Cavity wall inner skin at door opening?

[Peter500]

I'm building an extension (DIY) and I'm a bit unsure about the inner skin around the door opening. I've gone single skin for the door openings but I've realised that a lot of people seem to have it coming up to around 1 block (225) below FFL. I've attached a picture to show the two options. That's my build up in the picture (150 hardcore, 50 sand, 150 concrete, 150 PIR, 50 screed).

I can't work out what the inner skin does as there is nothing above it that it needs to support, but I can't imagine builders would spend the time/money doing it if it's not required. I would imagine a concrete fill would add strength to the outer skin when the oversite is being done but I would expect that to be fine once the oversite is finished.

 

[^woody^]

Lots of posts about door thresholds. Do a search

 

[Peter500]

Thanks Woody, I didn't know what to search for. I had a look and the drawings seem to all show blocks on the inner skin, although I'm still not sure what the reason for it is.

Thing thing that is making it a little more complicated for me is that I have a pipes going under my large door and a large lintel (approx. 1000mm span). I'm wondering whether a concrete lintel would be suitable and what spec I would need. I know that they need to be evenly loaded and I think normally have at least 3 courses of brickwork above them. I don't know whether having concrete straight on top of the lintel would potentially put too much load on the lintel in one spot? I don't know whether I should go for a cheap 65mm one or 140mm or whether neither are adequate?

 

[Notch7]

I'm building an extension (DIY) and I'm a bit unsure about the inner skin around the door opening. I've gone single skin for the door openings but I've realised that a lot of people seem to have it coming up to around 1 block (225) below FFL. I've attached a picture to show the two options. That's my build up in the picture (150 hardcore, 50 sand, 150 concrete, 150 PIR, 50 screed).

I can't work out what the inner skin does as there is nothing above it that it needs to support, but I can't imagine builders would spend the time/money doing it if it's not required. I would imagine a concrete fill would add strength to the outer skin when the oversite is being done but I would expect that to be fine once the oversite is finished.

its easier for brickies to run the blockwork all the way around with the same cavity.

generally the blockwork is knocked down to concrete oversite level at door openings and then the insulation is run to butt up to the outer skin. It can be supported if needs be but once the screed is down it usually doesnt matter.

one thing it does do is maintain a cavity all the way around, 225mm below DPC. that means any damp, cavity condensation or rain penetration just falls down the cavity to below floor level.


you can run oversite to the outer skin at doorways, but you need to make sure you have adequate DPM to avoid bridging

 

[Peter500]

its easier for brickies to run the blockwork all the way around with the same cavity.

generally the blockwork is knocked down to concrete oversite level at door openings and then the insulation is run to butt up to the outer skin. It can be supported if needs be but once the screed is down it usually doesnt matter.

one thing it does do is maintain a cavity all the way around, 225mm below DPC. that means any damp, cavity condensation or rain penetration just falls down the cavity to below floor level.


you can run oversite to the outer skin at doorways, but you need to make sure you have adequate DPM to avoid bridging

Thanks, it sounds like I can probably avoid the need for lintels in those areas . I'm not entirely sure that the cavity would let water run down as it's usually filled with concrete.

My DPM should hopefully keep all of the water out