Extensions Question

Tying in to existing brickwork/ continuous cavity in new + old walls

Hi there,

We are in the process of adding a single-storey extension to our house. Both have cavity walls but the builder has used wall plates to connect the two walls without actually opening the existing cavity to make the new and old cavities continuous. No damp-proof membranes appear to have been used either.

It does not appear to be required by building regs but is this best/recommended practice?

The new walls are now at roof height and the carpenter has already started on the roof rafters.

Any advice gratefully received.

6 Answers

Best Answer

If i have understood correct. There is no need to expose existing cavitys anymore to tie in, Simply, starter bars are drilled to the wall and you go from there. You do however need a stihl saw cut running up building in the cavity and a damp proof course put in place.

Hope that explains a little

Answered 18th Sep 2011

James White Brickwork

Member since 13 Dec 2010

He is correct in using the wall tie system to tie in old to new, however building control in my area insist of a vertical joint cut out, central of the 2 new walls, with a disc cutter, then insert a vertical dpc, held in to chase with clear silicon.

Answered 18th Sep 2011


Member since 29 Oct 2008


It is my belief that the cavities need to be continuous and uninterrupted to meet building regs. I certainly do it everytime and its something the building inspector will check.

Its ok to use wall starters but is again best practise to tooth out the brickwork on the existing building to tie in the new brickwork, it looks much better and also allows you to open up the existing cavity to join it to the new.

With regards to the damp proof membrane, this should have been used in the floor and then overlapped to tie in with the damp proof course before the superstructure was built. Again, this is essential and will be picked up by the building inspector!!

It might be worth having a chat with your builder to see if he has used a dpm and also whether your local building regs requires an uninterrupted cavity. Then get the building inspector out to check that he is happy with everything so far before you go any further.

Hope this helps.


Answered 18th Sep 2011

R.Hughes Building and Conservation Ltd

Member since 25 Feb 2009

Your builder should, at the very least,have cut a slot through the existing wall and inserted a vertical DPC. Ideally, he should have opened the wall to ensure a continuous cavity. What has your architect specified in the construction notes? Without a DPC or cavity there is a risk of damp penetration. This is something you should take up with the builder, as it shows lack of care and may be a pointer to sloppy work on other parts of the build. Also ensure that all required Building Control Inspections are carried out, or you may struggle to get a completion certificate.

Answered 18th Sep 2011

Steve Kennedy

Member since 2 Aug 2010

Not sure without seeing your house, but if the extension is single story there should be cavity trays in the existing wall above the roof. This is to stop penetrating damp (i.e from driving rain) above the roof from running down the cavity and dampening the now internal cavity wall inside.

Answered 5th Dec 2011

S. Murby General Builder

Member since 13 Oct 2011

Usually it is ok not to open the cavity as this can weaken the wall ,as long a you fit a vertical damp course.
To do this you need to run a cutter up the wall were the cavity would be then slot in the damp course,if you have any questions or your unhappy you could check with the building inspector he is there to help you and the builder if needs be .
Hope this helps

Answered 18th Sep 2011


Member since 15 Jul 2009

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