Joining new extension wall to existing wall

[browfish]

I have bought wall starter kits to tie my extension walls onto the corner of our existing house but i have 2 questions.

1. Should i cut away a section of wall to tie the inside skin to the old inside skin prior to cutting away the wall much later when i am water tight.

2. how should the 2 DPC's join? Do i scrape away the mortar above the existing slate an run the new plastc i few cm in and repoint?

Thanks

 

[noseall]

1. Should i cut away a section of wall to tie the inside skin to the old inside skin prior to cutting away the wall much later when i am water tight.

It depends. If the old part of the wall needs to project beyond the inner face, i.e. the knock through is not flush then leave alone. If however the knock out is flush, then it would be ok. It does mean that you will risk destabilising the remaining (external face) brickwork and this could be a problem when you drill and fix the outer skin starter.

2. how should the 2 DPC's join? Do i scrape away the mortar above the existing slate an run the new plastc i few cm in and repoint?

There was a time when BCO's used to insist we cut a brick out and physically lap dpc's but nowadays most are happy with them butted.

 

[^woody^]

Run a disk saw up the corner 1/2 brick in and remove that bit of external wall to allow you to tie to the existing internal leaf

Just turn up the new DPC 100mm or so up the wall. No need to lap to existing but don't just butt it up

Ideally the internal DPC should be lapped though

 

[noseall]

Run a disk saw up the corner 1/2 brick in and remove that bit of external wall to allow you to tie to the existing internal leaf

Fine, so long as the design does not ask for any buttressing i.e. that this piece of existing wall does not need to project beyond.

 

[browfish]

Shoudlnt need to, it is about 6m to the next butressing internal block work partition that comes off it.

 

[cotswoldbuilders]

Shouldnt be any need to cut out any bricks, bolt the furfix to wall.
Our BCO insist on cutting a slot central of furfix, about 30mm deep, insert a vertical dpc.
As the others have said, butt the horizontal dpc.

 

[^woody^]

Fine, so long as the design does not ask for any buttressing i.e. that this piece of existing wall does not need to project beyond.

Yes that's right, but it would be a poor designer who allowed an external wall to become part of an internal wall - even with just a slot for the vDPC

If there was some buttressing required then this should be formed from the internal leaf only

 

[garyo]

I'm in a similar situation. My wall will be flush on the inside - so no buttressing required.

On my outer leaf I'm toothing in the bricks for aesthetic reasons. For the inner leaf, I'm going to butt the blockwork up against the existing outer leaf (which is now slightly destabilised due to the toothing removing its full bond) with the intention that when I finally knock through I'll going to make-good the ~150mm gap that I'll end up with on the inner leaf.

I'm probably making work for myself but I don't really know how to size up the stability of what would become a free standing outer leaf (apart from the wall ties...) if I did it the efficient way...

 

[browfish]

Would you not end up with the remaining external corner being a cold bridge and a route of damp from outside to internal flush wall?

 

[theoldun]

Yes that's right, but it would be a poor designer who allowed an external wall to become part of an internal wall - even with just a slot for the vDPC

If there was some buttressing required then this should be formed from the internal leaf only

Not saying we agree with it, but afraid that's the way it is done these days Lots of BIs do not even ask for the 30mm slot for vertical dpc.
If you formed the buttress wall only from the inner skin, then what holds the outer skin up?
Regards oldun

 

[theoldun]

I'm in a similar situation. My wall will be flush on the inside - so no buttressing required.
.

Different ball game altogether. You are joining new to old on a straight pull. Run a saw down back wall half brick in. cut toothings dpc to joist, lift 1st lift face brickwork, jump inside run saw down back wall again about 400 from first cut, pull slot out and fix starter ties to back wall of inner skin thereby forming continuous cavity. Drill out or saw out inner skin on knock through.
The way we do it is to erect poly dust screen in side and cut internal blockwork down from outside the same size slot as external before building inner skin. You can then either tooth into existing block or use a starter.
Regards oldun

 

[^woody^]

If you formed the buttress wall only from the inner skin, then what holds the outer skin up?
Regards oldun

Like this

Untitled

• ^woody^
• 26 Apr 2012
• 1

If there was no butress, then the new internal wall (light beige) would just go straight up to the existing internal wall (grey)

The cavity would be continuous and leafs tied as per normal cavity walling

 

[browfish]

I have cut open the cavity this morning to fit my wall starter kits. The existing wall is filled 100% with a blown fibre glass. the new cavity will onlyl be partially filled. How do i keep the existing insulation in the existing wall?

 

[noseall]

Snug fit some dritherm cavity batts in there.