DPC below concrete floor

[rebuild]

Good morning,

I am looking for some advice regarding my proposed solution to repair a 1920's concrete floor.

The internal concrete floor is about 2 courses above the DPC. Consequently, the DPC was being bridged causing damp.

I have dug a 6" trench around the perimeter of the room, to decouple the wall from the floor. All of the damp in the walls has now cleared up.

I am about to start filling in the 6" trench, do you think the following is a suitable way forward:

a) 3 coats of synthaprufe liquid DPM to wall running 1 course below DPC and 2 courses above. To provide a waterproof barrier between the wall and floor.

b) Physical DPM upstand, using DPC 300mm. This is a belt and braces approach to prevent any part of the floor coming in to contact with the wall. It is also to prevent the liquid screed (to be applied over the top of the floor) coming in to contact with the water based liquid DPM.

c) A waterproof mix of sand, cement and SBR. To fill in the trench and provide another waterproof barrier.

To connect the DPC to the physical DPM upstand, I was thinking of using a butyl based mastic applied along the DPC to form a water & water vapour tight barrier.

Thoughts, comments, suggestions welcome.

Cheers

 

[Ivor Windybottom]

Looks good, I also have concrete floors without a membrane so will be looking at similar in future, thankfully I don't have a bridging issue as the DPC is at floor level in my case.

I haven't looked at it in detail yet, but I understand it's possible to epoxy coat the entire floor surface. May be relevant to you, as you'll still have that path from the soil up through the old concrete into the room. Seems like a sensible idea, as I say I haven't looked into it yet but understand it exists.

 

[rebuild]

Looks good, I also have concrete floors without a membrane so will be looking at similar in future, thankfully I don't have a bridging issue as the DPC is at floor level in my case.

I haven't looked at it in detail yet, but I understand it's possible to epoxy coat the entire floor surface. May be relevant to you, as you'll still have that path from the soil up through the old concrete into the room. Seems like a sensible idea, as I say I haven't looked into it yet but understand it exists.

Thanks, yes I plan to do exactly that once the floor to wall detail has been completed.
I will be creating a sandwich of Ardex NA screed followed by Ardex 1C DPM (epoxy) and a final Ardex NA screed

 

[JohnD]

How thick is the old concrete floor? I wonder if it would be feasible to dig it out, add a DPM and insulation, and lay new?

I suppose it has to match the height of floors in adjacent rooms, which are above DPC?

 

[rebuild]

How thick is the old concrete floor? I wonder if it would be feasible to dig it out, add a DPM and insulation, and lay new?

I suppose it has to match the height of floors in adjacent rooms, which are above DPC?

Hi John,
The concrete is about 2" thick. Yes, ideally the floors would all be dug out and replaced.
However, that is not currently an option.
Cheers

 

[Ivor Windybottom]

My only reservation is that you could cause sinking by removing the edges of the slab. This may be its strongest part, where it's resting on rubble sitting on the wall footing. All ours have sunk in the middle but are at original height at the skirtings, presumably as this is the only part that's firmly supported.

Do you have an issue with damp rising up the walls? If not then perhaps just doing the floor surface treatment may be less likely to cause sinking.

Edit: Saw you've already said the walls are damp. Internal walls too?

 

[rebuild]

Thanks for your input.
The edges will be well compacted and the 4:1 SBR mix should fill all gaps to provide support. I'm not too concerned as the slab seems fairly stable.

Yes all walls were damp. However, the external walls were by far the worst. Because the internal walls have a second DPC at skirting height.

 

[Ivor Windybottom]

Your new edges will undoubtedly hold themselves up. I'd worry about the old concrete then sinking due to no longer resting on the footings.

Have you compared the cost of all this repair process against just renewing it? It may not be that different with the amount of cost and complication, plus you'll know it will definitely be dry and won't sink.