How to waterproof 1960s concrete slab & basement?

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

Thank you Mitchell.

In lieu of getting a professional out, I think I’d prefer to do this myself considering how much has been done already. Would you have any recommendations for tackling this myself?

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

Good Evening @boxer-brass_09

😕 What a pain. And I am heading straight to getting a profesional to assess at the very least. When I did my garage, much like you I had "presumed" that brick walls wouldnt pass water. Yeah they do, cracks in mortar and just bricks being bricks lol I couldnt tell where the water was coming from and thought it may have come up through the slab.

It was coming from outside, either throught low level cracks in the brickwork or from up my patio. Water travels long distances and will always take the shortest path. If you have water coming up through the holes you drilled for the wallplates, that is a bit of a concern structually. It would mean that your concrete floor isnt too thick. (35mm to 45mm for the stud wall sitting on the concrete is my thoughts, drilling down even 80mm from flat concrete you should be in concrete still.

With your description of various ingresses of water, it sounds like it may be wise to go through and fix each issue, that said if the floor is a culprit it may be cheaper to redo the floor then try and fix the symptons. A professional will be able to give a better judged answer.

Dave

Nailbag

Hi @boxer-brass_09

Noting your post reply to @MitchellMc in not seeking the advice and services of a professional, bear in mind the living space is illegal. I only highlight this because insurance companies look for any excuse not to pay out, even if the causes aren't related. There are very strict guidelines and requirements for converting any space to a living one underground or not.

The photos supplied indicate severe water ingress which will compromise the sub-level building structure and more importantly electrical wiring.

My advice is to please take this very seriously and at the very least seek professional advice on how to rectify the issues. This is not going to be something an on-line forum will be able to resolve.

Nailbag.

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.

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

Hi Eric,

I had a friend who’s a builder take a look at it, and he said there were a few options for me.

1. Excavate the exterior (the besser brick walls aren’t waterproofed on the positive side), waterproof that and put ag pipe and drainage gravel to mitigate water coming in.

2. Take out the baseplate studs, put waterproofing on the slab, then damp proof course and H3 timber, then refit some villa board or wet area gyprock in the lower levels.

Would anyone have any advice on particular products to use?

Afternoon @boxer-brass_09

I do like the idea of waterproofing from the outside, My probelm tho is the effervescence on those blocks of the floor. It shows water is coming up through the blocks. It would be a shame to go to the expense and labour to do the walls like you are suggesting without mitigating the floor issue. The square blocks with nothing between them is another one that potentially has water issues as well.

Dave

Hi Dave,

The efflorescence on the flooring is from when I put down vinyl backed carpet tiles. When I realised what was happening between them and the slab they were gone pretty quickly and replaced with rugs. The only thing between the Davco K11 and them was Davco 173 floor leveller. It always thought it was a result of temperature differences between the slab, vinyl backed carpet tiles and the room itself.

When you say the concrete squares, do you mean the many smaller slabs that make up the big one?

Would you have any advice on any products to use, and/or an order to to it all in?

Thanks in advance!

Good Evening @boxer-brass_09

Yes I was talking about the many smaller slabs making up the larger one and sadly I dont know what you could use to seal between them as its only going to seal the top and not the sides. 😕

The only time ive seen efflorescence like yours was from water pushing through the concrete more then sitting on top and then draing away. I dont think the vinal backed tiles would cause that look. I have had the same issue with my garage right down to the vinyl backed carpet tiles getting super wet/going under and the concrete didnt have the efflorescence look (my water came through the soil outside and then through the brickwork.)

One of the other members may have a suggestion on how to seal the concrete blocks, but remember its going to be a multi pronged solution I would suggest.

Dave

In your second proposed option - Take out the baseplate studs, put waterproofing on the slab, then damp proof course and H3 timber, then refit some villa board or wet area gyprock in the lower levels. Unfortunately, I'm not aware of a product that can withstand extreme hydrostatic pressure. Your concrete floor is showing signs of strong water ingress and anything placed on its surface will meet up with that hydrostatic pressure and get peeled off.

Waterproofing is only effective if it is applied on the correct side or surface. In this instance it's highly probable that putting a waterproofing membrane on top of your floor will get pushed off.

Eric

Hi @EricL and @Dave-1

Thank you for your responses.

Davco/Lanko K11 is what I used for waterproofing the main area (I didn’t do below the studs because they’d been installed before I knew better). It’s a cementitious Class 3 membrane that reacts to water present and forms crystals, essentially creating a uniform slab. Sika K10 or Sikalastic are other options sold by Bunnings, or were last time I checked.

Excavating the external side and waterproofing is the ideal option, but not feasible right now as the aircon unit is on the ground not on the side of the building.

Afternoon @boxer-brass_09

As in interm measure then I would try and create a way the water will flow away from the edges of your building as much as possible until you go through the steps you want.

Dave

Hi @Dave-1,

There isn’t much I can do at the moment, unfortunately.

In terms of products that would be suitable, I’ve been looking at Sikalastic 1-Komponent and Dunlop Damp-proof Waterproofing. I’ve also come across Gripset Betta Water Plug.

Would you have any recommendations between the these, or any other products?

