Confused about Vertical DPCs at wall join

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Roberto2019

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Evening All. This seems to be one of the few topics that the Internet does not contain endless chapter and verse on... and that information that is available is less than clear.

I'm talking here about the vertical DPC at the join of a new cavity wall for a house extension to the existing cavity wall.

My building reg drawings, confirmed with the building inspector, suggest that a 60mm chase is cut into the existing brick outer leaf and a vertical DPC inserted. The bricks being 100mm wide, so the DPC will only be half way through the brick.

I don't get how this is worth doing? Surely it has to be all the way through the outer leaf to break the line that moisture can take, or it isn't worth doing at all? BC say that this would weaken the existing wall too much, as it is on a corner, which I can understand - but I would think a DPC only going half way is a waste of time?

I have found a few diagrams on Google (one of them by Woody, as it happens), which all show the DPC going all the way through. But how does this work with the wall weakening issue mentioned by BC?

Can anyone shed any light on this? Perhaps this is just a case of needing to switch my brain off, follow the drawings and don't worry too much about it..!

Many thanks
 
Surely a cut off /stihl saw blade isn’t quite big enough to go through the whole skin anyway so you can’t cut all the way through unless you do 2 cuts and smash a whole section out
 
Thanks.. yes I saw that other thread. It doesn't look conclusive though..? One of the diagrams I have seen is on the LABC website, which shows it cutting the whole way through, but this contradicts what the BC inspector is saying.

I simply don't understand how a 50-60mm deep DPC is better than nothing at all.

I think a disc cutter will just about get through 100mm at maximum.
 
Are you sure the architect didn’t mean a 60mm wide cut that was 100mm deep?

This vertical DPC stuff is just politics in my area. I asked BC if they wanted it doing and they just said no, despite all the books saying otherwise
 
I’ve never had to do it and not had first hand experience of it being done either, not that I’m saying that’s the right way, but deffo varies with building control region
 
I would have thought they meant 60mm wide, not deep? Are you having partial fill cavity insulation? (Just thinking that the cut may be that wide to allow for insulation too?)
 
Go with whatever BC are happy with?. Its probably 60mm in so it stops the water transfer on the face of the bricks.

My BC said I don't need a vertical damp inside the wall is pretty sheltered from the weather. I did run a DPC behind the wall starters. There seem to be many different ways of doing things but the most important is keeping BC happy so you get that certificate!
 
Roberto2019 said:
I simply don't understand how a 50-60mm deep DPC is better than nothing at all
Click to expand...

It is because when it rains the brickwork gets damp on the surface and doesn't penetrate deeply.

It depends really on how porous the bricks are how good or bad the mortar is, that determines how deep the damp penetrates.

I've seen plenty of conservatories done with no vertical DPC and zero damp on the plaster internally.


Building control often seem to want a 50mm slot cut all the way through these days....that acts as a thermal break.
 
Yes this is something that I can't seem to get to the bottom of either.

The old external wall that gets cut into is just that, an external non load bearing wall, thus surely it should be removed entirely in order for the cavity to be made continuous. It is no longer an external wall and should therefore be removed. This to me seems the correct way of doing it, however no builder in this day and age seems to do it this way.

The vertical cut and furfix of the new inner wall to the old external wall seems a bit of a cheap and nasty corner cutting exercise really.
 
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