I've been dealing with leaks in and around customers' bathrooms for years. The majority of them can easily be avoided, and if the worst does happen they're usually quite easy to fix — once you know what you're looking for.
I see it all the time: water leaking from bathrooms into the flat below, or causing damage to the downstairs living room. It's not cheap to fix, and even if you can claim on insurance there's always a hefty excess to pay.
In this guide I'll run through the three most common reasons for bathroom leaks and how to deal with each one. Before you start checking anything, take off the bath panel or look under your shower with a torch. Run the shower over the rims of the bath where it meets the wall, spray around the enclosure, then use a torch and your hands to feel for drips and wet patches. This will give you a good idea of where the leak is coming from.
Leak 1: Mouldy or failing silicone sealant
This is by far the most common reason for bathroom leaks. Silicone sealant is applied to every bath and shower on installation — it's the only thing forming a barrier between your appliance and the floor below. Over time it goes black, develops gaps, and stops doing its job.
Inspect the sealant
Get down to eye level and look closely for gaps, tears or sealant that's pulled away from the surface. Black colour and a damp smell are the giveaways. If your bathroom is leaking, this is the first place to check.
Remove all the old silicone
New silicone won't bond to old — you have to remove every bit of it first. Run a Stanley knife along the top of the sealant strip, then the bottom, and pull it away. Use a single blade to scrape off the remaining residue. It takes patience but it has to be done properly.
Apply the new silicone
Fill your bath before applying — the weight of water pulls the bath down slightly, and applying silicone with the bath full means it won't crack in use. Head to your local DIY store and pick up some anti-mould white silicone and a silicone gun. Cut the nozzle tip off with a Stanley blade, work out the bead size you need and cut the nozzle at 45 degrees at that point. Keep the nozzle almost flat to the surface, squeeze the trigger and slowly pull it away from you along the area to be sealed.
The masking tape trick
When I first started out I wasn't great at creating a neat bead, so I'd apply masking tape above and below the area to be sealed — leaving a gap for the silicone — then apply the bead. When the tape is removed you're left with a clean, straight line even if the application wasn't perfect. Worth doing if you're not confident. Smooth with a wet finger and leave 24 hours before using the bath or shower.
Leak 2: Mouldy or cracked grout
The second most common cause — and one I find myself repairing more and more. Check for hairline cracks, gaps or pin holes in the grout. If you've ruled out the silicone and suspect the grout, here's how to fix it.
Cut out the damaged grout
You can't apply new grout over old — it won't bond. I use a Bosch 180 E multi-tool with a grout blade. A Dremel also works well. I'd strongly advise against a grinder — you'll end up losing half the tiles along with the grout. Run the cutter along the grout lines to around 50% depth until you see the whiteness of the old grout underneath. Once the blackness is gone you can re-grout without cutting all the way through.
Mix and apply the new grout
Pick up anti-mould grout in the colour that matches your bathroom, a grout float and a mixing tub from your local DIY store. Wipe the area down with a damp cloth to remove dust. Mix the grout powder with small amounts of water until it forms a thick paste — it should stick to a trowel when turned upside down. Add the grout to the float and push it into the gaps using a diagonal motion. Sweep up the excess as you go, then smooth the lines by licking your finger and running it down each join.
Clean off and seal
Leave for 5–10 minutes then use a damp sponge at a diagonal angle to wipe down the grout and clean up the lines. Leave overnight to dry fully, then finish with a spray grout sealer — a can from Topps Tiles is around £16. Just spray over the grout lines and leave to dry. It forms an invisible extra barrier and is something a lot of DIYers skip, but it makes a real difference to how long the grout lasts.
Leak 3: Bath or shower waste
This is something I come across all the time. A leak from under the bath or shower where the waste joins the appliance. When it's first installed, silicone is applied both under the chrome section and below where the washer and hex nut join to the bath. Over time, movement and wear causes the silicone to fail — and that's when the leaks start.
Dismantle the waste fitting
Find the screw in the middle of the plug and unscrew it all the way out. This separates the chrome section on top from the part that screws into the trap. Unscrew the trap from the bottom and undo the hex nut that seals it to the bath (for a shower, just unscrew the lower part of the trap). Remove both the top and bottom sections along with the rubber washers — usually one on top and one underneath. Clean off all the old silicone using a Stanley blade or silicone remover.
Reassemble with fresh silicone
Apply a small bead of silicone around the plug hole on the ceramic or cast steel surface of the bath or shower. Drop the chrome section through from the top with the washer underneath it. From below, apply a thin bead around the washer that forms the seal under the bath (not needed for a shower trap). Screw the bottom of the chrome section into the black plastic section using the large screw — tighten just beyond hand tight. If silicone squeezes out at the sides, smooth it round the connection with a wet finger. Attach the bath trap to the black plastic section and tighten up.
Test before closing up
Run both taps and fill the bath with the plug in place. Check underneath for any leaks before you put the bath panel back. Better to find out now than after everything's been put back together.
All three of these leaks can be prevented by keeping an eye on the condition of your grout and sealant. Blackness is the obvious sign — if you catch it early, the fix is straightforward and cheap. Leave it, and it becomes much more expensive. Good luck with the job. If you're ever unsure or simply don't have the time, I'm happy to help — just give me a call on 07877 486 939. JHDS Plumbing & Tiling accepts no responsibility for damage caused following our guides. If in doubt, contact a professional before starting.
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Not confident doing it yourself? John is happy to help — no job too small in Stirling or Dunblane.