External wall built on top of internal screed DPM

[Sylvain]

So I am in the middle of converting a small barn and trying to track down some water ingress I discovered that the previous owner built the back wall of the barn onto the internal screed DPM which ends outside - see pictures attached. This is causing water ingress on the inside on the floor all along the back wall.

Just wondering what would be the best approach to solve this?

I was thinking to cut the DPM short and build/pour a thick concrete plinth (as wide as the slab so it clears it) with an angle so water can run off. Is that a good solution? If so, what prep/mix would be best to get a good bond and make it as water tight as possible?

 

[23vc]

I think that’s going to be pretty tricky to get right. I’m thinking lapping the excess DPM up the outside of the wall and if needed extending it by taping more to it, might be your best bet. And protecting it from getting damaged. But that’s only going to alleviate ground level moisture and not anything penetrating through the wall higher up…

 

[Deluks]

Channel out the screed internally, cut dpm, tape new dpm to old and run it up the wall.

You could also clad the wall somehow to keep water clear of it.

Another option would be to put a roof between the two walls, with the appropriate detailing and drainage.

But first, are you sure that the internal dpm and the external dpm are all the same piece?
They could have just used an offcut as a separate dpc.

 

[Sylvain]

Channel out the screed internally, cut dpm, tape new dpm to old and run it up the wall.

You could also clad the wall somehow to keep water clear of it.

Another option would be to put a roof between the two walls, with the appropriate detailing and drainage.

But first, are you sure that the internal dpm and the external dpm are all the same piece?
They could have just used an offcut as a separate dpc.

I am not 100% sure it's the same DPM but it's 95% sure for the following reasons:

- I can see the dpm sticking up inside against all wall but that back wall.
- We get a large water mark on the floor all along that back wall. While the wall is perfectly dry above.
- Internal height of the floor is a couple of inches above that the DPM/slab level outside.

Catching the dpm in side is something I have thought of but it's rather old and seems to break very easily so not entirely sure if it would be feasable to catch enough of it to tape a new piece on.

Instead of cladding, is there anything wrong with doing a cast base plinth out of concrete? Pouring a concrete plinth like the picture attached?

 

[Deluks]

Ok, yes definitely sounds dodgy.

The plinth thing is an out of date idea that used to be done to deal with dpc and 'rising damp' issues, results can be hit and miss and might just cause more problems. It is effectively making a 'bridge' from the ground to the upper wall above where the dpc is (or should be)

If you are saying that the dpm is old and fragile, then perhaps just bite the bullet and redo the lot. Floor up, new dpm, you could even have the benefit of insulating the floor while you are at it.
It might seem daunting but you'll at least have piece of mind, rather than fiddling and fussing with retrofit solutions that may or may not work.

 

[Sylvain]

Ok, yes definitely sounds dodgy.

The plinth thing is an out of date idea that used to be done to deal with dpc and 'rising damp' issues, results can be hit and miss and might just cause more problems. It is effectively making a 'bridge' from the ground to the upper wall above where the dpc is (or should be)

If you are saying that the dpm is old and fragile, then perhaps just bite the bullet and redo the lot. Floor up, new dpm, you could even have the benefit of insulating the floor while you are at it.
It might seem daunting but you'll at least have piece of mind, rather than fiddling and fussing with retrofit solutions that may or may not work.

I might be wrong but I am pretty sure the wall has no dpc...
I might try my luck with trying to catch (and extend) the dpm from the inside. If not then yeah pulling it all out and redoing it with insulation is one option (which admittedly was trying to avoid as I was also looking at UFH......)

BTW that back wall is a cavity wall without insulation, just 2 layers of concrete blocks seperated by 50mm void