If you're reading this because your tiles have cracked, broken loose, or started rocking — subfloor preparation is almost certainly where things went wrong.
I've been called to fix failed tiling jobs and the story is almost always the same: good tiles, good adhesive, terrible prep. This guide fixes that. Whatever your subfloor situation, I'll walk you through the correct preparation process.
Why Subfloor Prep Matters
A tile cannot fix a bad subfloor. No matter how good the adhesive is or how carefully you lay the tiles, if the substrate beneath is moving, soft, uneven, or contaminated — the tiles will fail.
The three things that kill tiles:
- Flex — the subfloor moves and the tile (which doesn't flex) cracks
- Poor adhesion — adhesive doesn't bond to a contaminated or untreated surface
- Unevenness — hollow spots cause tiles to rock and eventually crack under point loading
All three are subfloor problems. All three are fixable before you start tiling.
Step 1: Assess Your Subfloor
Before you buy any adhesive, grout, or tiles, get down and look at your subfloor properly.
Check for Flex
Walk every part of the floor. Any board that bounces, any area that gives slightly underfoot — mark it. For a concrete floor, jump on it. You should feel nothing. For a timber floor, some movement is expected; the question is how much.
Acceptable flex for tiling:
- Concrete: zero deflection
- Timber with backer board: maximum 3mm deflection over a 2m span
Check for Level
Lay a 1.2m spirit level across the floor in multiple directions. Note where the gaps are and how big they are.
For standard tiles (up to 400mm): Maximum 5mm deviation over 2m For large format tiles (600mm+): Maximum 3mm deviation over 2m
Check for Contamination
Look for paint, oil, wax, grease, plaster, or any surface that might prevent adhesive bonding. Run your hand across the surface — if it feels dusty, chalky, or slippery, it needs treatment.
Check for Moisture
In older properties especially, check for rising damp before tiling. A damp meter will give you a reading — tile adhesive over damp concrete will fail. Existing damp needs treating before you tile.
Concrete Subfloor
Concrete is the best substrate for tiling when it's properly prepared. Here's the process:
Clean and Degrease
Remove all paint, adhesive residue, dust, and contamination. Use a floor grinder or angle grinder for stubborn paint. Sweep thoroughly. Vacuum the dust.
For oil or grease contamination: apply a degreaser (white spirit or specialist tile cleaner), scrub, and rinse. Allow to dry completely before the next step.
Fill Cracks and Holes
Use a flexible filler for fine cracks (hairline to 3mm). For larger cracks or movement joints, use a specialist crack filler and consider installing a decoupling mat over the area to prevent movement transferring to the tiles.
Expansion joints in the concrete: Never tile over these. They exist because the concrete needs to move. Bridge them with a decoupling mat or fill with silicone and tile up to the edge.
Level the Surface
For high spots: grind down with an angle grinder.
For low spots and general unevenness: use a floor levelling compound (also called self-levelling screed). Products like Mapei Ultraplan, BAL Level It, and Ardex K15 are poured as a liquid and find their own level.
Key rule: When using a levelling compound, apply a primer first. Without primer, the compound will be too absorbent and dry too quickly near the surface, causing it to crack or powder. Use a water-diluted PVA or a proprietary tile primer.
Allow levelling compound to fully cure before tiling — check the manufacturer's data sheet (typically 3–24 hours depending on depth and product).
Prime
All concrete subfloors should be primed before tiling, even if they look clean and level. Primer achieves two things:
- Reduces the porosity of the concrete so it doesn't absorb moisture from the adhesive too quickly
- Improves adhesive bond strength
Products: Bal Prime AP, Mapei Primer G, Ardex P51. Dilute 1:3 with water, apply with a brush or roller, allow to go tacky (10–20 minutes). The floor should feel slightly tacky but not wet before you start tiling.
Timber Subfloor
Timber is the most common subfloor in UK homes and the most challenging to tile. The problem is movement — timber is hygroscopic (absorbs and releases moisture) and responds to temperature changes. This natural movement is what kills tiles on timber substrates.
There are two correct approaches:
Option A: Decoupling Mat (Recommended)
A decoupling mat (DITRA, DITRA Heat, or similar products) is a polyethylene mat bonded to the timber floor with adhesive. The mat's geometry creates a break between the tile and the timber, absorbing the timber's movement before it reaches the tile.
