CALCULATOR
Waterproofing membrane calculator
Estimate litres required for Gripset 38FC, Gripset Betta, and RLA WPM Class III. Sized for wet areas, balconies and podium decks across the Hunter coast.
Waterproof membrane coverage is calculated as litres per square metre per coat, multiplied by the number of coats. Most liquid-applied membranes call for two coats; balconies and exposed substrates usually need three. The calculator below estimates litres required and the right pack size to order for the three liquid membranes we carry.
How waterproofing coverage is calculated
Liquid membrane consumption is set by the wet film thickness the TDS calls for, and the number of coats. For wet-area undertile membranes, that’s typically 0.5–0.65 L/m² per coat at the dry film thickness required to achieve Class III to AS/NZ4858. Two coats is the minimum; three coats is industry standard for balconies and external decks.
- Wet areas (showers, bathrooms, laundries) — 2 coats. Use the recommended primer first; never skip it.
- Balconies + podium decks — 3 coats. Reinforce internal corners with bondbreaker tape (Elastoproof B50 or equivalent).
- Roof-mounted plant rooms + machine areas — 3 coats minimum, with reinforcing fabric at high-stress joints.
Always add a 10% reserve for tape lap, joint reinforcing, and small spills.
Typical coverage rates we use
| Membrane | L/m² per coat | Coats (wet) | Pack sizes |
|---|---|---|---|
| Gripset 38FC | 0.5 | 2 (3 for balcony) | 4 L, 15 L |
| Gripset Betta | 0.6 | 2 (3 for balcony) | 4 L, 15 L |
| RLA WPM Class III | 0.65 | 2 (3 for balcony) | 4 L, 15 L |
What changes real-world consumption
Porosity of the substrate
Fresh sand-cement screeds and porous AAC blockwork drink the first coat. Plan for an extra 0.1–0.15 L/m² on coat one — the porous primer reduces but doesn’t eliminate this.
Corner + junction reinforcing
Wall-to-floor corners and pipe penetrations need bondbreaker tape (Elastoproof B50) plus an additional embedded coat. For a typical 2 m × 2.5 m shower, that’s another 0.5–0.8 L on top of the flat-area total.
Application method
Brushed corners use noticeably more product than rolled flats. Reserve 15–20% for brush work around drains, hobs and corners.
Hunter-coast exposure
Salt-air coastal balconies (Merewether, Redhead, Salamander Bay) need three coats minimum and consume 30–40% more product than sheltered internal wet areas. Our balcony checklist walks through the detail.
Membranes we stock for the Hunter
Gripset 38FC
SBR fast-cure undertile membrane. Class III to AS/NZ4858 and AS4654. 4 L & 15 L tubs.
Gripset GP Primer
The primer that goes under 38FC. General purpose, water-based, low VOC. Don’t skip it.
RLA WPM Class III
Water-based SBR latex undertile membrane. Class III certified. 4 L & 15 L pails.
Frequently asked questions
How many coats do I need for a bathroom?
Two coats of liquid membrane over the recommended primer, with bondbreaker tape at all internal corners. Three coats if the bathroom is on an upper storey above habitable space, or if the substrate has movement potential.
Do I need primer under waterproofing?
Yes. Always. Primer seals porosity, promotes adhesion, and reduces the membrane consumption on coat one. Skipping the primer is the single most common cause of bond-failure callbacks we see.
Is the calculator right for balcony waterproofing?
The calculator sizes for two coats by default. For balconies, set coats to 3 and add 15% for corners and reinforcing. Our balcony waterproofing checklist walks through Hunter-coast specifics.
Which membrane should I pick — Gripset or RLA?
Both are Class III SBR membranes; the differences are application-specific. Our RLA vs Gripset post compares them job-by-job — fast-cure jobs lean Gripset 38FC; long-runtime indoor wet areas often lean RLA WPM.
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