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How to waterproof over rendered and plastered brick walls?

AlbertFUSCUS

Hi Community,

I am currently renovating a bathroom where the walls were made out of brick, rendered, plastered and painted.
My plan is to tile the whole bathroom, and will have to waterproof underneath the tiles.

I have peeled all the paint, as it was in bad shape, but the plaster underneath is in good condition. My idea was to apply "Crommelin 15kg Pro-500 Undertile Waterproofing Membrane" over the plaster, but reading the datasheet it says that it is not suitable for plaster substrates. Now, what would be the best way to proceed? Can I seal the plaster with "Dulux Precision Sealer Binder" and then use the Crommelin waterproofing membrane? Do I have to look at a different waterproofing membrane? None of the brands seem to indicate their membranes for plaster wall.

Additionally, could you please clarify the difference between these two products:

Crommelin 15kg Pro-500 Undertile Waterproofing Membrane
Crommelin 15L Shower Waterproofing Membrane

Thanks in advance for your help.
Kind Regards,
Albert

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Accepted answers

EricL

Hello @AlbertFUSCUS

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

Before proceeding any further, I suggest having your walls checked for asbestos for safety. If your walls contain asbestos, you'll need the services of an asbestos removal specialist. If the test is negative, you can continue with your renovation plans.

It's great to hear that you've decided to renovate your bathroom. Yes, you've read correctly, standard plasterboard is not ideal for bathrooms and other wet areas. Before replacing any wall linings, I'd recommend discussing your plans with a qualified builder. They can assess the existing wall construction, confirm that the correct substrate is being used for your particular application, and ensure the wall assembly complies with the National Construction Code (NCC) and the manufacturer's installation requirements before waterproofing and tiling begin.

For bathroom and shower walls, I'd suggest replacing the existing lining with https://www.bunnings.com.au/james-hardie-2-88m-villaboard-lining-2400-x-1200-x-6mm_p0714174 . This 6mm fibre cement sheet is widely regarded as the industry standard across Australia for lining shower and bathroom walls. It offers excellent moisture resistance, remains dimensionally stable, and provides a strong, durable surface capable of supporting the weight of ceramic and porcelain tiles far better than regular plasterboard.

When it comes to waterproofing, the Crommelin Shower Waterproofing Membrane is the product specifically designed for most DIY bathroom renovations. It's easy to apply, has excellent adhesion, remains permanently flexible, and provides everything needed for a compliant shower, bathroom or laundry waterproofing system when used correctly.

The Crommelin PRO-500 takes things a step further. Its polyurethane-modified formulation allows it to build film thickness more quickly, dry faster, and better accommodate structural movement and demanding conditions. It also complies with the external waterproofing standard (AS 4654), making it suitable for balconies, decks and other exposed areas where greater durability and weather resistance are required.

For a standard residential shower over Villaboard or Aquachek, I'd recommend using the Crommelin Shower Waterproofing Membrane. It was designed specifically for this application and will provide excellent performance when installed in accordance with the manufacturer's instructions using the appropriate primer, bond breaker and reinforcing fabric.

Please note that waterproofing licensing requirements vary across Australia. Some states and territories require waterproofing to be carried out by a licensed waterproofer, while others permit the work to be completed by a suitably competent person in accordance with the NCC and relevant Australian Standards. Before commencing any waterproofing work, always check the requirements with your local building authority or certifier. Where required, ensure the appropriate compliance certificates and documentation are obtained for your records. Incorrectly installed waterproofing can lead to water damage, expensive repairs, insurance complications, and may not satisfy statutory building requirements.

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

Here is a handy guide that will help - The Right Foundation For A Bathroom Renovation

Please remember to wear personal protection such as gloves, goggles and mask.

If you have any other questions we can help with, please let us know.

Eric

All comments

EricL

Hello @AlbertFUSCUS

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

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

Here is a handy guide that will help - The Right Foundation For A Bathroom Renovation

Please remember to wear personal protection such as gloves, goggles and mask.

If you have any other questions we can help with, please let us know.

Eric

Nailbag

Hi @AlbertFUSCUS

Before taking things further, it would be wise to check with your local building authority as to the legality in your state to do your own waterproofing. For example NSW and QLD require waterproofing to be conducted by a professional.

I'm in Vic where it's legal. And despite renovating several bathrooms, I have always used a certified waterproofer as the work comes with a warranty. This is because it's arguably the most crucial stage to ensure no issues down-track with internal ingress damage and insurance coverage.

If you decide to go down that path, then gaining the advice of the waterproofer will establish what exactly is required. But it sounds like pealing the wall back past the damaged sections and install new Villaboard as @EricL suggested would be the go.

Nailbag

AlbertFUSCUS

Thanks for the quick reply.

Maybe I didn't explain myself correctly. There's no plasterboard on the walls. It's rendered walls that have been skimmed with plaster. So I can't rip it and install cement sheets.

Here's a photo of a section of the wall where the old vanity pulled a piece of plaster as it was being removed so that you can understand better the current layers.

How would you waterproof it in preparation for tiling?

Thanks

Albert

MitchellMc

Hi @AlbertFUSCUS,

Many thanks for clarifying that it’s a rendered masonry wall with a plaster skim coat rather than plasterboard.

In that case, I wouldn’t waterproof directly over the plaster skim unless Crommelin confirms that exact build-up is suitable. The concern is that plaster can be soft, moisture-sensitive and prone to delamination, so even if the membrane sticks initially, the plaster layer itself may not be a reliable substrate once the bathroom is tiled and in use.

The alternative to this is to sheet over the walls with villaboard. Also, you might like to check whether waterproofing the walls is a requirement. You could find that, unless it is a shower recess or a splash zone, the walls might not need to be waterproofed.

As waterproofing failure can cause significant damage and insurance issues, a licensed waterproofer or tiler would also be best placed to inspect the wall in person and confirm the compliant preparation method.

Unfortunately, a plaster coating is not a suitable substrate to start with to apply waterproofing over. This is the same as, for a wet area, having standard plaster sheeting. That is also not acceptable. Your options for substrates are really cement render over masonry, Villaboard or Aquachek plasterboard.

Please let me know if you have further questions.

Mitchell

AlbertFUSCUS

Thanks Mitchell,

I think in my case the way to go would be to remove the plaster layer with a scraper, and then there's a few areas where I would have to reapply some render to have a nice smooth wall for the waterproofing membrane and tiles to sit on. I have too questions regarding render:

Can a layer of Acrylic render be applied over old cement render?
Will the Crommelin membrane work with an acrylic render substrate?

Thanks again for your help.

Albert

JacobZ

Hi @AlbertFUSCUS,

I've contacted the technical teams for both Cromellin and Dunlop to confirm that there wouldn't be any compatibility issues if you use https://www.bunnings.com.au/dunlop-20kg-multipurpose-acrylic-render_p0760242 over the cement render and then apply https://www.bunnings.com.au/crommelin-4l-shower-waterproofing-membrane_p0961670 over the top.

In saying this, the original cement render needs to be in good condition, well adhered to the surface beneath and not flaking or breaking off. If the cement render is not in sound condition, it would be best to remove it also, then assess the substrate to ensure it is in sound condition, before rendering and waterproofing.

As with the others above, I'd consider speaking with a certified water proofer just to ensure everything is up to standard as waterproofing failure can cause major damage. In a bathroom, it is always worth getting the waterproofing perfect to ensure you are as protected as you can be. certified water proofer, who will warrant their work, would be the best way to do that.

Let me know if you have any further questions.

Jacob