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I’ve got a late 1960s high set house, so the slab doesn’t have positive side waterproofing (pretty sure the original owners sorted the slab themselves).
The lower level isn’t legal height, but I’ve built another spare bedroom downstairs. Timber framed, gyprock sheeting, insulation. When the base plates for the stud walls were put in I didn’t put waterproofing down, so there is water coming up through the slab. The side walls have bitumen paint on them that seems to have worked. I initially did the same for the flooring, but with the rains back in early 2022, that was pushed up and had to be removed mechanically. Since then I put down Davco K11 - an alkali reactive cementitious waterproofing product. After Alfred came through recently, there was minor flooding coming through where the studs were drilled into the slab and under the studs. I know I’ll have to remove the baseplates of the studs and replace them.
I’m after advice on waterproofing products to use, whether or not the baseplates can be steel studs or I should go with timber again with adequate damp proof treatment between the timber and slab, or just any general advice. Waterproofing products would need to be negative side.
Thank you in advance!
Welcome to the Bunnings Workshop community @boxer-brass_09. It's terrific to have you join us, and many thanks for your question about waterproofing.
This definitely sounds like a complex issue, and I’d strongly recommend bringing in a professional—particularly a licensed waterproofer or builder—given the extent of the work likely required.
To waterproof the area effectively, you'd probably need to remove all internal walls so that a continuous membrane can be applied. That includes making sure all junctions with exterior walls are properly sealed, as water will find any weak point. You’ll also want to assess how water is entering in the first place—if it’s coming through the slab, you may need to look at external drainage improvements around the perimeter of the building to reduce hydrostatic pressure on the slab.
In terms of negative-side waterproofing, a product like Sika 10kg Sikalastic Waterproof Membrane could be suitable, as it's designed to resist water pressure from the inside. But again, application must be done thoroughly, with attention to every joint, penetration, and wall junction.
As for rebuilding the walls, you can go with steel framing to avoid rot in future, or if you choose to stick with timber, make sure to install a proper damp-proof course (DPC) or waterproof membrane between the slab and the base plate. Treated timber rated for ground contact is also a must in these situations. However, in the presence of moisture, even the steel frame (which is extremely thin) will rust and deteriorate quickly; perhaps even faster than timber.
All up, it’s a sizeable job, and one that really needs to be done once, and done properly—so definitely worth getting a pro’s help here.
Please let me know if you have any questions.
Mitchell
Hello @boxer-brass_09
I understand your desire to do this as a D.I.Y project, but due to the complex nature of the repair, I can only suggest what my teammate has recommended in regards to getting a professional to repair it. This leak is actually more involved than you realize as it has probably occurred under the walls as well, as it is a single solid slab. These sections under the wall will probably be unsealed as well and will therefore be likely leaking.
Let me call on our experienced members @Dave-1 and @Nailbag for their recommendations.
If you need assistance, please let us now.
Eric
Thanks Eric - much appreciated!
I took off the gyprock in the affected areas a while ago (this all happened during Alfred, as mentioned, so the photos were at the peak of the flooding).
The concrete slab isn’t uniform - it’s a whole lot of smaller ones that have been placed to form the larger one.
You can see in some of the photos where the water must’ve come up and brought some debris with it (not sure if that’s from carpenter ants or not - someone with far more knowledge than me is more than welcome to chime in!).
From closer inspection is doesn’t look like it’s coming up through the drill holes for the baseplates - it’s more in the corners where the slabs meet the walls.
Hello @boxer-brass_09
The white powdery residue on the surface of your concrete floor is called efflorescence. It's a phenomenon caused by moisture migrating through the concrete, bringing dissolved salts to the surface. This often happens when hydrostatic pressure is present which is water moving through the concrete. This automatically tells you that the concrete floor has no moisture barrier underneath. Installing a temporary sealer will not be an effective solution as it will only be a short-term fix.
I propose speaking to a concrete specialist and getting a detailed quote on what it would take to properly seal off your floor. You will at least have an idea of the materials and cost for the repair. Having an idea of what is involved will at least give you a starting point.
If you need further assistance, please let us know.
Eric
