Why does the grout in my floor keep cracking?

Regrouting a floor that keeps cracking is one of the most repeated wasted jobs in the house. The grout is not failing — it is reporting a movement problem that nobody has addressed.

What you'll need

• A grout rake or a multi-tool
• Flexible grout
• Silicone or a proprietary movement joint
• A straight edge
• A vacuum

Step by step

1. Map the cracks.Draw the floor and mark every crack. Perimeter cracks, cracks in a straight line across the room and scattered cracks in the field all mean different things.
2. Check the perimeter first.Where the tiling meets a wall, a doorway, a hearth or a kitchen unit, there should be a flexible joint, not grout. Rigid grout there cracks every time.
3. Look for a line across the floor.A crack running in a straight line usually follows a joint in the substrate — a screed bay, a change of construction, or the edge of a heated zone.
4. Test the floor for deflection.Bounce on the middle of the floor with a straight edge across it. Visible movement means a timber deck that is too flexible for tiling.
5. Check for underfloor heating.Heated floors expand and contract on every cycle. They need flexible adhesive, flexible grout and movement joints at the perimeter and in bays.
6. Rake out the failed grout.A grout rake or a multi-tool with a grout blade, down to the full depth of the joint. Grouting over cracked grout hides it for a fortnight.
7. Convert perimeter joints to flexible.Rake the perimeter grout out entirely and replace it with a matching colour silicone. This alone cures the majority of recurring perimeter cracking.
8. Use a flexible grout in the field.Modern cement grouts with a polymer additive tolerate far more movement than a plain cement grout. Over timber or heating, it is not optional.
9. Add movement joints in large floors.Tiled floors over about 40 square metres, or any dimension over about 8 metres, need intermediate movement joints, and they must go through the tile, adhesive and grout.
10. Stiffen a bouncy timber floor before regrouting.Noggins between joists, an additional layer of ply screwed at close centres, or a decoupling membrane. Otherwise the cracks return.
11. Consider a decoupling matting.A membrane between substrate and tile that absorbs small movements. It is the standard answer for tiling over timber or a cracked screed.
12. Clean and seal.Once the new grout is cured, seal it if it is a cement grout in a wet or heavily used area.

Tips

• Colour-matched silicone is made for exactly this. A perimeter joint in matching silicone is invisible and never cracks.
• If the floor was tiled directly onto floorboards, that is the cause. Boards move individually and no grout survives it.
• Cracks that appear in the first heating season after a floor is tiled are almost always thermal, not structural.

Common mistakes

• Regrouting without finding the movement — Grout is the weakest, most rigid part of the assembly, so it always cracks first. Replacing it without addressing the movement means the same crack in the same place within months, and the tiles crack next.
• Grouting the perimeter joint solid — The perimeter is exactly where a floor needs to expand. Filling it with rigid grout removes the only relief the floor has, so the compression goes into the field and can lift or crack tiles.

If it doesn't work

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