How to Caulk a Bathtub

Re-caulking a bathtub is an easy DIY job — about an hour of work plus 24 hours of cure time, under $15 in materials, no permit and no plumber. You cut out the old caulk, clean and dry the joint, fill the tub with water, and lay a single smooth bead of mildew-resistant silicone. The two secrets are filling the tub first (so the bead flexes instead of tearing) and using 100% silicone, not painter's caulk. There is no scenario where this needs a professional.

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Tools and materials

• 100% silicone caulk, mildew-resistant, kitchen-and-bath rated.
• A caulk gun, a caulk-removal tool or utility knife, and a caulk-softening gel for stubborn old beads.
• Rubbing alcohol, rags, painter's tape (optional, for crisp edges), and a caulk-finishing tool or your finger.
• Basic hand tools — see best plumbing tools.

Step 1: Remove all the old caulk

Cut along both edges of the old bead with a caulk tool or utility knife and peel it out in strips. Apply a caulk-softening gel to stubborn silicone and give it a few hours to let go. Scrape every last trace off the tub and wall surfaces — new silicone will not bond to old silicone, so any residue becomes a peel point.

Step 2: Clean and dry the joint

Wipe the joint with rubbing alcohol to strip soap film, mildew and old residue. Let it dry completely — silicone will not adhere to a damp surface. If there was mold, treat and dry the area fully. A clean, dry, residue-free joint is the foundation of a seal that lasts.

Step 3: Fill the tub with water

Fill the tub to near the overflow before caulking. The weight of the water pulls the tub down to its lowest, widest gap position, so the caulk cures in the open position and flexes with the tub instead of tearing when someone steps in. Leave the water in until the caulk has fully cured. Optionally, run painter's tape along both sides of the joint for a crisp edge.

Step 4: Apply a single smooth bead

Cut the caulk tube nozzle at a 45-degree angle to a small opening (about 1/8 to 1/4 inch), load the gun, and lay one continuous bead along the joint in a steady motion — do not stop and start, which leaves gaps and lumps. Then tool the bead: draw a caulk finishing tool or a wet fingertip along it once with light, even pressure to press it into the joint and smooth it concave. Remove the painter's tape immediately if you used it, before the caulk skins.

Step 5: Let it cure fully

Leave the tub filled and do not get the caulk wet until it has fully cured — check the tube, but plan on 24 hours for standard silicone. Drain the tub only after curing. Rushing this step, or draining the tub before the bead sets, is what ruins an otherwise good job.

Common mistakes

• Caulking a dry tub, so the bead tears when the filled tub drops under weight.
• Using acrylic latex caulk instead of 100% silicone in a wet joint.
• Applying over residue of old caulk, which prevents bonding and causes peeling.
• Getting the bead wet before it cures, ruining the seal.

When to call a pro

Caulking never needs a plumber, and there is no permit. But if you pull the old caulk and find soft, spongy wall board or ongoing water behind the tile, that is water damage behind the surround, and fresh caulk only hides it — a bathroom pro should open and repair the wall. Persistent leaking from the tub itself (not the joint) points to a drain or overflow gasket issue, which is a plumbing repair, not a caulking one.

Related: how to replace a shower head, how to unclog a bathtub drain, and how to replace a toilet wax ring for the base-of-toilet seal.