How to Waterproof a Shower Floor Hob to Stop Water Escaping

A leaking shower hob is one of the most common causes of water damage in NZ homes, and it's almost always down to inadequate or failed waterproofing. The good news is that with the right preparation, materials, and technique, a DIYer can waterproof a shower hob to a high standard — though there are situations where a licensed waterproofer is the smarter call.

Understanding the Shower Hob and Why It Leaks

The shower hob (also called a shower threshold or kerb) is the raised lip at the shower entry that contains water on the floor. It's a vulnerable spot because it sits at the junction of the shower floor, the wall, and the open entry — three different planes meeting in one place. Water pressure, movement, and poor membrane application at these junctions are the usual culprits when leaks occur.

Under the NZ Building Code, specifically Clause E3 (Internal Moisture), wet areas like showers must be waterproofed to prevent moisture damaging the building structure. Getting this right isn't just good practice — it's a legal requirement.

What You'll Need

• Waterproofing membrane (liquid-applied, such as products from Ardex or Mapei)
• Reinforcing fabric/tape (fibreglass mesh tape for corners and junctions)
• Primer (as recommended by your membrane manufacturer)
• Paint brush and roller suitable for membrane application
• Sandpaper or grinder for surface preparation
• Flexible sealant (silicone, compatible with your system)
• PPE: gloves, safety glasses, and adequate ventilation

Always check the manufacturer's technical data sheet for your chosen membrane — application rates, drying times, and compatibility with your substrate (concrete, fibre cement, or GIB Aqualine) will vary.

Step-by-Step: Waterproofing the Shower Hob

1. Prepare the Surface

All surfaces must be clean, dry, and structurally sound. Remove any old sealant, grout, or failed membrane. Sand or grind back any high spots. Fill cracks or voids with a suitable filler and allow to cure fully. Any movement or flex in the substrate will cause the membrane to fail prematurely.

2. Prime the Substrate

Apply the manufacturer's recommended primer to the hob and surrounding floor and wall areas (typically 100–200 mm up the wall and onto the floor). Allow to dry as directed — skipping this step is a common DIY mistake that leads to poor adhesion.

3. Embed Reinforcing Tape at All Junctions

This is the most critical step. Apply a first coat of membrane, then immediately embed fibreglass mesh tape into the wet membrane at:

• The internal corner where the hob meets the shower floor
• The internal corners where the hob meets the walls
• The top of the hob
• The external corner at the shower entry (hob face meeting the bathroom floor)

Smooth the tape flat with no bubbles or wrinkles. These junctions are where movement and stress concentrate, and the tape bridges minor cracking.

4. Apply Multiple Coats of Membrane

Once the tape coat has cured, apply at least two full coats of liquid membrane over the entire hob and the required surrounding area, allowing each coat to dry between applications. Most manufacturers specify a minimum dry film thickness — check your product's data sheet. Typical coverage is around 1–1.5 kg/m² per coat, but this varies.

Extend the membrane at least 150 mm up the walls from the shower floor, and ensure full coverage over the top and face of the hob.

5. Seal the Hob Entry with Flexible Silicone

The junction between the hob and the bathroom floor outside the shower is a movement joint — never grout this. Once tiling is complete, this joint must be filled with a flexible, mould-resistant silicone sealant. Products from Sika are widely used for this purpose in NZ. Choose a colour to match your grout for a neat finish.

6. Allow Full Cure Before Tiling

Do not tile over the membrane until it has fully cured — check the data sheet for minimum cure times, which can range from several hours to 24+ hours depending on temperature and humidity. NZ's variable climate means curing can take longer in cooler or humid conditions.

Membrane Comparison at a Glance

For a shower hob specifically, a liquid-applied membrane with fabric reinforcement at junctions is the most widely recommended approach in NZ trade practice.

Common Mistakes to Avoid

• Skipping primer — leads to delamination
• Not reinforcing junctions — the number one cause of hob leaks
• Insufficient membrane thickness — always check the data sheet
• Grouting the movement joint at the hob entry — grout will crack and allow water in
• Tiling too soon — wet membrane under tiles will fail

When to Call a Professional

Waterproofing a shower is not restricted work under NZ law, so a competent homeowner can do it. However, you should seriously consider hiring a qualified waterproofer (find one on tradedirectory.co.nz) if:

• You are renovating a rental property (Healthy Homes compliance — see Tenancy Services)
• The existing structure shows signs of rot, mould, or significant water damage
• Your shower is tiled over a timber subfloor
• You are unsure about the substrate type or condition
• A building consent is involved — check with your local council whether consent is required for your specific project

A licensed waterproofer will also be able to provide a producer statement or warranty, which can be valuable if you're selling the home. For guidance on what work requires consent, visit MBIE Building Performance.

Sources & Further Reading

• MBIE Building Performance — Building Code Clause E3
• BRANZ — Wet Area Waterproofing Guidance
• Ardex NZ — Waterproofing Products
• Mapei NZ — Waterproofing Systems
• Sika NZ — Sealants and Waterproofing
• GIB — Wet Area Systems