The bead of caulk where the tub meets the tile is the only thing keeping water out of the wall. When it cracks, pulls away or goes black with mould, water gets behind the tile every time someone showers, and the eventual repair bill is measured in thousands. Re-caulking costs about $15, takes two hours including removal, and the difference between an amateur bead and a professional one comes down to five techniques this guide explains.
Key takeaways
- Remove all the old caulk. New caulk over old caulk fails within months.
- Use 100% silicone labelled kitchen & bath / mould-resistant. Acrylic latex "tub and tile" caulk is easier but cracks sooner.
- Fill the tub with water before caulking so the joint is at its widest. Drain after the caulk cures.
- Cut the nozzle small (3–5 mm), run a steady bead, and tool it in one pass with a caulk tool or a finger dipped in soapy water.
- Let it cure 24 hours (check the tube) before showering. Re-caulk every 3–5 years.
Does it need re-caulking?
- Cracks or gaps between the caulk and the tub or tile.
- Caulk that has pulled away and you can lift with a fingernail.
- Black spots inside the caulk that do not clean off (mould has grown under the surface).
- Caulk that is yellow, brittle or crumbling.
- A tub that moves slightly when you step in. The caulk may be intact, but movement will break it soon; the tub needs to be shimmed or supported first.
Tools and materials
Removal
- Utility knife with fresh blades
- Plastic caulk removal tool or a plastic putty knife
- Caulk softener/remover gel (optional, for old silicone)
- Rubbing alcohol and clean rags
- Shop vacuum or brush
Application
- 100% silicone kitchen & bath caulk (one 300 mL tube does a standard tub twice)
- Caulking gun (a dripless one is worth $15)
- Painter's tape (optional, for perfect lines)
- Caulk finishing tool, or dish soap and water in a cup
- Paper towels, lots
Choosing the caulk
| Type | Pros | Cons | Use it for |
|---|---|---|---|
| 100% silicone (kitchen & bath) | Fully waterproof, flexible for years, mould inhibitors, lasts 5–10 years | Harder to tool, cannot be painted, strong smell while curing | Tub-to-tile joint, shower base, around faucets and spouts. The right choice. |
| Siliconized acrylic latex | Easy to apply, water cleanup, paintable | Shrinks, cracks in 1–3 years in wet joints | Baseboards, trim, dry areas. Not the tub. |
| Hybrid / polyurethane | Very strong adhesion, paintable, flexible | Expensive, hard to remove later | Freestanding tub to floor, stone surrounds |
Colour: white for white tubs and white grout; clear for coloured tile and freestanding tubs on wood or tile floors; "almond" or "biscuit" to match older fixtures. Match the grout colour, not the tile.
Step 1: Remove the old caulk completely
- If the old caulk is silicone and stubborn, apply a caulk remover gel and wait the time on the label (usually 2–3 hours). It softens the bond.
- Score along the top edge (against the tile) and the bottom edge (against the tub) with the utility knife held at a low angle. Do not gouge the tub; acrylic tubs scratch easily.
- Pull the bead out. Long strips come away in one piece if you scored well.
- Scrape residue with the plastic tool. On tile, a razor scraper held flat is fine; on the tub, plastic only.
- Wipe the whole joint with rubbing alcohol. Silicone residue is invisible and nothing sticks to it, so wipe twice.
- If there is mould in the joint, treat it with hydrogen peroxide or diluted bleach, then let it dry.
- Vacuum the joint and let it dry completely, ideally overnight. Silicone applied to a damp joint will not bond.
Step 2: Fill the tub
This is the step most people skip and the reason most DIY caulk fails. A tub with 150 kg of water and a person in it settles a millimetre or two. If you caulk it empty, the joint stretches every time it is used and tears within months. Fill the tub at least halfway (or stand in it while you work, if you can reach) and leave it full until the caulk cures.
Step 3: Tape (optional but recommended)
Run painter's tape along the tile about 3 mm above the joint and along the tub about 3 mm below it. Press the edges down firmly. The tape gives you dead-straight edges and takes the pressure off your tooling. Remove it immediately after tooling, before the silicone skins.
Step 4: Apply the bead
- Cut the nozzle at a 45° angle with a small opening, about 3–5 mm. You can always add; you cannot easily remove. Puncture the inner seal with a nail or the gun's rod.
- Load the gun, squeeze until caulk just reaches the tip, and release the pressure.
