How to fill a void in bathroom floor?

Question

Hi all,

I have been fixing a bathroom shower leak and had to remove a rotted door moulding that was embedded into the floor mortar. The rotten moulding appeared to have waterproofing applied to its exterior after it was attached to the door frame, so it was part of the bathroom waterproofing. I want to fill the void and replace the door molding above the existing tile level but am concerned about the gap created in the waterproofing. Do you have any suggestions for waterproofing and filling the void. Should a waterproofing coat be applied into the void before filling? Would you use a waterproof filler to fill the void (eg. an epoxy grout or hydraulic cement based product like Water Plug)?

Any thoughts or suggestions welcome.

Thanks.

MitchellMc · Accepted Answer

Welcome to the Bunnings Workshop community @sportsport. It's sensational to have you join us, and many thanks for your question about filling a void.

That’s a bit of a tricky situation. In a standard setup, waterproofing should be applied continuously over the subfloor and turned up the walls or vertical surfaces to create a sealed "bath" effect. In your case, it sounds like the waterproofing was done after the door frame was installed, possibly running up to or just over it.

Now that you’ve had to remove part of that structure, there’s likely been a break in the waterproofing layer—which is definitely something to be cautious about. You’re right to be thinking carefully about how to repair it.

It would be wise to waterproof the entire recessed or exposed area before filling it, but whether that patch will meet the required standards really depends on the product used and how it’s applied. Most liquid membranes do allow for repairs, but these typically need to be done as a continuous coat with proper overlap onto the existing membrane. That’s something a licensed waterproofing specialist would need to confirm, especially to ensure it remains compliant with the relevant codes.

As for filling the void, a standard mortar would be fine, but only after you’ve ensured the area is properly waterproofed. Using a waterproof filler alone isn’t a substitute for a continuous membrane layer.

I’d recommend having a waterproofing professional take a look and advise the best way to bring the area back up to standard—especially since it’s in a wet zone.

Please let me know if you have any questions.

Mitchell