Good Evening @boxer-brass_09

I just had look at the Dunlop 4L Damp-Proof Waterproofing you mentioned and the first part that jumped out at me is

"An acrylic brush-on membrane, Dunlop Damp-Proof Waterproofing will prevent rising damp, efflorescence and will resist negative hydrostatic pressure"

This I thnk means that it would go on the soil side of the wall to stop water coming through.

I have only used a thick black tar like substance to stop water traveling through the soil then brickwork into my garage It worked but it was messy. I had also permantly removed the soil from against the outside of the wall.

Fixing water ingress issues in garage and in particular Gripset Betta 4L Bitumen Rubber Waterproofing Membrane This is the black tar like substance, Its meant to go on the soil side so water dosnt get a chance to seap through the brickwork. I only include it as an example of how a negative hrdrostatic material works. A positive one (which I dont know of any 😕 )you could paint on the inside of a wall to hold back the water.

I dont have any other knowledge of types of products that you could use. @EricL , @MitchellMc may.

Dave

As I mentioned in the beginning of this discussion, Sika 10kg Sikalastic Waterproof Membrane (1-Komponent) would be a suitable product to use. It's suitable for inside waterproofing of negative water pressure of walls and floors in basements.

Remember, the entire floor needs to be coated, even under the walls.

Mitchell

Thank you @MitchellMc

I spoke to a concrete specialist who recommended using that and Xypex in the problem areas and wall/floor joints. I can’t find who sells Xypex in Brisbane, or if there is a similar product sold by Bunnings?

As that product is not something we sell @boxer-brass_09, it's difficult for me to suggest an equivalent. Sikalastic can also be used on joints between walls and floor when used in conjunction with Sika SealTape S. However, in speaking with Sika again, since the moisture is pushing up from below, they recommend enlisting the services of a professional.

Mitchell

Thanks @MitchellMc

I called him back to ask for clarification, and he specified it being a “crystalline growth compound”, and suggested Drizoro and Fosroc as equivalent products

That would be Drizoro Maxseal Super. You'd need to speak with our Special Order team in-store, and they'll try to order it in for you. It's a trade-level product and not currently available for Special Order. Remember that to have its crystalline effect work, it needs to be applied over concrete, not a previous membrane.

Mitchell

Hi @MitchellMc

That’s really helpful, thank you!

It’d be going in the wall/floor joints and problem areas that don’t any existing waterproofing, so that sounds good. I imagine I’d then go over the top of that with Sika 1-Komponent?

@MitchellMc How does that compare to Drizoro Maxplug?

The Maxseal Super is a standalone product @boxer-brass_09. However, you could potentially apply the 1-Komponent over it.

MaxPlug is primarily designed for active leaks and patch repairs. I also don't believe it is a crystalline growth compound, as I couldn't find a reference to that. The Drizoro technical team advised that the Maxseal was the preferred product for your circumstance.

Mitchell

Thanks @MitchellMc - much appreciated!

Am I over complicating this by potentially using all of these products, or are they necessary due to the complexity of the issue? How do they compare to say Dunlop Damp Proof, or something made by Crommelin?

I’ve taken up the baseplate of one of the walls and found the problem area/s. The main one being where the brick wall meets the slab (the wood was blackened with mould - the area has since been sprayed with Selley’s mould killer), and there were a few very small centipedes running around). The black you can see on the bricks is Sika BE. That section of the wall is incidentally where the external concrete drainage gutters meet the house from the very steep driveway and run along the front of the garage,

There’s no evidence of water coming up from where the concrete screws are drilled in the the slab.

Given the amount of work you're doing, you should do it once with the correct product. That being Maxseal Super, if water is pushing up from below. You have quite a serious problem, and something like Damp Proof isn't capable of resolving it. That's more used for rising damp, whereas you have water penetration under pressure.

Mitchell

Thank you @MitchellMc

I need to put down floor leveller too, what sequence should I carry out each step?

What should I do in terms of connecting the DPC baseplate to the slab - should I use dynabolts (if so, should I pre drill holes in the slab and then include those in the waterproofing), or would something like liquid nails be sufficient?

Is Sika 1 Komponent sufficient in this situation, or would I be better off with Drizoro Max Seal? (I’ve already bought some Sika 1 Komponent)

As mentioned earlier, it would be best to speak with a builder or waterproof expert regarding these questions, as they fall outside the typical advice we can access from the manufacturer. When waterproofing a slab, all penetrations through it should be sealed. That could be done with a polyurethane like Sikaflex 11FC.

I've been advised that Sika 1 Komponent would not be the best product to use on a flat surface that has pressurised water beneath it. It could be fine to use in these circumstances, but Sika can't confirm entirely due to the unknown variables, like water pressure.

Mitchell

Hi @MitchellMc

I’ve just had a site visit from a company representative from one that specialises in crystalline cementitious waterproofing, he was very helpful but recommended getting further advice on products that would be more suitable for wall/floor joints between concrete slabs and brick, specifically one that has a bit of flex considering it’s a joint,

Would Drizoro Maxseal Super still be suitable for this, or would Maxseal Flex be more appropriate?