Why Is My Shower Leaking and Causing Damage?

A leaking shower is one of the most common — and most damaging — problems in a New Zealand home. Water finds its way through failed seals, cracked tiles, or faulty waterproofing membranes, then silently soaks into the framing, substrate, and lining behind your walls. Left unchecked, it can rot timber, corrode fixings, and create the perfect conditions for mould. The good news is that many causes are identifiable early, and some minor fixes are well within a homeowner's reach.

Why Shower Leaks Happen

Showers are one of the most demanding wet areas in a home. They must comply with NZ Building Code clause E3 (Internal Moisture), which requires wet areas to be waterproofed so water cannot penetrate to adjoining spaces. When that waterproofing system fails — even partially — damage follows quickly.

Common root causes include:

• Failed or missing silicone sealant at junctions (floor-to-wall, wall corners, around the tray)
• Cracked or missing grout between tiles, allowing water to reach the substrate
• Deteriorated waterproofing membrane beneath tiles (especially in showers over 10–15 years old)
• Poorly installed or cracked shower tray or base
• Leaking shower mixer or waste fitting (plumbing connections)
• Inadequate falls on the shower floor, causing water to pool and find weak points
• Substrate damage — swollen or delaminated fibre-cement sheet or compressed fibre board behind tiles

BRANZ research consistently identifies wet area waterproofing failures as a leading cause of building defects in NZ homes.

Symptoms and What They Tell You

How to Diagnose the Problem Step by Step

Step 1 – Do a visual inspection

With the shower dry, examine all silicone joints at the floor-wall junction, wall corners, and around any penetrations (waste, mixer). Look for gaps, cracks, discolouration, or mould that won't clean off. Check grout lines for cracks or missing sections.

Step 2 – Tap test the tiles

Using a coin or knuckle, tap each tile systematically. A hollow sound indicates the tile has debonded from the substrate — a sign water has already got in underneath.

Step 3 – Plug and flood test

Block the waste with a rubber plug or bag of water, fill the shower floor with 20–30 mm of water, and mark the water level with tape. Leave for 30–60 minutes and check whether the level drops. If it does, water is escaping through the floor or waste fitting. Also check the adjoining wall or ceiling during this time.

Step 4 – Check the waste fitting

Remove the waste cover and inspect the seal between the waste body and the waterproofing membrane. This is a very common failure point, particularly in older showers.

Step 5 – Check supply pipes (plumbing)

If water appears below the bathroom or the leak only occurs when the shower is running (not during a flood test), the issue may be a leaking supply pipe or mixer connection rather than the waterproofing. This is restricted plumbing work — see below.

What You Can Safely Fix Yourself

Some minor maintenance is well within a confident homeowner's ability:

• Re-siliconing joints: Remove old silicone completely using a silicone remover tool and solvent, clean and dry the surface thoroughly, then apply a fresh bead of mould-resistant bathroom silicone. Products from Sika or similar are widely available at Mitre 10 or Bunnings. Allow full cure time (usually 24–48 hours) before using the shower.
• Re-grouting: Rake out cracked or crumbling grout with a grout saw, clean the joints, and apply fresh grout. Seal with a penetrating grout sealer once cured.
• Improving ventilation: Ensure your extractor fan is working properly and venting to outside — poor ventilation accelerates sealant and grout deterioration.

Important: These fixes only address surface-level issues. If the waterproofing membrane beneath the tiles has failed, re-grouting and re-siliconing will not solve the underlying problem.

What Requires a Licensed Professional

Restricted plumbing work

Any work on supply pipes, waste pipes, or mixer/tapware connections is restricted work under the Plumbers, Gasfitters and Drainlayers Act 2006. It must be carried out by a licensed plumber registered with the Plumbers, Gasfitters & Drainlayers Board (PGDB). Doing this work yourself is illegal and could void your insurance.

Waterproofing membrane replacement

If the membrane has failed, the tiles must generally be removed, the substrate assessed (and replaced if damaged), and a new compliant membrane installed before retiling. This is specialist work. A qualified waterproofer will apply a system — such as those from Ardex or Mapei — that meets NZ Building Code E3 requirements. If the work is significant, it may require a Licensed Building Practitioner (LBP); check at lbp.govt.nz.

Structural damage

If framing timbers are rotted or compromised, structural repairs must be assessed and carried out by qualified tradespeople.

⚠️ Safety note: Mould resulting from long-term leaks can include species harmful to health. If you find extensive mould growth inside wall cavities, seek professional assessment before disturbing it. See WorkSafe NZ guidance on mould in buildings.

How to Prevent It Recurring

• Inspect silicone and grout annually and touch up any deterioration before it becomes a leak
• Squeegee or wipe down shower walls after use to reduce moisture load
• Run the extractor fan during and for at least 10 minutes after every shower
• Avoid harsh chemical cleaners that degrade grout and silicone over time
• When retiling or renovating, insist on a compliant waterproofing system installed by an experienced waterproofer — don't cut corners on membrane quality or coverage
• For rental properties, note that Tenancy Services Healthy Homes standards require adequate ventilation in bathrooms

When to Call a Professional

Call a qualified waterproofer or tiler if:

• Your flood test shows water is escaping through the floor or walls
• Tiles are hollow, loose, or lifting
• You can see or smell mould inside wall cavities
• There is visible damage to the ceiling or wall on the other side of the shower
• The shower is more than 10–15 years old and has never been resealed or inspected

Call a licensed plumber if the leak is coming from supply pipes, the mixer, or the waste connection.

A qualified local waterproofer or tiler can assess the full extent of the damage and carry out compliant repairs. You can find experienced tradespeople in your area on tradedirectory.co.nz.

Sources & Further Reading

• BRANZ – Wet area waterproofing guidance
• MBIE Building Performance – NZ Building Code clause E3
• Plumbers, Gasfitters & Drainlayers Board – find a licensed plumber
• Licensed Building Practitioners – find an LBP
• Ardex – waterproofing systems
• WorkSafe NZ – mould in buildings