Bathtub grout takes more abuse than any other grout in your home, soaking up daily steam, standing water, soap scum, and the constant flex of a tub that shifts slightly every time it fills and empties. When bathtub grout cracks, darkens, or crumbles, it is rarely random; it is telling you something about how the joint was built and maintained. Understanding why grout fails at the tub is the first step to fixing it right and keeping it fixed. As a tile contractor, I have torn out more moldy tub joints than I can count, and nearly every one came down to the same handful of mistakes. Here is how to avoid them.
Sanded Versus Unsanded Grout
Your first decision is grout type, and it hinges on joint width. Sanded grout contains fine sand that adds strength and resists shrinkage, making it the right pick for joints 1/8 inch and wider, common with larger wall tiles. Unsanded grout is smooth and sticky, ideal for tight joints of 1/16 to 1/8 inch and for polished or glass tile that sand would scratch.
For most tub surrounds with standard 1/16 to 3/32 inch joints, unsanded grout is the correct choice. If your tile has wider joints, use sanded. Getting this wrong causes real problems: sanded grout jammed into a hairline joint cracks, and unsanded grout in a wide joint shrinks and pulls away. Read the joint before you buy the bag.
Why Grout Cracks and Molds at the Tub
The dark line where the bottom row of tile meets the tub is the trouble spot, and there is a physical reason. A filled bathtub can hold 40 to 80 gallons of water, adding hundreds of pounds that flex the tub downward. As the tub moves and the wall stays put, that joint opens and closes with every bath. Rigid grout cannot handle repeated movement, so it cracks.
Once a crack opens, water wicks behind the tile, feeds mold, and the black staining spreads. Grout is also cementitious and porous, so even an intact joint absorbs moisture and grows mildew if it is not sealed. Two forces, movement and moisture, conspire at exactly the tub line, which is why that spot always fails first.
Caulk, Not Grout, at the Change of Plane
Here is the single most important rule in this whole guide: the joint between the tub and the tile, and any inside corner where two walls meet, should be caulk, not grout. These are “changes of plane,” and they move. Flexible silicone or a quality tub-and-tile caulk absorbs that movement; rigid grout cannot and will crack every time.
Use grout on the flat field where tile meets tile on the same plane. Use caulk at:
- The horizontal tub-to-tile joint
- Vertical inside corners where two walls meet
- The joint where tile meets a window or fixture
Choose a 100 percent silicone or a mildew-resistant siliconized caulk rated for wet areas, ideally color-matched to your grout. If your existing tub joint is grouted and cracking, that is your culprit, and the fix is to remove it and switch to caulk.
Regrouting a Tub Surround Step by Step
When field grout is stained, cracked, or crumbling beyond cleaning, regrouting restores the surround. The process:
- Remove old grout: Rake out the failed grout to at least half the tile depth with an oscillating multi-tool or a carbide grout saw. Vacuum the dust.
- Clean the joints: Scrub out soap scum and old sealer, then let everything dry fully overnight.
- Mix fresh grout: Blend to a peanut-butter consistency and let it slake for 10 minutes.
- Apply: Force grout into joints with a rubber float held at a 45-degree angle, working diagonally across the tiles.
- Clean the haze: Wait 15 to 30 minutes, then wipe with a barely damp sponge, rinsing often. Buff off haze the next day.
- Caulk the tub joint: After the grout cures, run fresh silicone along the tub-to-tile line and corners.
Do not grout the tub joint even if it looks tidier. Fill the bathtub with water before caulking the bottom joint so the tub sits at its lowest, then caulk; this prevents the joint from tearing open under load later.
Sealing Grout for the Long Haul
Cementitious grout is porous and needs a penetrating sealer to resist water and mildew. Wait for new grout to cure fully, typically 48 to 72 hours, then apply a quality penetrating silicone or fluoropolymer sealer with a small brush or applicator bottle, wiping excess off the tile face.
Reseal a tub surround every one to two years, or when water stops beading on the grout lines. Epoxy grout is a maintenance-free alternative that never needs sealing and resists stains and mold, though it costs more and is harder to install. For a heavily used family tub, epoxy grout in the field plus silicone at the joints is the most durable combination you can build.
Grout Colors and Common Mistakes
Color choice affects how much maintenance you sign up for. Bright white grout looks crisp on day one but shows every speck of mildew and soap scum, and it is nearly impossible to keep pristine at a tub. A mid-tone gray or greige hides staining and reads clean far longer between deep cleanings. Match your caulk to the grout color so the tub joint disappears rather than framing the tub in a mismatched line.
A few mistakes I see repeatedly: grouting over a joint that was never cleaned of old soap film, so the new grout never bonds; skipping the slaking step and mixing grout too wet, which weakens it and washes out color; and wiping the haze with a soaking sponge, which pulls binder out of the joints and leaves them pitted. Work with a barely damp sponge, rinse it constantly, and resist the urge to overwork the fresh grout.
Cleaning Mildew and Stains
Black spots in tub grout are usually surface mildew, and you can often clean rather than replace. Spray the joints with a bathroom mold remover or a homemade mix of one part bleach to three parts water, let it dwell 10 minutes, then scrub with a stiff nylon brush and rinse. For stubborn staining, a baking soda and hydrogen peroxide paste worked into the joint and left 15 minutes lifts a lot of discoloration.
Ventilate the room and wear gloves when using bleach, and never mix bleach with ammonia products. If the black returns within days no matter how you scrub, the mold is growing behind the grout through a cracked joint, and cleaning alone will not fix it. That is your signal to rake out and regrout, then caulk the moving joints properly.
What Bathtub Grout Work Costs
Costs are modest for a job this important. A bag of quality grout runs $15 to $30, a tube of silicone tub caulk $6 to $12, and a bottle of penetrating sealer $15 to $25, so a full DIY regrout of a standard tub surround costs $50 to $80 in materials plus a grout saw or multi-tool blade.
Hiring a pro to regrout and recaulk a tub surround typically runs $300 to $700 depending on size and how much old grout must come out, with epoxy grout at the higher end. Given that a failed tub joint can leak water into the wall and cause thousands in rot and mold damage, staying ahead of your bathtub grout with regular sealing and prompt recaulking is some of the cheapest insurance in the bathroom.