Process:
- Clean and degrease the timber
- Ensure all boards are firmly fixed — knock in any squeaky nails, add screws where needed
- Fill large gaps between boards with filler or thin ply
- Apply S1 flexible adhesive to the timber with a 6mm notched trowel
- Press the mat into the adhesive, butt-joining sections tightly
- Allow to cure, then tile over the mat using S1 adhesive
Decoupling mats are particularly good on older suspended timber floors that have some historic movement. They're also the right choice under underfloor heating mat systems.
Option B: Tile Backer Board
Cement-based backer board (Jackoboard, Hardiebacker, Marmox board) screwed directly to the timber provides a more rigid, tile-friendly surface than bare timber.
Process:
- Check the timber for flex — add noggins or additional boards to stiffen where needed
- Fix 6mm or 12mm backer board using screws at 200mm centres and ring-shank nails at board edges
- Fill screwhead recesses with tile adhesive
- Tape all joints with self-adhesive fibreglass tape and apply a thin skim of adhesive over the tape
- Allow to cure, then tile with S1 adhesive
When to use backer board vs decoupling mat: On a stiff, well-fixed timber floor with minimal movement, backer board is straightforward. On older, springier floors, a decoupling mat is more forgiving.
What NOT to Do
- Never tile directly onto chipboard. Chipboard expands significantly when wet. Bathroom tiles laid directly onto chipboard will fail — sometimes within weeks.
- Never tile onto OSB. Similar movement characteristics to chipboard.
- Never tile onto timber without S1 adhesive. Standard C1 adhesive on timber is a guaranteed failure.
Existing Tiles
Sometimes you'll want to tile over existing tiles. This can work, but only in specific conditions:
Check the existing tiles are fully bonded. Go around the entire floor tapping each tile with a coin or knuckle. A solid tap = bonded. A hollow sound = the tile is loose. Any loose tiles must be removed before proceeding.
Check the combined height won't cause issues. Adding 8–12mm (tile + adhesive) to the floor may cause problems at doorways, with skirtings, or with adjacent flooring. Measure and check before committing.
Apply a bonding primer to the existing tile surface before tiling over. BAL Bond SBR or similar products improve adhesion to the glazed tile surface.
Use S1 adhesive — you're now tiling on top of a tiled surface, which has some flexibility characteristics. Rigid adhesive will fail.
Wet Room / Bathroom Subfloor
Any bathroom floor will get wet. A wet room floor will get extremely wet. The preparation process needs to account for moisture.
For standard bathrooms with a bath or shower tray: Standard preparation as above. Ensure the bath/tray is correctly bedded and sealed. Prime with a waterproofing primer at the perimeter.
For wet rooms and walk-in showers: Full tanking is required. Apply a waterproof membrane over the entire floor (and up the walls to at least 300mm — ideally the full wall height in the shower zone). Products like BAL Waterproof Plus, Mapei Mapelastic, or Schlüter DITRA-DRAIN are the professional standards.
Apply the membrane in two coats, paying particular attention to corners and changes of plane. Allow full cure before tiling.
Underfloor Heating
If you're installing underfloor heating (UFH) under tiles, the sequence and preparation are slightly different:
- Prepare the subfloor as above
- Install UFH mat or cable as per manufacturer's instructions
- Apply a thin self-levelling compound over the UFH (follow UFH manufacturer's recommendation — some products don't require this)
- Tile over with S1 or S2 adhesive rated for UFH
- Leave the UFH off for the adhesive and grout cure period (typically 7 days minimum)
- Commission the UFH gradually — don't run it at full temperature immediately
FAQ
Do I need to prime before tiling? Yes, always on concrete. On new plasterboard or backer board, use a diluted primer or PVA. Primer costs almost nothing and prevents adhesive failures that can cost hundreds of pounds.
Can I tile onto painted floors? Only if the paint is completely stable — no flaking, no movement. Test by scraping: if paint comes off in sheets, strip it back. Any paint that could delaminate will take your tiles with it.
My subfloor is uneven — can I just use more adhesive to level it? No. Using thick adhesive beds to compensate for an uneven floor creates weak spots (thin at one side, thick at the other) and risks tile failure. Level the subfloor first with a levelling compound.
How long before I can tile over a levelling compound? Depends on the product and depth. Most compounds are ready for tiling in 3–24 hours. Check the data sheet and err on the side of caution — rushing a levelling compound is the third most common cause of failed tiling (after not priming and not using S1 on timber).
What You'll Need
- Floor primer (Bal Prime AP or Mapei Primer G)
- Levelling compound if needed (Mapei Ultraplan or BAL Level It)
- Decoupling mat or backer board for timber substrates
- S1 flexible adhesive for any substrate with movement potential
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