How to apply waterproofing membrane on acrylic render?

AlbertFUSCUS

Hi Everyone,

I am currently renovating my bathroom which has rendered brick walls and I am planning on waterproofing it using Crommelin undertile waterproofing membrane. I assume the current render is a cement based render, but when removing the old tiles, some chunks of render fell off, so I will have to touch it up and I was planning on using acrylic render to do so. Now, here's my questions:

• Can the Crommelin membrane be applied on Acrylic render? The specification sheet mentions cement render, but not sure if it's compatible with both.
• The current render has a few 6mm drill holes. What's the best approach to waterproof? Fill them with Crommelin bond breaker sealant and then apply the membrane as usual?

Thanks in advance for your help.

EricL

Hello @AlbertFUSCUS

Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about the water proofing membrane.

Since the existing walls are already cement-rendered brick, I would suggest sticking with a cement-based render for the repairs rather than introducing acrylic render. This will give you a consistent substrate and should make preparing the wall for the Crommelin Undertile Waterproofing Membrane more straightforward. Although some waterproofing products can be used over acrylic-modified surfaces with the appropriate preparation or primer, if Crommelin's specification specifically nominates cement render and does not clearly nominate acrylic render, I wouldn't assume the two are interchangeable.

For the areas where chunks of the existing render have come away, remove any remaining loose or drummy material and patch these sections with cement render, allowing it to cure properly before waterproofing. Using cement render also avoids potentially needing a special primer or additional compatibility checks between the acrylic render and waterproofing membrane.

For the existing 6mm drill holes, I would also recommend filling these with cement-based render or a suitable cementitious repair material rather than filling the entire holes with bond breaker sealant. Bring the repairs flush with the surrounding wall and allow them to cure. Once the wall is sound, even and properly prepared, the waterproofing membrane can then be applied over the repaired areas as part of the complete waterproofing system. Crommelin bond breaker sealant is better reserved for locations where a flexible bond breaker is actually required, such as wall-to-floor junctions, corners and other changes of plane, rather than ordinary holes in the face of the render.

Just a friendly reminder that bathroom waterproofing is regulated building work in parts of Australia, and the requirements around who can carry it out vary between states and territories. Before applying the waterproofing yourself, I recommend checking with your local council or relevant state building authority to confirm the rules and regulations that apply in your area. This is particularly important if the bathroom work will eventually need to be inspected and pass compliance.

Let me call on our experienced members @Nailbag and @DIY4Knuckleheds for their recommendations.

If you need further assistance, please let us know.

Eric