Damp foundation in 1930s house

benners321 · Post by **benners321** » Sat Sep 16, 2023 4:42 pm

Hi folks,

I'm renovating my annexe which is in the basement and underground. The previous owner had done the heavy lifting a couple of decades ago when they turned the cellar into a livable space. The house was built in the 1930s in the South West.

As we started moving some pipework for the new bathroom, we noticed that the concrete was incredibly wet 5cm under the surface. So wet, that one can basically keep digging into it with bare hands. I don't think this has been a problem yet, as the top bit is dry.

The pipework hasn't been used for some time, so I don't think a leak causes the dampness. I will drill a few more holes in different places to confirm that it's not just an issue around the pipework.

I'm assuming the foundation has been wet for many years.

This situation is outside my DIY comfort zone. As far as I understand it I have three options:
1. Do nothing and continue with the renovations and hope the dampness doesn't rise any further
2. Damp-proof course on top - does that really work, what are rough costs per m^2?
3. Somehow replace all or parts of the foundation - what are rough costs per m^2, is it worth it?

Any other options? What would you do next? Any thoughts?

I'm looking for general advice/ideas/knowledge on how to resolve a wet foundation conundrum!

Someone-Else · Post by **Someone-Else** » Sat Sep 16, 2023 6:37 pm

You really need to find out WHY it's wet. Suggestions. Is there a leaking pipe you have yet to find, is it coming in from outside, is there a high water table, is one part wetter than another, dig down "deep" and see what you find. (Dig a hole in the garden, see does it fill with water) are there any walls that are wet.

Hoping something doesn't happen is never a good idea, as if it does happen, you will have wish you had done something about it, on the other hand if it doesn't happen you are lucky.

Damp course stops water passing through it, it may also act as a raft and float, you may need tanking, you might need a sump and pump depending on what you find with the holes.

I believe burying plastic pipe in concrete is never good as if the concrete moves, it will damage the pipe, it should be covered in shingle first.

But wait and see what others say.

Cost? never ask on ANY forum "how much is / will be"

Reason:
Person D says £307
Person J says £96/hour
Person M says £2,500+VAT

So you have 3 "costs" but you have no idea if any are right, and you STILL have to get a suitable person round to see the actual job, best not ask.

benners321 · Post by **benners321** » Sun Sep 17, 2023 9:31 am

Thanks for your two cents!

I'll drill a few more holes in the foundation in various places to figure out whether it's a problem everywhere or just in that corner sounds like a good next step.

Who would be the specialist who could help with this, and provide a quote? Construction companies, ordinary builders?

aeromech3 · Post by **aeromech3** » Sun Sep 17, 2023 12:30 pm

I somewhat doubt the original floor concrete was as deep as the underside of those drain pipes, more likely just infill and being a cellar below ground it is likely nearer the water table; the 50mm concrete screed is shallow.
What floor are you going to put down?
You have untreated studding so at least roll out a strip of DPC under the bottom plate, you can buy 30mtr for £5!
Better still consider a DPM over the whole floor and new screed, even a liquid epoxy DPM, but take it up the walls about 100mm.

benners321 · Post by **benners321** » Sun Sep 17, 2023 4:24 pm

Thanks for your thoughts!

I somewhat doubt the original floor concrete was as deep as the underside of those drain pipes, more likely just infill and being a cellar below ground it is likely nearer the water table

There is a rebar just underneath the pipes. Wouldn't that indicate we reached the middle of the concrete layer? That said, it definitely feels like infill .. it's super coarse, and one can quite literally dig with bare hands.

What floor are you going to put down?

Tiles (with EFH)

Better still consider a DPM over the whole floor and new screed, even a liquid epoxy DPM, but take it up the walls about 100mm.

Thanks, that makes sense. I probably have to take the studs down again... but would be worth it in the long run!

Just to be clear - the order of layers would be DPM, EFH, Screed, Tiles. Correct?

benners321 · Post by **benners321** » Sun Sep 17, 2023 8:04 pm

One more question: Reading up on Epoxy DPMs, they mention it's not a suitable solution for when there is hydrostatic pressure. How would I know whether there is too much hydrostatic pressure for a DPM?