Extention, non continuous cavity.

[B1lly L]

Hi all,
I am in the process of self building a small single storey extention, currently building up to DPC.
I will be using wall starters and Dom to join both (block) skins to the original back of the house, which will be completely knocked through right up to the original sides of the house.

I’m currently stuck as to continuing the cavity between the old and new. If I cut even a blade width slot in the wall to maintain a cavity it will weaken the corner and leave me no support for the RSJ.

What are my options to prevent damp/cold bridging on the section of wall that span the external and internal walls?

 

[noseall]

Where will the steel beam ultimately sit?

Can you rotate the image next time please.

 

[B1lly L]

Drawing shows/states centrally over the cavity of the rear wall with a 400x100mm padstone. To me that means, half on the original inner skin and half on the return section on the rear outer skin, hence my concern of separating this from the rest of the wall. If I cut the cavity out the padstone will be on a 4” square pile of bricks essentially.

 

[stuart45]

Can you get the plans up?
I'm guessing the red below the cut lines is being removed, so the new inner skin will come across the cavity and be tied into the inner skin on the left.

DSC00033

• stuart45
• 18 Aug 2019

 

[matlob]

If the house is two storey I don't think you can knock through the entire wall width with just a rsj. I think you can only knock through 2/3 of the wall ensuring there is enough width either side to sit the beam on and maintain structure. It could be all knocked through if using a frame though... Not 100% sure but that is what an SE has told me recently as doing the same.

 

[B1lly L]

Plans are vague.

Tying the new inner into the old inner is possible, but would involve me making a big hole in the back of the house months before it can then be made water tight again.

B1,b2 and B3 are all RSJs replacing walls. Hatched line are all new walls.

 

[stuart45]

Plans are vague.

Tying the new inner into the old inner is possible,

View attachment 169763

How else were you thinking of doing it?
https://www.contractortalk.com/attachments/f90/46283d1302641945-deep-footings-phplzylizpm.jpg

 

[noseall]

It looks to me as though your new and existing blockwork will not line up in any case.

I would also refer to the engineers' calc's and look to see if there are any recommendations regards tying in, buttressing, size of piers etc.

 

[cdbe]

This is mine:

Everything behind and to the left of the level will go and my new block work will meet the 100mm which is left thus continuing the cavity.
On my other side there is a return as it steps in from the boundary but the principle is the same (everything to the right of the inner starter will go):

You do of course need to chop big holes in the back of your house! My right hand side is being held up by a piece of 4x2 (the hole wouldn't normally be that big but I needed the bricks elsewhere!):

And the left hand by a 100mm pier next to the window frame:

Support for my steels on the straight cavity side is via a "spreader" padstone lengthwise on the inner leaf, which I suppose is essentially what your 400x100 padstone is.

 

[B1lly L]

Cheers Cbde, pics help loads. Rethinking what I was planning now.

Any tips for waterproofing the hole for a few months?

 

[noseall]

Any tips for waterproofing the hole for a few months?

We usually use shutter ply and studs...

Temp1

• noseall
• 19 Aug 2019

Temp2

• noseall
• 19 Aug 2019