How To Seal A Bath
Fresh silicone keeps water out of the walls and floor around your bath and the job needs little more than a steady hand.
The short answer
To seal a bath, clean and dry the joint, then fill the bath with water so it settles to its loaded position. Run two lines of masking tape 6 to 12 mm apart, cut the silicone nozzle at 45 degrees and lay one steady bead from the far corner. Smooth it with a wet finger, peel the tape and leave the sealant 24 hours to cure.
Any bath that sits against a wall needs a bead of silicone along the joint, unless it's freestanding or shaped so it doesn't touch one. Without that seal, splashes run straight down the gap and soak into the wall and the floor under the tub, where they can sit unnoticed for months. Left long enough, that water leads to mould, stains on the ceiling below and rotten timber around the bath.
Sealing a bath is a job most people can manage with a handful of basic tools, as long as the prep is done properly. The photos in this guide come from sealing a brand-new bath that still had its label inside. If you're redoing an old seal, strip it back first with our guide to removing old bathroom sealant, because fresh silicone won't bond to the old layer.
Tools And Materials For Sealing A Bath
- Bathroom silicone sealant: A mould-resistant formula made for wet areas
- Caulking gun: Any sturdy gun that feeds the tube without jerking
- Utility knife: For cutting the nozzle cleanly
- Masking tape: Two lengths that run the full edge of the bath
- Paper towels: Silicone is sticky, so keep a roll close by
- Sealant applicator: Optional, since a wet finger does the same job
The silicone is the part that has to cope with steam, water and cleaning sprays for years, so it's worth paying a little more for a good tube. Our pick of the best bathroom sealants covers the ones we'd trust around a bath or shower tray.
How To Seal A Bath Step By Step
1. Clean And Dry The Bath Rim And Wall
Start by cleaning the rim of the bath and the bottom of the wall where the silicone will sit. Even a brand-new bath carries dust and residue from being fitted, and silicone won't grip through it. Wipe off any soap scum, then give both surfaces time to dry completely, because a damp joint is the main reason a fresh bead lifts.
Rushing this stage leads to mould growing behind the silicone and wobbly edges where it hasn't stuck. If you find black spots along the old line, our guide on getting rid of bathroom mould explains how to kill it off before you reseal.
2. Fill The Bath With Water
Fill the bath before you seal it. The weight of the water pulls the tub down slightly and widens the gap at the wall, which is exactly the position it sits in every time someone climbs in. Seal it empty and the silicone gets stretched the first time the bath is used, which is when cracking and peeling begin.
Keep the water in while the silicone cures, then drain it once the sealant has set.
3. Tape Both Sides Of The Joint
Run one line of good masking tape along the bath and a second along the wall, leaving a strip between them for the silicone. A gap of 6 to 12 mm is a sensible guide, and the right width depends on how wide the joint is and how bold you want the finished bead to look.
Take your time with the tape, as it decides how straight the line will be. Keep the gap the same width all the way along and press both edges down firmly so silicone can't creep underneath.
4. Load The Caulking Gun And Cut The Nozzle
Slot the tube into the caulking gun and check the plunger moves freely. Then use a utility knife to cut the nozzle 2 to 3 cm down from the tip at a 45-degree angle. That angled cut lets you hold the gun low along the bath and push the bead into the corner rather than leaving it sitting on top.
5. Run The Bead From The Far Corner
Start in the corner furthest from you and work back towards yourself so you never have to reach over wet silicone. Hold the nozzle close to the joint, as in the photo below, and squeeze the trigger gently and evenly. The sealant can take a few squeezes before it starts to flow, so give it a moment before you move along.
Aim for one continuous pass down each edge. If you do have to stop, pick up exactly where the bead ends rather than going back over silicone that's already down.
6. Smooth The Silicone And Wipe Off The Excess
Straight from the gun, the bead looks lumpy and uneven, which you can see in the photo above. Smoothing presses the silicone into the joint and gives it a neat, slightly hollowed shape that sheds water instead of holding it.
We prefer a wet finger for this, drawn along the joint at an angle in one steady stroke. A sealant applicator does the same job if you'd rather keep your hands clean, but you can manage without one. Wipe the silicone that builds up on your finger onto a paper towel as you go, or it ends up smeared across the bath.
