How to Fix a Leaking Internal Corner in Your Shower

A leaking internal corner in your shower is one of the most common moisture problems in NZ homes, and if left untreated it can cause serious damage to framing, linings, and subfloors. The good news is that if the leak is simply failed silicone sealant, it's a manageable DIY job. However, if the waterproofing membrane underneath is compromised, you'll need a licensed builder or tiler.

Why Shower Corners Leak

Internal corners are a high-movement zone. The wall and floor expand and contract at different rates, which means rigid grout cracks over time. The most common causes of a leaking corner include:

• Failed or mouldy silicone sealant — the most frequent culprit
• Cracked or missing grout that has allowed water to reach the substrate
• Damaged or poorly applied waterproofing membrane behind the tiles
• Movement in the building structure causing tiles or linings to shift

Under the NZ Building Code, wet areas like showers must be waterproofed to comply with Clause E3 (Internal Moisture). If your shower was built or retiled without proper waterproofing, a surface re-seal will only be a temporary fix.

Tools and Materials You'll Need

• Utility knife or oscillating multi-tool
• Grout rake or small chisel
• Stiff brush and cloth
• Mould-killer spray (e.g. a bleach-based cleaner)
• Neutral-cure silicone sealant rated for wet areas (look for bathroom/kitchen grade)
• Masking tape
• Silicone smoothing tool or a wetted finger

You can find suitable silicone sealants at Bunnings or Mitre 10. Choose a colour that matches your grout, and always check the product is suitable for continuous wet areas.

Step 1 — Diagnose the Problem First

Before you reach for the silicone, work out what you're actually dealing with.

• Press gently on the tiles near the corner. If they flex or feel hollow, the adhesive or substrate behind them may be wet and failing — this goes beyond a DIY re-seal.
• Check the grout lines for cracks, gaps, or discolouration.
• Look at the existing silicone — is it peeling, cracked, black with mould, or missing entirely?
• Run the shower for a few minutes, then check underneath (if accessible) or in the adjacent room for signs of moisture.

If the problem is isolated to failed silicone or cracked grout, carry on. If you suspect deeper waterproofing failure, skip to the "When to call a professional" section below.

Step 2 — Remove All Old Sealant and Grout

This step is critical. New silicone will not bond properly over old silicone.

1. Use a utility knife or oscillating multi-tool to carefully cut out all the old silicone from the corner joint.
2. If grout has been used in the corner (a common mistake — corners should always have a flexible sealant joint, not rigid grout), rake it out with a grout rake.
3. Clean the area thoroughly with a mould-killer spray and let it dry completely — ideally 24–48 hours without using the shower.
4. Wipe down with a dry cloth to remove any dust or residue.

Important: Never apply new silicone over mould or damp surfaces. The bond will fail quickly and the mould will return.

Step 3 — Apply New Silicone Sealant

1. Run masking tape along both sides of the corner joint to get a clean, straight line.
2. Cut the silicone cartridge nozzle at a 45-degree angle to match the width of your joint (usually 4–6 mm).
3. Apply a smooth, continuous bead of silicone into the corner, working from top to bottom.
4. Smooth the bead with a wetted finger or silicone tool in one firm stroke.
5. Remove the masking tape immediately before the silicone skins over.
6. Allow to cure fully — check the manufacturer's instructions, but most bathroom silicones need at least 24 hours before water contact.

Sika and other reputable brands offer neutral-cure silicones well suited to tiled shower environments. Avoid acetoxy-cure ("vinegar-smell") silicones on natural stone.

Step 4 — Address Any Cracked Grout

If grout lines elsewhere in the shower are cracked or porous, water will continue to find its way in even after you've re-sealed the corner. Rake out any damaged grout and re-grout using a waterproof grout product. Mapei and Ardex both offer quality grout products available through tile and trade suppliers in NZ.

Preventing Future Leaks

• Never grout internal corners — always use flexible silicone sealant.
• Re-inspect and re-seal shower corners every 3–5 years as part of routine home maintenance.
• Ensure your bathroom has adequate ventilation to reduce moisture build-up — this is also a requirement under the Healthy Homes Standards for rental properties.
• BRANZ has good guidance on wet area construction and maintenance best practice.

When to Call a Professional

Call a Licensed Building Practitioner (LBP) or qualified tiler if:

• Tiles are loose, hollow-sounding, or moving
• You can see or smell mould in the wall cavity or subfloor
• The shower base or liner appears damaged
• Water is appearing in an adjacent room or ceiling below
• The shower is more than 15–20 years old and has never been re-waterproofed

In these cases, the waterproofing membrane likely needs to be replaced — a job that involves stripping tiles and relining the shower to meet current NZ Building Code requirements. This is restricted building work if it affects the weathertightness of the home, meaning it must be carried out or supervised by an LBP.

You can find qualified local builders and tilers on tradedirectory.co.nz.

Sources & Further Reading

• MBIE Building Performance — NZ Building Code
• BRANZ — Wet Area Guidance
• Licensed Building Practitioners (LBP)
• Tenancy Services — Healthy Homes Standards
• Ardex NZ — Tile & Waterproofing Products
• Sika NZ — Sealants & Adhesives