How to Recaulk a Bathtub or Shower
Updated September 18, 2026
Can I do this myself?
Safe to DIY
Yes, recaulking is a safe beginner job. Ventilate well, handle razor blades carefully, and call a pro if the wall behind the caulk is soft or mould keeps coming back.
The line of caulk where the bathtub meets the wall is a small thing that does a big job. It stops water getting behind the tiles and under the tub, where it can rot framing, loosen tiles, and grow hidden mould. When caulk cracks, peels away, or turns black, it is time to replace it.
Recaulking properly is less about the caulking gun and more about preparation. New caulk will not stick to old caulk, soap scum or damp surfaces, and it will not stop mould that is already growing underneath. The steps below take you through fully removing the old bead, cleaning and treating the joint, drying it thoroughly, and laying a neat, watertight bead of 100% silicone.
Plan the job for a time when you can leave the tub unused for at least 24 hours (longer in cool or humid conditions), because silicone needs time to cure before it gets wet.
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Signs of the problem
- Caulk is cracked, shrunken or pulling away from the tub or wall
- Black or dark spots on or under the caulk that do not scrub off
- Gaps between the tub and the tiles
- Water stains on the ceiling below or on the floor beside the tub
- Loose tiles or crumbling grout near the tub edge
Common causes
Age and wear
Most commonEven good silicone hardens and loses its grip over several years of hot water, cleaning products and movement. Eventually it cracks or peels away from the surface.
Mould growth
Most commonBathrooms are warm and humid. Mould can grow on the surface of caulk and in the gaps underneath it. Once it has spread into the caulk itself, cleaning will not remove the stains, and the caulk should be replaced.
Wrong product or poor application
SometimesAcrylic or painter's caulk is not designed for constant wetting and fails quickly around tubs. Caulk applied over old caulk, soap scum or a damp surface will not bond well and peels early.
Tub movement
SometimesTubs, especially acrylic and fibreglass ones, flex slightly when filled. If the caulk was applied with the tub empty, the joint can pull apart when it is loaded with water. Filling the tub before caulking reduces this.
Safety first: before you start
- Critical:Ventilate the bathroom throughout: open a window and run the extractor fan. Silicone gives off acetic (vinegar-like) fumes as it cures, and mould removers and solvents need fresh air.
- Critical:Never mix bleach-based mould removers with ammonia, vinegar, or acidic cleaners. Use one product, rinse thoroughly, and let it dry before using anything else.
- Wear gloves and eye protection when using mould cleaners and solvents, and a mask if you are sensitive to mould.
- Use a plastic scraper or a razor blade in a holder, and cut away from your body. Loose razor blades cut fingers easily.
- Work from a non-slip surface and do not stand on the tub edge.
- Keep solvents such as isopropyl alcohol away from flames and heaters.
Tools & materials
Tools
- Caulking gun
- Plastic caulk removal tool or putty knife
- Utility knife or razor blade scraper (in a holder)
- Painter's tape
- Rags and paper towels
- Old toothbrush or small scrub brush
- Rubber gloves and eye protection
Materials
- 100% silicone sealant labelled for kitchen and bath, with mould resistance
- Caulk remover (optional, for stubborn caulk)
- Isopropyl (rubbing) alcohol or mineral spirits
- Mould and mildew cleaner or diluted bleach solution
- Foam backer rod (for gaps wider than about 6 mm/quarter inch)
Step-by-step fix
- 1
Cut and pull out the old caulk
Run a utility knife or razor scraper along both edges of the old bead, where it meets the tub and where it meets the wall. Then hook the end with a caulk removal tool or putty knife and pull it out in strips. Work gently on acrylic or fibreglass tubs, which scratch easily.
Warning: Keep the blade low and pull it toward you, not into the tub. A slipped blade can gouge enamel or cut your hand.
- 2
Remove every trace of residue
Scrape off remaining bits with a plastic scraper. For thin silicone film, apply a caulk remover and wait according to the label, then scrape again. Finish by wiping the joint with isopropyl alcohol or mineral spirits on a rag. New silicone will not stick to old silicone, so this matters.