7. Peel The Tape And Let The Silicone Cure
Pull the masking tape off while the silicone is still wet, drawing it away from the bead at a low angle. You're left with two crisp, straight edges and no smears on the tiles or the bath. Then leave everything alone for at least 24 hours so the sealant can cure fully before anyone has a bath or shower.
We filmed the Short below while sealing for this guide, and it shows the bead going on and being smoothed.
Sealing Bath, Shower & Other Fittings Is Much Easier With A Quality Sealant & We've Tested Multiple!
Sealing A Bath With A Large Gap
A wide gap between the bath and the wall is too much for silicone on its own. These gaps come from a bath that was fitted badly or a wall that isn't straight, and a thick bead pushed into one sags and splits before long.
Refitting the bath tighter to the wall is the proper fix, although it's a bigger job that involves the waste, the taps and the panel. Where that isn't practical, fill the gap first with a bonding material and adhesive or fit a flexible caulking strip along the joint, and seal over the top once it's firm.
Keeping Bath Silicone Clean And Free Of Mould
How long a bath seal lasts comes down to the prep and how damp the room stays, so look along the joint for lifting edges or dark spots whenever you clean the bath. Silicone also stays white for much longer when it isn't left wet. Clearing the steam with the window or fan after every bath and wiping the joint dry after a shower both help. When surface spots do appear, a mould remover that's safe on sealant can lift them, but black mould growing underneath the silicone means it's time to cut it out and reseal.
The same tape, bead and smooth method works on a shower tray, a basin or a worktop upstand. If a new tray is going in, our guide to installing a shower tray shows where the sealant goes once the tray is bedded down.
Quick Questions And Answers
How long does bath silicone take to dry before you can use the bath?
Leave bath silicone at least 24 hours before using the bath or shower so it can cure all the way through. The exact time depends on the brand, how thick the bead is and how warm and well ventilated the room is, so check the tube as well. Keep the water in the bath during that time so the joint cures in its loaded position.
Should you fill the bath with water before sealing it?
Yes, fill the bath with water before you seal it. The weight pulls the tub down and opens the gap at the wall to the size it'll be whenever the bath is in use. Silicone applied to an empty bath gets stretched the first time it's filled, which is a common cause of cracked and peeling sealant.
Can you put new silicone over old bath sealant?
No, new silicone doesn't bond properly to old silicone, so a fresh bead laid over the top peels away and lets water in behind it. Cut out the old sealant with a sharp blade, remove the residue left on the bath and tiles, clean off any mould and let the joint dry before you apply new silicone.
Why does the silicone around my bath keep going mouldy?
Bath silicone goes mouldy when it stays damp for long periods, especially in a bathroom with poor ventilation or where the joint was dirty when it was sealed. Mould that grows into the silicone can't be scrubbed out. Reseal with a mould-resistant bathroom sealant, then clear steam after bathing and wipe the joint dry to slow it coming back.
Is masking tape really necessary when sealing a bath?
You can seal without it, but the line will only be as straight as your hand. Two strips, one on the bath and one on the wall, give the bead a fixed width to fill, and peeling them while the silicone is wet leaves clean edges with nothing smeared on the tiles.
Should I use a wet finger or a sealant applicator to smooth the bead?
Either works. We reach for a wet finger, pulled along the joint at an angle in one stroke, and an applicator suits anyone who'd rather not get silicone on their skin. Whichever you use, keep a paper towel close for wiping off the build up.
Why start sealing from the far corner of the bath?
So you never lean over silicone you've just laid. Working back towards yourself also keeps you moving in one direction, which makes a single continuous pass down each edge much easier. If you do stop, carry on from exactly where the bead ends.
What do I do if the gap between my bath and the wall is too wide for silicone?
Close the gap before you seal it, as a thick bead on its own just sags and splits. Refitting the bath tighter to the wall is the proper fix, but that means disturbing the waste, taps and panel. The simpler route is a bonding material and adhesive or a flexible caulking strip in the gap, with silicone run over it once it's firm.
Richard has spent years renovating properties and turning them into rentals and Airbnbs. Like a lot of people he started out as a beginner, and these days there isn't much DIY he won't take on.