Why does the grout keep cracking in the corners of my shower?

Two walls meeting at a corner expand, contract and settle in different directions. So does the joint where tiling meets a bath or tray. Grout has almost no elasticity, so it is the wrong material for any of them — and this is the single most common tiling mistake in domestic bathrooms.

What you'll need

• A grout rake or an oscillating tool with a grout blade
• Sanitary silicone matched to the grout colour
• Masking tape and a smoothing tool
• Eye protection

Step by step

1. Identify every joint that should be flexible.Wall to wall internal corners, wall to ceiling, wall to bath or tray, and around any change of plane or material. All of those are movement joints.
2. Confirm the cracking follows those lines.Cracks in the middle of a tiled field mean something different — usually substrate movement or a failed board — and need investigating rather than resealing.
3. Rake the grout out of the corner.Use a grout rake or an oscillating multi-tool with a grout blade, working carefully so as not to chip the tile edges. Wear eye protection.
4. Get right back to the tile edges.Leaving a sliver of grout in the corner gives the new sealant a poor surface to bond to and keeps the rigid bridge in place.
5. Vacuum and dry thoroughly.Grout dust is the enemy of adhesion. Vacuum, wipe with a solvent, and let it dry completely before sealing.
6. Choose a sealant that matches the grout.Most grout manufacturers sell a matching sanitary silicone. Colour-matched corners are the difference between a job that looks intentional and one that looks repaired.
7. Mask both sides and apply one continuous bead.Corners are highly visible. Two lines of tape at the intended width, one pass, tool concave, then pull the tape immediately.
8. Do the tray or bath joint at the same time.It is the same principle and the same product, and doing them together gives a consistent line all the way round.
9. Let it cure fully before use.24 hours minimum, longer in a cold bathroom.

Tips

• Colour-matched silicone from the grout manufacturer looks far better than white in a coloured grout scheme.
• If cracks appear in the middle of a tiled area rather than at corners, the problem is the substrate, not the grout.
• Rake, do not chisel. Chisels chip glazed edges and every chip shows against a fresh sealant line.

Common mistakes

• Regrouting the corner instead of sealing it — Grout has no elasticity. Whatever caused the movement is still there, so the new grout cracks along the same line, usually within a heating season.
• Applying silicone over the cracked grout — The silicone is only as sound as what it is stuck to. Cracked grout continues to move and break up underneath, taking the bead with it.

If it doesn't work

Questions people ask