Can I paint over mouldy grout instead of cleaning it?

No — painting over mould without killing and removing it first will not solve the problem. The mould will continue to grow beneath the surface and will eventually break through. Always clean and treat the mould first, allow the surface to dry fully, and then consider a mould-resistant paint or sealer if appropriate.

How often should I replace bathroom silicone?

Quality bathroom silicone that is well-maintained can last many years, but as a general guide it's worth inspecting it annually. Replace it when you notice cracking, loss of adhesion, deep discolouration that won't clean off, or any gaps forming between the silicone and the tile or bath surface.

Is mould in my bathroom a health risk?

Yes, mould can pose health risks — particularly for people with asthma, allergies, or compromised immune systems. Common bathroom moulds can irritate the respiratory system and skin. Addressing mould promptly and improving ventilation is important for a healthy home environment.

What's the difference between mould-resistant silicone and standard silicone?

Mould-resistant (or anti-fungal) silicone contains additives that inhibit the growth of mould and mildew on the surface of the sealant. It won't prevent mould entirely if ventilation is poor, but it significantly slows regrowth compared to standard silicone. Always choose a bathroom-grade, mould-resistant product for wet areas.

Do I need a building consent to re-waterproof my shower?

Re-waterproofing a shower as part of a like-for-like repair generally does not require a building consent, but if you are making changes to the layout or structure, consent may be needed. Check with your local council or visit MBIE Building Performance at building.govt.nz if you're unsure. Waterproofing work in wet areas must still comply with NZ Building Code clause E3.

Stop Mould Growing in Bathroom Silicone & Grout NZ

Mould in bathroom silicone and grout is one of the most frustrating problems NZ homeowners face — and given our damp climate, it's incredibly common. The good news is that with the right approach, you can remove existing mould and take practical steps to stop it coming back for good.

Why Does Mould Grow in Bathroom Silicone and Grout?

Mould thrives where there is moisture, warmth, and a food source — and bathrooms tick all three boxes. Silicone sealant and grout are particularly vulnerable because they are porous or textured enough to trap soap scum, body oils, and mineral deposits, giving mould spores exactly what they need to take hold.

In New Zealand, many homes — especially older ones — have inadequate bathroom ventilation, which means steam and condensation linger long after a shower. BRANZ research consistently highlights poor ventilation as a leading cause of moisture damage and mould growth in NZ homes.

Assessing the Problem First

Before reaching for the bleach, take a good look at what you're dealing with:

Surface mould on grout or silicone that is still intact and well-adhered can usually be cleaned and treated.
Mould that has penetrated deep into silicone — where the silicone looks grey or black all the way through — means the silicone needs to be removed and replaced entirely.
Cracked or crumbling grout should be raked out and regrouted, as gaps allow water to get behind tiles and cause far more serious damage.
If you notice soft walls, swelling cabinetry, or a persistent musty smell even after cleaning, you may have a deeper moisture problem that needs professional assessment.

How to Remove Mould from Grout

What you'll need: rubber gloves, safety glasses, an old toothbrush or grout brush, a spray bottle, and a suitable cleaner.

Ventilate the room well — open windows and run the extractor fan.
Apply a mould and mildew remover or a diluted bleach solution (follow product label directions carefully) to the affected grout. Allow it to dwell for the recommended time.
Scrub firmly with a stiff grout brush or old toothbrush.
Rinse thoroughly with clean water and dry the surface.
Once dry, apply a grout sealer to help repel future moisture and staining. Products are available from Mitre 10 and Bunnings.

Safety note: Never mix bleach with other cleaning products, especially ammonia-based cleaners — this produces toxic fumes. Always work in a well-ventilated space and wear gloves and eye protection.

How to Remove and Replace Mouldy Silicone

If the silicone is deeply discoloured or has lost its bond, cleaning alone won't fix it. You'll need to remove it and apply fresh sealant.

Use a silicone removal tool or sharp utility knife to carefully cut and peel away the old silicone. Take care not to scratch tiles or the bath/shower surface.
Remove any residue with a silicone remover product and a cloth.
Clean the joint thoroughly and allow it to dry completely — this step is critical.
Apply mould-resistant bathroom silicone using a caulking gun, tooling the bead smooth with a wet finger or caulking tool.
Allow to cure fully before exposing to water (check the product's cure time — typically at least 24 hours).

Look for silicone labelled "mould resistant" or "anti-fungal" — these contain additives that slow mould growth. Sika and other reputable brands offer suitable bathroom-grade products.

Preventing Mould from Coming Back

This is where most homeowners go wrong — they clean the mould but don't address the underlying cause. Here's how to keep it away:

Improve Ventilation

Run your extractor fan during every shower and for at least 15–20 minutes afterwards. Under the Healthy Homes Standards, rental properties must have adequate bathroom ventilation — but it's good practice for owner-occupiers too.
If your fan is old or underpowered, consider upgrading to a higher-capacity model.
Open a window when possible to allow fresh air circulation.

Wipe Down Surfaces After Use

A quick wipe of tiles, the shower screen, and silicone joints after each use dramatically reduces the moisture available for mould.
A squeegee kept in the shower is one of the simplest and most effective tools you can own.

Keep Grout Sealed

Re-seal grout every year or two with a quality grout sealer. This reduces porosity and makes cleaning much easier.

Use a Bathroom-Specific Paint

If your bathroom walls are painted, use a moisture-resistant paint with mould inhibitors. Resene and Dulux both offer bathroom-specific paint ranges formulated for humid environments.

Check for Leaks

Sometimes persistent mould is a sign of a slow leak behind the wall. If mould keeps returning in the same spot despite good ventilation and cleaning, it's worth investigating further.

When to Call a Professional

Some situations go beyond a DIY fix:

Widespread mould or water damage behind tiles or walls requires a qualified waterproofer or tiler to strip back, dry out, and re-waterproof the area to meet NZ Building Code clause E3 (internal moisture).
Re-waterproofing a wet area (shower, bath surround) is specialist work — improper waterproofing can lead to costly structural damage.
If you suspect a plumbing leak is contributing to the problem, this must be investigated and repaired by a licensed plumber. Plumbing work in NZ is restricted work under the Plumbers, Gasfitters and Drainlayers Board — don't attempt to open walls or repair pipes yourself.

For peace of mind, find a qualified local waterproofer or tiler on tradedirectory.co.nz who can assess the situation and carry out any remedial work to the required standard.

How Do I Stop Mould Growing in Bathroom Silicone and Grout?