- 3
Check for mould and damage
Look closely at the joint and the grout above it. Clean any mould with a mould and mildew cleaner or a diluted bleach solution applied with a toothbrush, following the label. Rinse with clean water. Press on the tiles and wall near the tub. If they flex, feel soft, or mould keeps reappearing from behind, there may be water damage in the wall.
- 4
Let the joint dry completely
Moisture trapped under new caulk causes mould and poor adhesion. Leave the joint to dry for at least several hours, ideally overnight, with the fan running or a window open. A hair dryer on low can speed this up, used carefully and kept well away from water.
- 5
Fill the tub and tape the edges
Fill the tub with water. The extra weight makes the tub settle into its loaded position, so the caulk will not be stretched later. Apply painter's tape along the wall and the tub edge, leaving a gap about 3 to 6 mm (an eighth to a quarter inch) wide over the joint. For gaps wider than about 6 mm, push foam backer rod into the joint first.
- 6
Apply the silicone bead
Cut the nozzle at a 45-degree angle to make a hole slightly smaller than the gap. Load the gun and start in a corner. Squeeze steadily while moving at an even pace, pushing the bead into the joint. Try to do each side in one continuous run.
Tip: Silicone skins over within about 10 minutes, so work on one wall at a time and do not stop halfway.
- 7
Smooth the bead and remove the tape
Immediately smooth the bead with a caulk finishing tool or a gloved finger dipped in soapy water, using one smooth pass. Wipe excess off the tool as you go. Then pull the tape off at a 45-degree angle away from the joint while the silicone is still wet.
- 8
Let it cure before using the tub
Leave the water in the tub until the silicone has set, then drain it. Most silicone needs 24 hours before it gets wet, and some need longer in cool or humid conditions. Check the tube for the exact cure time and keep the room ventilated while it cures.
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How to check it's fixed
- The new bead is continuous with no gaps, bubbles or thin spots
- The caulk is firmly bonded to both tub and wall when pressed gently after curing
- No water appears below or behind the tub after the first few showers
When to stop and call a professional
- Tiles or wall board near the tub are loose, soft, or crumble when pressed
- Mould keeps returning within weeks, which may mean moisture in the wall
- There are water stains on the ceiling below or the floor near the tub is soft
- The gap between tub and wall is very large or the tub moves noticeably
- Grout is widely cracked or missing and tiles need resetting
How to prevent it
- Run the extractor fan during showers and for 20 to 30 minutes after.
- Squeegee or wipe down the tub edge after showering to reduce standing water.
- Clean caulk regularly with a mild bathroom cleaner to stop mould taking hold.
- Inspect caulk every few months and touch up small gaps before they grow.
Frequently asked questions
What type of caulk is best for a bathtub?
100% silicone sealant labelled for kitchen and bathroom use is the best choice for tubs and showers. It stays flexible, resists water and mould, and lasts much longer than acrylic or latex caulk.
Can I put new caulk over old caulk?
No. New silicone does not bond to old silicone, and any mould under the old bead stays trapped. Remove all the old caulk and clean the joint before applying new sealant.
How long does silicone caulk take to cure?
Most silicone sealants are touch-dry in under an hour and ready for water after about 24 hours. Cool or humid rooms can slow curing. Always follow the time on the tube.
Why does my new caulk keep turning black?
Black spots are usually mould. If they appear soon after recaulking, the joint may not have been fully cleaned or dried, or moisture is getting in from behind. Improve ventilation, and if it keeps coming back, have the wall checked.
Should I fill the tub with water before caulking?
Yes, it helps. A full tub settles slightly, so caulking it in that position means the bead will not stretch and crack when the tub is used. Drain it only after the silicone has set.
This guide is general information, not professional advice. Homes differ, so follow your product manuals and local codes. If anything looks different from what's described, or you feel unsure, stop and call a qualified professional. In an emergency, leave the area and call your local emergency number.
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