Wet area liquid membrane thickness key

This article, based on BRANZ’s monthly technical newsletter Guideline, explores why dry film thickness is sometimes insufficient for waterproofing membranes 

We have heard of issues around installation of waterproofing membranes in wet areas, particularly tiled showers. In fact, it’s so prevalent that the Government has said it will introduce a new LBP waterproofing licence specifically to avoid the large number of issues. One of the problems is that, where a liquid membrane is used in some installations, it is applied too thinly.  

Key Building Code compliance considerations:  

• B2 Durability – the membranes must have a service life of a minimum 15 years. 

• E3 Internal moisture – surfaces of building elements likely to be splashed must be constructed in a way that prevents water splash from penetrating behind linings or into concealed spaces. 

E3/AS2 Internal wet-area membrane systems references the Waterproofing Membrane Association Inc (WMAI) Code of Practice for Internal Wet Area Membrane Systems. The referenced 2020 edition is available for free download in E3/AS2 while an updated 2024 edition can be purchased from the WMAI website. 

The Code of Practice says that one of the factors for successful liquid membrane application is “ensuring it achieves the film build required by the supplier’s handling/installation instructions”.  

Manufacturer/supplier requirements affecting thickness typically include: 

• A minimum wet film or dry film thickness. 

• The minimum number of coats that must be applied (very often two coats). 

• Application rate (such as 1.5L per m2). 

A wet film thickness gauge is an easy way to ensure adequate membrane coverage during the application of each coat. These gauges are readily available, are inexpensive and provide a simple robust method to confirm the application requirements are met.  

While building consent authorities commonly require an inspection after application of the membrane and before tiling for work that requires building consent, it can be difficult for inspectors to effectively check the thickness of the membrane. 

BRANZ recommends that installers take photographs as they go to record what they are doing. 

Many liquid membranes for internal wet areas carry a BRANZ Appraisal. Check details and requirements in both the manufacturer’s installation documents and the Appraisal before applying a membrane. There are many requirements to meet beyond just achieving the right thickness. 

BRANZ practical advice 

Before applying liquid-applied membranes, prime as required using a primer recommended by the manufacturer. 

Membranes must be applied according to the manufacturer’s specified number of coats to achieve dry film thickness, and be allowed to fully cure between coats and before flood testing (generally at least 24 hours at 20°C). Don’t apply heat to speed the curing process as it may cause the membrane to skin while remaining uncured below the skin. Ensure the membrane has adhered to the outlet flanges. 

Glass fibre-reinforced resins are less flexible than other liquid-applied membranes, so they must have a rigid substrate. Bond breakers may be required at changes of direction. 

Waterproofing LBPs incoming 

A new LBP licence class for waterproofing is due to be announced this year. The changes were first announced in April 2025. When the licence class was announced, Building and Construction Minister Chris Penk said: “(We are) establishing a new waterproofing licence class for LBPs so consumers can be confident those completing wet area bathrooms and level-entry showers are suitably qualified and accountable for their work.”