Mould in the shower: is it the fan, the grout or the waterproofing?

Mould in the shower means moisture is staying in the space or getting into it, and cleaning alone will not fix that. Health Canada's guidance is that mould typically grows indoors where moisture is present and will reappear unless the cause of the moisture is found and solved. The fan, the caulk and grout lines, and the waterproofing behind the tile are three places to check.

Start with what mould needs

Health Canada notes that mould will typically grow indoors if moisture is present, and that it can begin to grow within 48 hours in an area with excessive moisture. The same guide says mould will reappear unless the cause of the moisture problem has been identified and solved. Scrubbing treats the symptom. In a shower, the useful question is where the moisture is coming from or why it is not drying out.

Check the fan and the humidity

Health Canada's prevention checklist covers the fan in some detail. It says to use an exhaust fan when you shower or bathe and to keep it running for at least 30 minutes afterward. It also says to maintain the fan so there is adequate air movement and it vents outside, to inspect the outside vent routinely, and to keep indoor relative humidity between 30 and 50 per cent. The checks that follow from that:

• Is the fan running during the shower and for at least 30 minutes after it?
• Does it visibly move air, and does the duct actually vent outside?
• Is the outside vent clear of lint and other build-up?
• Is the room's humidity staying within 30 to 50 per cent?

If you are thinking about replacing or adding a fan, whether a permit or current code requirements come into play depends on the scope of the work and on the municipal building department's determination. Building code provisions differ between editions, so confirm what applies with your municipal building department before you buy or install anything.

Check the caulk and grout lines

Corners, the tub edge and the floor-to-wall joint are good places to inspect for cracks, gaps and caulk that has lifted from the tile. Tile industry guidance published in TileLetter says movement joints are needed at changes of plane, specifically interior corners. It describes a flexible sealant meeting ASTM C920 as the best type of material for those joints. The same article mentions TCNA Handbook method EJ171 in connection with movement joints at perimeters. Standards are revised periodically, so check the current edition and the tile and sealant manufacturers' instructions before choosing a material or repairing a joint.

If mould keeps coming back on a grout or caulk line after you clean it, treat that as a prompt to look at the moisture source rather than to clean harder. Health Canada's Healthy Home Guide suggests considering a professional when mould keeps coming back after cleaning.

Consider the waterproofing behind the tile

Manufacturers of tile waterproofing systems are direct on this point. Schluter Systems states that tile and grout are not inherently waterproof, that water can pass through grout lines into the structure, and that a waterproofing membrane belongs behind the tile. If that layer or its details have failed, moisture can be present somewhere you cannot see.

Health Canada's guide says that many small patches of mould in one area or throughout the home can point to a larger moisture issue. The Healthy Home Guide suggests inspecting for stains or discolouration on floors and walls and checking for a musty, earthy odour.

A checklist to work through

1. Fix the routine. Run the fan during the shower and for at least 30 minutes after it.
2. Confirm the fan works. Check air movement, the duct route and the outside vent.
3. Watch the humidity. Aim for 30 to 50 per cent.
4. Inspect every caulked joint. Look for splits, gaps and edges lifting from the tile.
5. Check the surrounding surfaces. Look for stains, discolouration or a musty odour on adjoining floors and walls.
6. Note whether the mould returns. Recurrence after cleaning is a reason to get help.

Cleaning it safely

Health Canada recommends cleaning up visible mould regardless of the type of mould present. For small areas, Health Canada's mould infographic advises soap and water on non-porous surfaces for patches under about one square metre. It lists drywall, cardboard, paper and untreated wood as porous materials, and glass, metals, leather, plastics and vinyl as non-porous. It does not address grout joints specifically, so do not assume the simple approach suits a stubborn grout line.

Skip the bleach. The infographic says there is no need for it, and Health Canada's full guide in PDF form says never to use bleach to clean up mould.

For protection, the main Health Canada guide recommends a well-fitting disposable N95 respirator, along with goggles and gloves. If you have health concerns that you think are linked to mould, the guide says to discuss them with a healthcare professional.

Cleaning only helps if the cause is dealt with afterward. Dry the area, correct the ventilation habit and repair failed joints before the mould has a chance to return.

When to call a professional

Health Canada's Healthy Home Guide suggests considering a professional for large areas of mould or when it keeps coming back after you clean it. The infographic likewise says to hire a professional if the affected area is larger than a small patch. Because tile and grout are not the waterproof layer, a suspected membrane problem is a question for a qualified contractor to assess before anything is covered up.

If you are weighing a repair against a rebuild, see the bathroom renovations page. Permit needs and building code requirements depend on the scope of work, so confirm them with your municipal building department before starting.