixing pattern and adhesive requirement — the manufacturer's specification governs.
Allow for the extra height before you commit.Overlay plus adhesive plus tile is a real build-up. Check doors, thresholds, the toilet outlet and any step. It is much cheaper to find out now.
Screw the overlay down at close centres, staggered from the deck joints.Overlay joints must not coincide with the joints in the deck below, and the screw pattern is dense — the maker states the centres. Screw heads must sit below the surface.
Prime if the overlay calls for it.Plywood in particular is absorbent and its manufacturer or the adhesive maker will specify a primer.
Use a flexible adhesive rated for timber substrates.Look for a deformable classification, usually S1 or S2. Standard rigid adhesive on timber is one of the most reliable ways to crack a floor.
Get full adhesive coverage under every tile.Voids are worse on timber than anywhere else because the floor moves. Comb, press and slide, and back-butter.
Leave a movement gap at the perimeter and use flexible grout.Gap all round, filled with flexible sealant and covered by the skirting. Flexible grout in the joints. Rigid grout on a timber floor is a crack waiting to be blamed on something else.

Tips

A decoupling membrane is thinner than a plywood overlay and is often the answer where height is tight, but it does not add stiffness — it only handles in-plane movement.
If the floor is chipboard and it has ever been wet, it will have swollen at the joints. Tiling over swollen chipboard is a poor idea; replace the affected sheets.
Smaller tiles cope better with a timber floor than large format ones, because each tile spans less of the movement.
In a bathroom, deal with the waterproofing at the same time — a timber floor that gets water into it fails twice: rot and cracked tiles.

Common mistakes

Tiling straight onto floorboards — Individual boards move independently, expand and contract across their width, and cup. Tiles bridge several boards, so every board movement is transferred into a rigid sheet of ceramic. Cracked grout appears within months and cracked tiles follow.
Using a thin plywood overlay screwed at wide centres — A thin sheet at sparse fixings flexes between the screws, which is exactly the movement you are trying to eliminate. The point of the overlay is stiffness and continuity, and both depend on thickness and fixing density.
Using standard adhesive and grout to save money — The whole build-up is designed to accommodate movement, and rigid adhesive and grout are the two components that cannot. It is the cheapest part of the job to get right.

If it doesn't work

Grout cracking along one line months after laying

Cause: A joint in the deck or overlay below, moving — Fix: Rake the grout out along that line and form a flexible joint there instead. If tiles have cracked too, the substrate needs opening up.

Tiles hollow and lifting in one area of the floor

Cause: Deflection over a weak span, or an overlay joint that has moved — Fix: Lift the tiles and inspect. Usually the floor needs stiffening from below before re-tiling.

Floor creaks under the new tiles

Cause: Boards not screwed down before overlaying — Fix: The creak is movement, so it will crack something eventually. It cannot be fixed from above once tiled — which is why the fixing-down stage matters.

Questions people ask

Plywood or backer board?

Backer board is dimensionally stable and unaffected by water, which makes it the better choice in a bathroom; plywood is stiffer for the same thickness and often cheaper, which makes it attractive where water is not the issue. Both need to be fixed to the manufacturer's pattern, and neither adds much structural stiffness to the joists themselves.

Can I tile onto a suspended timber floor at all?

Yes, it is done routinely and successfully, but the floor has to be stiff, and older houses often have joists sized for the loads and spans of their day rather than for tile. If in doubt about the structure, that is a question for someone who can look at the joists.

How much extra height will it add?

It depends on the overlay and the tile, and it is worth measuring the actual materials rather than estimating — the combination of overlay, adhesive bed and tile adds up faster than people expect, and it is the doors and the threshold to the next room that suffer.