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Placement of bifold / slider - cavity wall foundation

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Hi all


Wanted to gauge the best approach as to where to site a bifold or slider within the depth of the wall, when one is contending with a 100mm cavity.


The wall detail is below. This TF is being put onto existing foundations from our conservatory, and thus I am building it over the brick / block and 100mm cavity already there.


From foundations (which have been checked by SE) there is 2 courses of dense concrete block and 6 courses of facing bricks, then DPC. 3 courses are above the ground. 


Interestingly the concrete infill has been done up to 4 courses of bricks - see pic below which shows the current state of it (current conservatory upvc / glazing not removed yet) ........... and yep, going to need to fix the DPC / DPM that the previous lot have hacked out!!


I am aware of armatherm, compacfoam and phonotherm as solutions for supporting a bifold over a cavity, however, given the state of the current concrete infill, I would need chip some out and then use some sort of liquid self level to level this off to use compacfoam / armatherm, and phonotherm would raise the door 25mm which affects threshold height.


If I stick them on the outer leaf, there is the minor issue of floor tiling over the block / cavity area. This was done previously over the cavity with a cavity closer by prior owners and all the tiles had cracked.


If I stick them on the inner block, I need and huge sill and will have huge reveals.


Thoughts welcome.









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16 minutes ago, Dave Jones said:

marmox blocks under the bifold. dpc and conc the cavity behind it. run flooring/screed over the top.


So get some 100mm marmox blocks and put them in the cavity area, and backfill the remainder with concrete, and thus sit the bifold over the cavity area?

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On 09/05/2024 at 14:53, Dave Jones said:

no sit the billfold's on top of the marmox, they are a thermal break. mass fill cavity, insulation over top, screed on top of that.


I decided they are going on the outer skin, so reveal matches the windows.


I managed to remove the inner skin of blocks across the bifold width today and levelled off the cavity infill to match that of the floor slab.

I then cut down some new blocks to the level of the concrete slab / infill and have cemented them in.


Tomorrow I will lay DPM, fit some perimeter insulation and take 100mm PIR across to the outer skin, slip sheet over top then screed it.




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