Cure for gap between DPC & DPM?

Hello all.

Been looking for a while and found some useful stuff, however, would be grateful for professional opinion on a specific damp problem.

I'm doing a lot of work on my 1963 built bungalow, it has cavity walls but the outer leaf is irregular local stone and the cavity has been backfilled to support this when it was built. Not a lot I can do about that of course, but during various phases of work I have ascertained that the DPC would appear to be the full width of the walls ie. spans inner leaf/cavity/outer leaf. The DPM appears to be liquid applied between the slab and the screed.

The problem therefore is that the two do not meet and damp can penetrate from the join between the slab and the wall, just below/behind the skirting board. This leads to mould on the walls behind furniture.

I will be replacing the storage heaters with central heating and have new windows with trickle vents, but wondered if there was anything that could be done to effectively seal this gap at the floor/wall junction. It's not really feasible to dig up the floor around the whole of the internal perimeter, but thought there might be a drill and inject type system that would be a start. I was about to ring the damp 'specialists' but thought I would gauge opinion on here first.

Much obliged for any constructive input!

Paul.

Paul,
How high up the wall has the cavity been backfilled?

stuart45 said:

Paul,
How high up the wall has the cavity been backfilled?

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It's all the way up. The back of the stone is so irregular that, even though it has wall ties, it has been infilled with smaller pieces and some cement in places to bind it all together.

That was built as a solid wall, rather than a cavity. It was cheaper than using stone right through the wall. Solid stone walls were often built with a brick backing and the gap filled with rubble and mortar. Some of the problem may be condensation. Mould is often a sign of this.
How thick are the walls?

Yes you are right of course, whatever it looks like it is effectively a solid wall. I have put a couple of pictures up to help.

The inner leaf is 100mm block, the 'cavity' plus outer leaf a further 270mm.

This section is where I am knocking through for a set of french windows, and I have removed the 'infill' from the back of the outer leaf ready to knock through when the doors are on site.

Hopefully the other photo' shows the problem with the DPC. Where it finishes under the inner leaf next to the slab, when left overnight, the dust at this edge is damp.

I know that I won't necessarily cure all my problems with a 'solid' wall but if this breach can be rectified to some extent it would help. View media item 10110 View media item 10111

The damp could be condensation. A lot of DPMs in the 60's and 70's were cut off flush with the slab and not lapped with the DPC. BRE recommended 3 coats of black to seal the break. In your case that would mean cutting up some of the floor, which is not what you want to do.

Is there any other alternative? Maybe pulling off the skirting, drilling down and injecting some wonder cure that spreads and seals? Isn't this what they do in older, solid wall properties that don't have any DPC?

Thanks for your input Stuart, all advice gratefully received.

Injection wont work for moisture coming horizontally across the wall, and it is impractical to insert a physical DPC.

Therefore the best option is to apply a barrier to the inner surface of the blocks.

A sand cement render may be enough, or a liquid product such as RIW or Superprufe.

But in either case you will have a thermal bridge (cold spot) where the wall is solid at the bottom and cooler than the wall above with an open cavity, and this may allow condensation to form which you may mistake for penetrating dampness. In this case the answer will be to dry-line the wall too.

^woody^ said:

But in either case you will have a thermal bridge (cold spot) where the wall is solid at the bottom and cooler than the wall above with an open cavity, and this may allow condensation to form which you may mistake for penetrating dampness. In this case the answer will be to dry-line the wall too.

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Its a solid wall all the way up so this shouldn't be a problem.

Allowing for the fact that the gap may not be the primary cause of any damp/condensation, would it not be good practise to address it anyway whilst the house is undergoing some renovation? And would injection actually achieve this? From what I can see I think the DPC & DPM are about 10mm apart in both planes, so about 15mm as the crow fly's, assuming the slab has been painted to the edge.

The GF had an old house treated with an injected DPC for mortgage purposes and says that they drilled about every two feet or so and apparently the fluid spreads over time. If that were the case surely 10/20mm would be achievable. The only alternative I can see is cutting back a couple of inches of screed and as you guy's have said, lap a liquid DPM up the block over the joint where the DPC sits. But thirty or forty metres of that doesn't appeal if there is a reasonable cost option. conversely, I don't want to get ripped off if injection isn't going to do the job properly.

As far as any other causes, like I said, double glazing with trickle vents (which we use) and central heating, and we may have to live with it, it's not a big problem, but just wanted to address anything obvious whilst I had the chance.

Thanks for your input guys, it's appreciated.