Everyone knows what I'm talking about when the grout between bathroom tiles starts to turn black and smell. It's unsightly, leaves an odour and — more seriously — causes water ingress that can lead to real structural damage. In over 15 years as a plumber and tiler, mouldy grout and failing silicone is the single leading cause of serious leaks and water damage to the flats below.
When grout in a shower or around a bath gets mouldy and cracked, water gets behind the tiles into the plasterboard — which soaks it up like a sponge. This can go unnoticed for years until the downstairs ceiling comes in. I've seen it so many times. Most people don't realise how much damage is quietly building up, or how easy it is to fix before it reaches that point.
A lot of people waste a significant amount of money retiling when regrouting would solve the problem completely — for a fraction of the cost. Regrouting takes a few hours, leaves the bathroom 100% watertight, and makes it look and smell brand new again. This guide covers exactly how to do it yourself, and also explains why getting a professional might actually save you money in the long run.
Tools you'll need
All available from your local DIY store or Topps Tiles:
How to regrout — step by step
Before you start, put on your dust mask, gloves and goggles. There's a lot of fine dust involved in cutting out grout.
Cut out the old grout
Attach the grout removal blade to your Bosch or Dremel multi-cutter and slowly work along all the affected grout lines, cutting until you can see the original colour again. Depending on the severity, you may want to cut all the way back to the plasterboard — but in most cases cutting to about 50% depth and removing the black discolouration is sufficient.
Clean down thoroughly
Wipe away all the dust from cutting using a damp sponge, making sure to get right into the grout lines. Any dust or debris left in the joints will stop the new grout bonding properly — don't rush this step.
Mix the grout
Mix your grout following the manufacturer's guidelines until it forms a firm, smooth paste. Using your mixing knife, scoop some onto the grout float and work it in using diagonal sweeps — pushing the grout into the lines and going over each one a few times to make sure it's filled to the full depth. Not filling the full depth is a common mistake that leads to cracking and water getting back in.
Wipe down and smooth
Leave for 10–15 minutes (check the manufacturer's guidelines — timing varies by product), then wipe down with the rough side of the grout sponge. You can also run a finger down the lines to smooth them off. Don't leave it too long before wiping — grout that's been left to go too hard will leave a permanent haze over the face of the tiles that's very difficult to remove.
Leave to dry and seal
Leave for 24 hours, then finish with a translucent spray grout sealant — available from Topps Tiles or any good tile store. Just spray over the grout lines and leave to dry. This forms an invisible barrier that keeps the grout watertight for years and stops the mould coming back.
Renewing the silicone at the same time
If the silicone around the bath or shower tray is also looking black and mouldy, it makes sense to do it all in one go while you're at it.
Remove all the old silicone
Use a Stanley knife to cut along the top and bottom of the existing bead, then a single blade to scrape out the rest. New silicone won't stick to old, so every bit has to come out.
Apply masking tape
Mask off above and below where the new bead will go — this is the trick that ensures a clean, straight line even if your application isn't perfect. Cut the tip off the silicone tube, attach the nozzle, gauge the bead size needed and cut the nozzle at a 45-degree angle.
Apply and smooth
Hold the gun at 45 degrees and move slowly along the line, squeezing the trigger steadily. Click the gun off whenever you need to stop. Remove the masking tape while the silicone is still wet, then wet your finger thoroughly and run it smoothly along the bead — stopping to re-wet your finger if any silicone starts sticking. Leave overnight before using the bath or shower.
When to call a professional instead
Regrouting is a DIY job — but there are good reasons why getting a professional to do it can actually work out better value:
Not sure whether your bathroom needs regrouting or retiling? Send John a WhatsApp photo to 07877 486 939 and he'll give you a straight answer — free, no obligation. See the regrouting service → JHDS Plumbing & Tiling accepts no responsibility for damage caused following our guides. If in doubt, contact a professional before starting.
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