How to Calculate Waterproofing Membrane Quantities for a Wet Area

Calculating waterproofing membrane quantities for a wet area is straightforward once you understand which surfaces need coverage and how the NZ Building Code defines those zones. Getting your quantities right before you buy saves money, avoids mid-job trips to the supplier, and ensures you meet the requirements of NZS 4858:2004 (Wet Area Membranes) and the Building Code clause E3 (Internal Moisture).

Understand What Needs to Be Waterproofed

Under the NZ Building Code and MBIE Building Performance guidance, wet areas include showers, baths, laundries, and any floor that receives direct water. The surfaces that must be waterproofed typically include:

• The entire shower floor (including the shower tray or former)
• Wall upstands — a minimum of 1,800 mm high in a shower enclosure, or to the full height of tiled walls
• Floor-to-wall junctions — a critical zone prone to movement and leakage
• Around penetrations — waste outlets, pipes, and fixings
• Bathroom floors outside the shower if they are tiled and could receive water

Always check your specific membrane manufacturer's technical data sheet, as some products have their own minimum coverage requirements. Ardex and Mapei both publish detailed application guides for their waterproofing systems.

Step 1 — Measure the Floor Area

Start with the floor plan dimensions. Measure the length and width of the wet area in metres and multiply them together.

Floor area (m²) = Length (m) × Width (m)

For a shower measuring 900 mm × 900 mm: 0.9 × 0.9 = 0.81 m²

For an irregular shape, break it into rectangles, calculate each, and add them together.

Step 2 — Calculate Wall Upstand Areas

Measure the perimeter of the wet area walls that require waterproofing, then multiply by the required upstand height.

Wall area (m²) = Perimeter (m) × Upstand height (m)

For a 900 × 900 mm shower with three waterproofed walls to 1,800 mm:

• Perimeter of three walls = 0.9 + 0.9 + 0.9 = 2.7 m
• Wall area = 2.7 × 1.8 = 4.86 m²

If you are waterproofing a full bathroom floor with a 100 mm upstand on all walls:

• Measure the full room perimeter and multiply by 0.1 m

Step 3 — Add Junction and Detail Areas

Floor-to-wall junctions, internal corners, and penetrations require reinforcing tape or fabric embedded in the membrane. While these don't add much to your membrane area calculation, they do affect how much detail membrane or bandage tape you need to order.

• Measure the total linear metres of all internal corners and floor-wall junctions
• Add approximately 300 mm extra per penetration (waste outlet, pipe, etc.)

Step 4 — Total the Areas and Apply a Wastage Factor

Add your floor area and wall upstand area together to get your gross wet area:

Total area = Floor area + Wall upstand area

Using the shower example: 0.81 + 4.86 = 5.67 m²

Now apply a wastage factor. For wet area membranes, a standard allowance is:

For our shower: 5.67 × 1.10 = 6.24 m² of membrane required.

Step 5 — Convert to Product Quantities

Check the coverage rate on your chosen membrane product's data sheet. Coverage rates vary significantly between products — liquid membranes are typically applied in two or more coats, and the data sheet will specify litres per m² per coat.

Example (guide only — always check your product's data sheet):

• If a product covers approximately 1 m² per litre per coat, and two coats are required
• You need roughly 2 litres per m²
• For 6.24 m²: approximately 12–13 litres total

Products from Sika, Ardex, and Mapei are commonly used in NZ and are available from PlaceMakers, Mitre 10, and Bunnings.

Account for Building Consent Requirements

Wet area waterproofing in a new build or significant renovation may require a building consent. Under Schedule 1 of the Building Act, some minor repair and maintenance work may be exempt, but always confirm with your local council before starting. MBIE Building Performance has clear guidance on when consent is needed.

Where consent is required, your membrane system must comply with NZS 4858 and the work may need to be inspected before tiling proceeds.

When to Call a Professional

While a competent DIYer can apply many liquid membrane systems, there are good reasons to engage a professional:

• Quantity surveyors can prepare accurate material take-offs for larger projects, helping you budget and procure correctly — find one on tradedirectory.co.nz
• Licensed tilers and waterproofers understand NZS 4858 requirements and can ensure your system is code-compliant
• If your wet area involves plumbing alterations (moving a waste outlet or supply pipes), this is restricted work under the Plumbers, Gasfitters and Drainlayers Act and must be carried out by a licensed tradesperson registered with the PGDB
• For larger bathroom renovations requiring consent, an LBP may need to carry out or supervise the work

Safety note: Some membrane products contain solvents or isocyanates. Always read the Safety Data Sheet, ensure adequate ventilation, and wear appropriate PPE as directed by WorkSafe NZ.

Sources & Further Reading

• MBIE Building Performance — Building Code clause E3
• BRANZ — Wet area construction guidance
• Ardex NZ — Waterproofing systems
• Mapei NZ — Waterproofing products
• Sika NZ — Construction waterproofing
• WorkSafe NZ — Safe use of construction chemicals