How to Get Rid of Mould in the Bathroom for Good

Bathroom mould is one of the most common household problems in Australia. Adelaide's warm, humid conditions during summer and the damp, cool conditions of winter create near-ideal conditions for mould growth in bathrooms — particularly in rooms with inadequate ventilation.

Most people's response is to spray bleach on the visible mould, see it lighten or disappear, and consider it dealt with. Within weeks it's back. Then the cycle repeats.

This keeps happening because spraying bleach on mould surface-treats a problem with a root cause that hasn't been addressed. This post covers both: how to properly remove existing bathroom mould, and what actually prevents it from returning.

---

Understanding Bathroom Mould

Mould is a fungus. In bathrooms, the most common types are Cladosporium, Aspergillus, and in some conditions, Stachybotrys (the notorious "black mould"). For the purposes of bathroom cleaning and health, the genus matters less than the location and extent.

What mould needs to grow:

• Moisture
• Warmth
• Organic material to feed on (grout, sealant, soap scum, dust)
• Poor air circulation

Bathrooms provide all four in abundance. The question is which of these can be reduced.

Where bathroom mould most commonly grows:

• Grout between tiles (particularly shower grout)
• Silicone sealant around the shower base, bath and basin
• The rubber seal of the washing machine (if in bathroom/laundry)
• The exhaust fan cover
• The ceiling — particularly in corners near the shower
• Windowsills and frames

---

Treating Existing Mould: Surface vs Penetrating

The most important distinction in mould treatment:

Surface mould sits on the outer surface of grout or sealant. It can be killed and removed with cleaning. This is what the bleach treatment addresses — and when done properly (not just a quick spray and wipe), it's effective.

Penetrating mould has grown into the porous material — through the grout surface and into the body of the grout, or through the outer layer of silicone. When you spray bleach and the mould stain doesn't change colour or lighten at all, the mould has penetrated below the surface. Surface treatment won't reach it.

Test: Apply bleach to the mouldy area and leave for 10–15 minutes. If the staining lightens significantly, it's predominantly surface mould and is treatable. If the staining doesn't change at all, the mould has penetrated and the grout or sealant may need to be replaced.

---

Step-by-Step: Treating Surface Bathroom Mould

What You Need

• Household bleach
• Spray bottle
• Old toothbrush or stiff grout brush
• Microfibre cloths
• Rubber gloves
• Ventilation (open window, exhaust fan on)

On Grout

1. Ventilate the bathroom — windows open, exhaust fan on. Bleach fumes are irritating and the space is small.

2. Apply bleach directly to the mouldy grout lines. For horizontal surfaces (floor grout), spray directly. For vertical surfaces (wall tile grout), apply with a cloth or cotton wool ball to prevent runoff.

3. Leave to dwell for 10–20 minutes minimum. This is where most DIY mould treatments fail — the product is wiped off before it has had time to work. The bleach needs sustained contact time to penetrate the grout surface and kill the mould.

4. Scrub with a stiff grout brush or old toothbrush. Work along the grout line, not across the tiles.

5. Rinse thoroughly with water.

6. Assess: if significant staining remains, repeat the application and extend the dwell time.

For particularly stubborn mould: apply bleach paste (mix bleach with a small amount of bicarbonate of soda to create a thick paste that stays on vertical surfaces) and leave for 30–60 minutes.

On Sealant

Mould in sealant (the silicone bead around the shower base, bath edges and basin) is more difficult. Silicone is less porous than grout but mould can grow on its surface and in crevices.

1. Apply bleach directly to the mouldy sealant. Use cotton wool balls or strips of tissue soaked in bleach pressed against the sealant — this maintains contact with the product and prevents runoff.

2. Leave for 30–60 minutes.

3. Remove the cotton wool and wipe the sealant with a damp cloth.

If the sealant has mould that doesn't respond to extended bleach treatment — or if the sealant is cracked, lifting or deteriorating — it needs to be removed and replaced. Mouldy, damaged silicone cannot be permanently cleaned; it needs to come out.

Replacing sealant: Use a silicone removal tool or sharp blade to cut away the existing sealant. Dry the area completely. Apply new mould-resistant silicone (specifically formulated for bathrooms — it contains a fungicide) using a caulking gun. Smooth with a damp finger or tool. Allow to cure for 24 hours before water contact.

On the Bathroom Ceiling

Ceiling mould in bathrooms is common and requires care on a soft surface like plaster or painted surfaces.

1. Mix a diluted bleach solution (1 part bleach to 4 parts water).
2. Apply to the affected ceiling area with a cloth or sponge on a handle — do not pour bleach onto a ceiling directly.
3. Leave for 15 minutes.
4. Wipe clean with a damp cloth.
5. Once completely dry, apply a mould-resistant ceiling paint to the affected area.

If the ceiling mould is extensive, or if it returns quickly after treatment, the underlying cause is likely a ventilation problem (inadequate exhaust fan) or a leak from above. Address the root cause before repainting.

---

Prevention: How to Stop Mould Coming Back

This is the part most people skip — and why their mould comes back. Treating existing mould without addressing the root causes is a cycle.

1. Fix the Ventilation

Bathroom mould is fundamentally a moisture problem. The moisture comes primarily from shower steam that isn't adequately extracted.

Exhaust fan requirements:

• The fan must be rated for the volume of your bathroom (check the product label — capacity is measured in litres per second)
• It must be turned on before you shower and remain on for at least 15–20 minutes after you finish
• The fan duct must vent to outside, not into the roof cavity (this is a common installation error that causes roof cavity mould problems)

If you don't have an exhaust fan: A small portable dehumidifier in the bathroom significantly reduces moisture levels. Open a window after showering.

Check your existing fan: If the fan is running but the bathroom still steams up, the fan may be undersized, the duct may be blocked, or it may vent into the roof rather than to outside.

2. Squeegee After Every Shower

A 20-second squeegee of the shower walls and glass screen after every shower removes the majority of water from tile and glass surfaces. Less standing water means dramatically reduced moisture and less surface for mould to establish on.

This single habit reduces mould growth in shower areas significantly.

3. Keep Surfaces Dry

Wipe down the basin area after use. Don't leave damp towels on the floor or draped over bathroom surfaces. Hang bath mats to dry between uses rather than leaving them flat on the floor.

4. Treat Grout and Sealant Preventively

Clean grout monthly with a tile cleaner. Apply a grout sealer annually — sealed grout is significantly more mould-resistant than unsealed grout.

Consider using mould-resistant silicone sealant when sealant is next replaced. It makes a genuine difference in high-moisture areas.

5. Address Any Water Leaks

A slow leak from the shower base, around plumbing fittings, or from the bath can introduce persistent moisture behind tiles and walls. This creates mould inside the wall that produces musty smell in your home and eventually becomes visible on the tile surface. If mould keeps returning in the same location despite surface treatment, consider whether there's a leak behind the surface.

---

When It's Beyond Cleaning

If mould:

• Returns within days or weeks of cleaning
• Covers an extensive area of ceiling or multiple walls
• Is accompanied by musty smell that persists after cleaning
• Has caused visible damage to plaster, tile backing or building materials

...you may be dealing with structural mould from moisture inside the wall or ceiling. This requires a builder or mould remediation specialist — not a cleaning service. Surface cleaning doesn't address mould inside building cavities.

---

For bathroom mould treatment as part of a deep clean or end-of-lease service, contact PF Cleaning Adelaide. We assess what's cleanable and what needs more than cleaning — and we're honest about the distinction.

---

Written by Victor Jacon — Master Cleaner & Home Detailing Specialist, Founder of PF Cleaning Adelaide.