DPC and DPM joint importance

[BobS]

We are a couple of months into a renovation and extension of a 1980s house that feels closer to a complete rebuild. Currently we are digging out the concrete floor slab, ready to install a new one with UFH continuing into the extension.

 

Because of the intention to pour a new slab in one, our builder has already begun building the extension walls without any damp proof membrane lapped in. They have used a DPC but pointed over most of it on the inside wall, thus there is nothing to attach a new membrane to. Their logic is that because the original membrane is broken it wasn’t worth putting one into the new walls.

 

However, the old house not only has the remains of DPM in the walls but the DPC hangs out about 30mm, which seems better than nothing. The new walls have little visible DPC and no overhang, so my concern is the new extension is now worse than the existing house.

 

I am wondering how important the joint between DPC and DPM is or can the new DPM just continue up the wall past DPC level?

 

Also, is pointing over the DPC on the inside wall a problem that needs correcting?

 

Thanks

[tonyshouse]

If you are worried you can paint it with “blackjack” to 75mm below dpc making contact with it. Then simply overlap the dpm on that 

 

plenty of people don’t?

[BobS]

Thanks for getting back to me. Could damp still travel up the mortar that covers the DPC, behind the black jack and come out further up the wall?

 

I realise lots of old houses don't have proper damp proofing so not sure if this should be a real concern. Just don't want to spend all the money on finishing walls and floors if there is a chance of a damp problem.