How to repair and waterproof a leaking suspended terrace?

charlyj1234

Hi Workshop community,

I’m seeking some guidance on a challenging waterproofing and surface repair project for our external suspended concrete terrace.

Context / Background:

We currently have an active water leak coming through this terrace and into the lounge room directly underneath. To investigate, we have just pulled up all the marine carpet that was covering the area (put down by the previous owners).

Longer-term, we plan to build an extension over this space, but for now, we need to waterproof the terrace to protect the slab, stop the water seeping into the house, and make it a usable, trafficable painted surface.

Pulling up the carpet revealed a bit of a mess (see attached photos):

• There is a green layer (looks like carpet glue).

• Then there looks to be a cement/tile screed layer underneath the green glue, but large chunks of it have shattered and come away with the carpet. I think it’s screed because it’s different thicknesses and it breaks apart pretty easily

• Then there are strange light blue/turquoise spots where the screed has bubbled up and failed completely. I suspect this might be moisture coming up from the suspended slab below? The slab below has pebblecrete by the looks of it.

• I also found a piece of what looks like steel reo sticking up through the surface.

I want to do this right so I don't trap moisture in the slab. I've broken down a plan into a few steps and would love your product recommendations and advice.

My Questions:

1. Surface Prep & Glue Removal:

There is green residue all over the terrace (a mix of old glue for the marine carpet that was applied). Does this need to come up completely down to the bare concrete or tile screed before I can do any repairs or apply new waterproofing? If so, what is the best method or product to remove it?

2. Treating the 'Blue Spots' and Moisture:

You can see light blue patches where the screed has bubbled and failed. Assuming this is moisture in the slab, how should I best treat these specific spots to stop the moisture before I put anything new down? Is an epoxy moisture barrier required here?

3. Filling the Screed Gaps:

As you can see, large sections of the old screed layer have come up, leaving deep gaps. Once the moisture areas are treated, what is the best patching/leveling compound to fill these gaps and make the floor level again so it's ready for waterproofing?

4. Exposed Rebar:

There is a piece of metal reo sticking up. My plan is to grind it down flat. Once ground, what should I use to treat it (rust converter?) and patch over it so it doesn't rust internally and create a new leak?

5. Waterproofing and Topcoat:

Once the gaps are filled and the surface is prepped, the idea is to paint on a waterproof solution over the entire terrace. What is the best exterior, trafficable waterproofing membrane for this? And what should I paint on top of that to give it a bit of grip/anti-slip so it's safe to walk on?

Any advice on the proper sequence of events and products would be incredibly helpful.

Thanks in advance!

JacobZ

Hi @charlyj1234,

Welcome to the Bunnings Workshop community, it is fantastic to have you with us.

Before getting too far into products or process, the first step should really be to understand exactly where the leak is coming from. Water can travel quite a distance from the entry point before it shows up inside, so what you’re seeing internally isn’t always directly below the source. It may be worth engaging a leak detection specialist to pinpoint the entry point and confirm whether it’s a surface failure, a joint issue, or something within the slab itself. That way you’re solving the actual cause rather than just applying a bandage over the top and hoping it fixes things.

After determining the specifics of the leak, it would be worth speaking to a waterproofing specialist in order to design a solution. While your proposed steps may work, there is likely further complexity to the issue that I can't know for sure based on your images alone. Having a waterproofing specialist come out to inspect the slab prior to commencing work would certainly be worthwhile.

With these things being said, once you move into prep, the existing glue will need to be removed as you can't guarantee a waterproofing membrane will stick to it. Scraping and grinding are typically the only effective methods here, and in most cases this process will take the remaining screed with it, so this needs to be factored in. While that might feel like you’re going backwards, it’s often necessary to get back to a clean, sound base before anything new goes down.

I can't say for certain what the blue spots are, but they look more like old concrete paint, tint or a spilled chemical of some sort. I would be speaking with your waterproofer about how best to address this.

Once you are back to the concrete slab, it’s fairly common to find a rough surface with minor cracking. That in itself isn’t necessarily a problem and can often be addressed with the right preparation. The bigger consideration is that a level concrete surface, even if it looks fine, won’t naturally shed water. Without proper fall, water will tend to sit and pool, which can contribute to the kind of failures you’re seeing now. So, the focus shouldn’t just be on making it flat, but on how the surface drains and how it ties into the waterproofing system. This could mean resurfacing the concrete so there is adequate drainage away from your home towards a drainage point, or it could mean working with the existing concrete slab to simply prepare it for waterproofing. I would be discussing this with a licenced waterproofer or drainage plumber to ensure things are done right.

With the exposed reo, which could also just be something like an old dynabolt, the typical approach would be to grind it back flush with the surface, then coat it with a galvanising spray to help prevent future corrosion.

When it comes to product selection, you need to be looking at a full system rather than individual products as most membranes will have a primer and things like reinforcing bandage that must be used in conjunction. The closest membranes I could find to fit your requirements were ARDEX WPM 909 and Sikafloor®-400 N Elastic, which are both exterior grade, trafficable membranes with slip resistant properties. These are not basic products and consequently we do not keep them in stock. We could certainly order them in for you, but they would not be available off the shelf, so I would suggest speaking with your waterproofer prior to purchase of your membrane.

I am sorry about the wall of text, but there was a lot to address. I don't want to completely turn you away, but there is a lot more complexity to your issue than there might seem at face value. Long term, I think it is important that this is done right and that will unfortunately mean speaking with professionals.

Let me know what you think and if you have further questions, please don't hesitate to ask.

Jacob