- Start in a back corner. Hold the gun at about 45° and push the nozzle along the joint (pushing forces caulk into the gap; pulling lays it on top). Move at a steady speed with steady pressure. Aim for a bead slightly bigger than the gap.
- Do one wall at a time, then tool it, before moving to the next. Silicone skins in 5–10 minutes.
- Release the gun's pressure at the end of each run so it stops oozing.
Step 5: Tool the bead
- Mix a few drops of dish soap in a cup of water. Wet your finger or the caulk tool.
- In one smooth pass, run the tool along the joint with light, even pressure, forcing the caulk into the gap and smoothing the surface to a slight concave. Do not go back and forth; every pass after the first makes it worse.
- Wipe the excess on a paper towel every 20–30 cm. Re-wet the finger.
- Pull the tape off immediately, at a 45° angle away from the bead.
- Fix any small ridges left by the tape with one more light pass of a wet finger.
Step 6: Cure
- Most silicones are tack-free in 30–60 minutes and water-ready in 24 hours. Some fast-cure products say 3–4 hours. Check the tube; do not guess.
- Keep the bathroom ventilated. Silicone releases acetic acid (that vinegar smell) as it cures.
- Drain the tub after the cure time, not before.
- Do not touch or clean the bead for 48 hours.
Special cases
Freestanding tub
A freestanding tub needs caulk only where the base meets the floor, and only for hygiene (so water does not collect underneath), not for waterproofing. Use clear silicone, keep the bead small, and leave a gap of a few centimetres at the back so any water that gets under the tub can escape and you can tell if the drain is leaking. Do not caulk between the tub and the wall; leave the designed gap.
Shower base and glass
The base-to-wall joint is done exactly like a tub. Where the glass panel or frame meets tile, caulk the outside of the frame only, never the inside bottom track; the track has weep holes that must drain into the shower.
Tub spout, faucet trim and shower arm escutcheon
Caulk around the top and sides, leaving the bottom open so any water inside the wall drains into the tub instead of pooling.
Very old or moving tubs
If the tub flexes when you stand in it, caulk will fail no matter how well it is applied. Support the tub from below (foam or mortar under an acrylic tub, shims under a steel one) before caulking.
Fixing common mistakes
| Problem | Cause | Fix |
|---|---|---|
| Bead is lumpy and smeared | Nozzle cut too big, or too many tooling passes | Let it cure, cut it out, redo with a smaller opening and one pass |
| Caulk peels off the tub after a few weeks | Old silicone residue, or joint was damp | Remove, clean with alcohol twice, dry overnight, redo |
| Crack along the middle of the bead | Tub was empty during caulking, or acrylic latex caulk was used | Redo with silicone, tub filled |
| Black spots return in months | Mould left in the joint, or non-mould-resistant caulk | Redo; treat joint with peroxide first; run the fan after showers |
| Silicone smeared on tile face | No tape, or wiped while wet | Let it cure, then peel or scrape with a plastic blade. Silicone remover for the haze. |
When the tub itself is the problem
If the tub is chipped, rusted through at the drain, stained beyond cleaning or simply too small, no caulk job will make you love it. A new freestanding tub starts under $800 and, in a renovation, replacing the tub is when the wall can be properly waterproofed behind it. See our renovation cost guide for what a tub replacement runs in Ontario.
Shop freestanding bathtubs at MyBath
Acrylic freestanding tubs from 55" to 71", plus matching floor-mount fillers. Delivered across Ontario, free on orders over $600.
Frequently asked questions
Can I caulk over old caulk?
No. New silicone will not bond to old silicone, and any mould underneath keeps growing. Remove it all, clean with alcohol and start fresh.
Should I fill the bathtub before caulking?
Yes. A full tub sits at its lowest point, so the caulk is applied with the joint at its widest and is compressed rather than stretched in use. Leave it full until the caulk has cured.
How long does bathtub caulk take to dry?
Tack-free in about an hour, fully cured and water-ready in 24 hours for most 100% silicones. Some fast-cure formulas allow water exposure in 3–4 hours. Follow the tube.
What is the best caulk for a bathtub?
100% silicone labelled for kitchen and bath with mould and mildew resistance. It stays flexible and waterproof for 5–10 years. Avoid acrylic latex in wet joints.
How do I get a smooth caulk line?
Small nozzle opening, painter's tape on both sides, steady bead, then one single pass with a wet finger or caulk tool. Pull the tape immediately.
How often should I re-caulk a bathtub?
Every 3–5 years, or as soon as you see cracks, gaps or black spots inside the caulk.
