How to remove black mold from shower grout and caulk

Black spots on shower grout and caulk are usually mold or mildew, and they grow in the warm, damp conditions that a shower creates. Mildew is the flat, surface growth that looks gray or brown. Mold is darker and it grows deeper into the material, which is why it is so hard to shift. Both feed on the soap scum and the oils left on the walls after every shower. Cleaning them takes some patience, but the methods below work well on the grout and the caulk of a typical bathroom, and they will not damage the tile or the tub.

The most important thing is to ventilate the room before you start. Open a window or switch on the exhaust fan, and keep it running while you work. Wear rubber gloves and eye protection, and do not use two cleaning products at the same time. Mixing bleach with ammonia, vinegar or any other acid can release gases that are dangerous to breathe.

What you need

• Rubber gloves and safety glasses
• A stiff nylon or bristle brush, or an old toothbrush for the corners
• Mild dish soap and warm water
• 3 percent hydrogen peroxide from the pharmacy
• Baking soda, for a paste on stubborn grout
• Optional: a bleach-based or mildew cleaner, used alone and as its label directs
• A microfiber cloth and a spray bottle
• A caulk remover and a new tube of mold-resistant silicone caulk, if the caulk needs replacing

Testing the mold

Before you choose a method, look closely at the spots. Surface mildew wipes off with soap and water, and it leaves the grout clean underneath. Mold that has grown into the grout, or that stays dark after a soapy scrub, needs one of the stronger methods below. Spots that come back quickly after cleaning are a sign that the moisture problem has not been fixed.

How the calculator works

The mold patch calculator works out the area of a patch of mold and says whether it is within the 10 square feet that the US Environmental Protection Agency (EPA) gives for cleaning mold yourself. Enter the patch’s length and width in feet. It gives the area in square feet and compares it with that limit.

Area is length x width. With the default patch of 3 ft by 2 ft, the area is 6 square feet, which is within the limit, so the steps in this guide apply. A patch of 4 ft by 3 ft is 12 square feet. That is over the line, and the calculator points you to the EPA’s fuller guidance and to the section on when to call a professional.

The figure comes from the EPA’s page Mold Cleanup in Your Home. It says that if the moldy area is less than about 10 square feet, roughly a 3 foot by 3 foot patch, you can in most cases handle the job yourself, and that a larger area calls for its fuller guidance or a professional. That fits what this guide says about size. A small patch is the job it describes, done with a brush, hydrogen peroxide and good ventilation. Mold that covers a large area is a sign of a hidden leak or a serious moisture problem, and cleaning the surface will not fix it.

For an uneven patch, measure its longest and widest points. The rectangle around it is larger than the patch, so the answer errs on the side of caution. Area is not the only test. Whatever the size, mold behind the tiles, tiles that sound hollow, a wall that feels soft, or mold that appears after a leak from a pipe or a seal means a call to a plumber or a mold specialist.

The steps for grout

1. Wash the walls and the grout with warm water and a little dish soap, using a cloth. This removes the soap scum that holds the mold in place, and it shows you how much mold is left.
2. Rinse the area and check it again. If the spots have lifted, scrub the grout lightly with a brush, rinse, and dry. You may be finished.
3. If dark spots remain, spray 3 percent hydrogen peroxide directly onto the grout. Do not rinse it off. Leave it for ten to fifteen minutes, then scrub each line with the stiff brush, working from the top of the wall down. Hydrogen peroxide is gentle enough for most sealed grout, and it does not leave a bleach smell.
4. For heavy staining, make a paste from baking soda and hydrogen peroxide, about three parts soda to one part peroxide, and spread it over the grout. Leave it for fifteen minutes, then scrub and rinse thoroughly.
5. Rinse the walls with clean warm water, using a fresh cloth or a sponge. Make sure no residue of the paste or the peroxide is left on the surface.
6. Dry the grout with a clean towel, and leave the door open or the fan running until the area is completely dry.

If you use a bleach-based mildew cleaner, follow the label, spray it on the grout alone, keep the room ventilated, and rinse it off thoroughly. Bleach does not reach deeply into porous grout, so it often lightens the surface and leaves the roots of the mold behind. That is why the spots come back after a few weeks. Hydrogen peroxide or a scrub with a detergent usually does the job better, with less risk.

Cleaning the caulk

The caulk around the bath and the shower tray takes mold differently from grout. Silicone caulk resists water well, but mold can grow on its surface, and if the caulk has shrunk or cracked, mold can grow beneath it.

1. Clean the surface with the same methods as the grout. Use a little more hydrogen peroxide, and leave it for fifteen minutes before scrubbing with a small brush.
2. Look at the caulk closely. If it is still black after cleaning, press a fingernail into the bead. If it is soft, stained through, or pulls away from the surface, replace it.
3. To replace caulk, cut the old bead out with a sharp craft knife, taking care not to scratch the tub or the tile. Remove every trace of the old caulk, then clean the gap with a caulk remover and let it dry completely.
4. Apply a new bead of mold-resistant silicone caulk, following the tube’s directions for the width and the smoothing. Smooth it with a wet finger or a caulk tool, and leave it to cure for the time the label gives, usually a day, before you use the shower.

Do not use paint or sealer on the caulk to hide mold. The mold will keep growing underneath the coat, and the paint will peel.

Walls, corners and the ceiling

The corners of a shower are where water sits, and the ceiling above the shower is where condensation drips. Clean both on the same day as the grout. Use a soft brush and a little hydrogen peroxide on the corners, then wipe the ceiling with a cloth dampened in mild soapy water. For mold on the ceiling, the same method works, but make sure the area is well ventilated, and stand on a stable step so you do not drip the cleaner into your eyes.

If mold returns to the same spot on the ceiling within days, the exhaust fan may be too weak for the room, or it may be blocked. A fan that does not vent to the outside, or one that is noisy and barely moves air, is not doing its job, and fixing the fan is the only lasting cure.

Keeping the grout clean

The best way to deal with mold is to stop it growing.

• Squeegee the walls, the glass screen and the tub after every shower. Water left on the surfaces feeds the mold.
• Run the exhaust fan for twenty minutes after you finish showering, and keep the door open or the window cracked.
• Wipe the grout in the corners and along the bottom lines once a week, using a cloth and a mild soap.
• Reseal the grout once a year, or as the product label advises. Sealed grout resists staining and water much better than bare grout.
• Keep a bottle of hydrogen peroxide or a mild mildew spray in the cabinet, so a spot can be treated before it spreads.

Most regrowth starts in the corners and along the lower grout lines, where water pools and does not dry quickly. A quick squeegee and a fan run stop it before it starts.

Safety

Wear gloves and eye protection for all of this. Do not mix cleaners. If you use bleach, never combine it with vinegar, ammonia or another cleaning product, and keep the window open. If you feel dizzy, have chest tightness or cough hard while cleaning, leave the room and get fresh air, and call a doctor if the symptoms do not clear.

When to call a professional

Mold that covers a large area, or mold that has spread behind the tiles, is a sign of a hidden leak or a serious moisture problem. Cleaning the surface will not fix it, and the mold may keep returning. If the tiles sound hollow, if the wall feels soft, or if the mold appears after a leak from a pipe or a seal, call a plumber or a mold specialist to find the source. The US Environmental Protection Agency puts the line between a small area and a large one at about 10 square feet, roughly a 3 foot by 3 foot patch: below that you can in most cases handle the job yourself, and above it the agency’s fuller guidance or a professional applies.

For small areas of mold on grout and caulk, a regular routine of drying, ventilation and a yearly reseal is enough to keep the bathroom clean and fresh.