Cracked grout along the edges and corners of your shower is one of the most common tiling problems NZ homeowners face — and the good news is it's very fixable. The root cause is almost always movement: shower walls, floors, and fixtures expand and contract at different rates, and rigid grout simply can't cope with that flex. The correct long-term solution is to replace the grout in those areas with a flexible silicone sealant, not to re-grout with the same product.
Why Grout Cracks at Shower Edges
Grout is a cementitious material designed to fill the gaps between tiles on a flat plane. It is rigid when cured. Shower edges — where the wall meets the floor, where a tiled wall meets a shower tray, or where tiles meet a fixture like a tap flange — are movement joints. These spots experience:
- Thermal expansion and contraction as hot and cold water cycles through daily
- Structural movement in the building framing (especially in timber-framed NZ homes built to NZS 3604)
- Differential movement between dissimilar materials (e.g. a fibrous cement sheet wall versus a ceramic tile versus a acrylic shower base)
When rigid grout is used in these joints, it cracks — often within months. Once cracked, water gets behind the tiles, leading to mould, substrate damage, and potentially costly waterproofing failures. The NZ Building Code clause E3 requires internal wet areas to be waterproof, so keeping these joints sealed is not just cosmetic — it's a compliance matter.
The Right Material: Flexible Silicone Sealant
The industry-standard fix is a sanitary-grade silicone sealant applied to all change-of-plane joints and perimeter edges in a shower. Look for products labelled as:
- Mould-resistant / anti-fungal (essential in NZ's humid bathrooms)
- Sanitary silicone (formulated for wet areas)
- Paintable or colour-matched options if aesthetics matter
Well-known brands available at NZ hardware stores include products from Sika and others stocked at Bunnings and Mitre 10. Try to match the silicone colour to your existing grout as closely as possible — most ranges offer white, off-white, grey, and beige.
Step-by-Step: Removing Old Grout and Applying Silicone
1. Remove the old grout or failed silicone
- Use a grout rake, oscillating multi-tool, or a sharp utility knife to carefully remove all the old material from the joint. Be patient — leaving any old grout behind will cause the new silicone to fail.
- For old silicone, a silicone remover product helps soften it before scraping.
2. Clean and dry the joint thoroughly
- Wipe the joint with methylated spirits or isopropyl alcohol to remove soap scum, grease, and residue.
- Allow the joint to dry completely — at least 24–48 hours. Silicone will not bond properly to a damp surface. This is critical in NZ's cooler, more humid climates.
3. Mask the edges
- Apply painter's tape along both sides of the joint for a clean, professional finish.
4. Apply the silicone
- Cut the nozzle at a 45° angle slightly smaller than the joint width.
- Apply in one smooth, continuous bead. Don't stop and start.
- Tool the joint immediately with a wet finger or a silicone finishing tool, pressing the sealant firmly into the gap.
- Remove the tape while the silicone is still wet.
5. Cure time
- Most sanitary silicones are touch-dry in 1–2 hours but need 24–48 hours before the shower is used. Check the manufacturer's datasheet for the specific product you use.
Key Joints to Always Seal with Silicone
| Location | Why it moves |
|---|---|
| Floor-to-wall corner | Differential movement between floor slab/substrate and wall framing |
| Shower tray perimeter | Acrylic/fibreglass tray flexes under load |
| Around tap flanges and fixtures | Penetrations through the tile plane |
| Internal wall corners | Two wall planes move independently |
| Where tiles meet a glass screen | Glass and tile expand at different rates |
Preventing the Problem in New Installations
If you're tiling a new shower or renovating, ensure your tiler follows best practice from the outset:
- Waterproof the substrate properly before tiling — BRANZ has good guidance on wet area construction.
- Use movement joints (silicone, not grout) at all changes of plane and at regular intervals in large tiled areas.
- Ensure the substrate (e.g. GIB Aqualine or James Hardie wet-area sheet) is correctly fixed and waterproofed to Building Code requirements.
- Check whether your shower installation required a building consent — some wet area work may be exempt under Schedule 1, but always confirm with your local council before starting.
When to Call a Professional
While replacing grout with silicone at shower edges is a manageable DIY job for a careful homeowner, call a licensed tiler or builder if:
- Tiles are loose, hollow-sounding, or coming away from the wall — this suggests water has already penetrated behind the tiles and the substrate may be damaged.
- You find mould or soft, crumbling material behind removed tiles.
- The shower tray or liner appears to be leaking (water stains on the ceiling below, or wet subfloor).
- You're unsure whether the original waterproofing was done correctly.
Waterproofing failures can be expensive to remediate and may affect your home's compliance with the NZ Building Code. A qualified tiler or Licensed Building Practitioner can assess the extent of any damage and carry out repairs to a standard that protects your home long-term. You can find experienced local tilers on tradedirectory.co.nz.
Safety note: When working in a confined shower space, ensure good ventilation — silicone solvents and cleaners can cause irritation. Check WorkSafe NZ guidance on safe use of chemical products in enclosed spaces.
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