is 200mm cavity block and brick wall acceptable

Discussion in 'Builders' Talk' started by Parttimechippy, Jun 7, 2018.

  1. Parttimechippy

    Parttimechippy New Member

    Hi, Is it ok to build a cavity wall with a 200mm cavity?

    I have a 9" solid party wall, and in order to build right up close to it with a 50-100mm cavity, I would need to underpin the wall.
    The existing footing is 250 below ground, 250 deep and not deemed suitable for the internal blockwork skin we are proposing.

    An alternative suggested by BC, is to build a new footing (450 wide, 1m deep) next to the existing and then I can build on the edge of that alongside party wall. But the existing footing protrudes 200mm from party wall so it would leave a 200mm cavity.

    So starting point is, would a 200mm cavity be acceptable?
     
  2. Parttimechippy

    Parttimechippy New Member

    these images may help explain what I mean! footing options image.png plan view.png
     
  3. rogerk101

    rogerk101 Active Member

    I don't see why not, but it seems like a waste of space to me.
    If it were an external wall, then you could stuff 200mm worth of insulation in there, but if it's just a space between your wall and your neighbour's wall, there doesn't seem much point in adding that much thermal insulation ... unless you want to turn it into sound insulation.
     
  4. Parttimechippy

    Parttimechippy New Member

    Hi roger, the point isn't to add more insulation, I would ideally just have a 50mm gap as originally planned. but that means 4m of underpinning, so for the sake of losing 150mm in the kitchen, seems the simpler option
     
  5. Parttimechippy

    Parttimechippy New Member

    I've since read that a wall like that would be classed as independent so might need 150mm blocks? kitchens getting smaller...
     
  6. rogerk101

    rogerk101 Active Member

    In a semi-detached rental home that I own, I had exactly the same sort of situation you describe.
    There was a really horrible flat roof extension sticking out the back of my house, created on the cheap by the people from whom I bought the house. There was a gap between our extension and the extension sticking out the back of the other half of the semi. The gap was always damp, collected rubbish, and was a complete waste of space (about 400mm).
    I spoke with the neighbours and suggested removing my extension's wall and eliminating the gap altogether, thus improving the thermal performance of the two houses (by joining them together at the extensions in the same way as they were joined together as the original house. I was the one doing the work, so I was the one benefiting from the extra space in my extension. We would have shared it had they been willing to move their wall.
    We ended up gaining over 600mm of space in the extension, vastly improved the thermal efficiency of both houses, removed a source of damp, and everyone's happy. Could you not do something similar?, i.e. could you not just forget about your wall altogether and just use the outside of their wall as the inside of yours? You'd still probably want to dot and dab some plasterboard on your side or something similar, but you sure wouldn't need any new footings.
     
  7. Isitreally

    Isitreally Well-Known Member

    Will they not let you underpin the area that is taking the new block/Brick work.

    2018-06-07 12.45.52.png
     
  8. Thanks for sharing this planning.
     
  9. Parttimechippy

    Parttimechippy New Member

    I suggested partly underpinning say 300 wide directly under 100mm block wall but was told by BC that it would create an uneven load so would have to be full underpin or new independent footing (albeit quite close to existing.
     
  10. egon999

    egon999 New Member

    overpin the existing foundation. dig down to below the bottom of the existing foundation and extend it to approx. 450mm wide, then concrete the excavation but flood the concrete up and over the existing foundation. the new and old concrete will act as one and you could then build your new inner leaf closer to the existing wall.
    ive done a cross section. bit blocky but it shows what I mean
     

    Attached Files:

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  11. Parttimechippy

    Parttimechippy New Member

    Hi egon, I like the overpin idea, will pitch that.
    Annoyingly I think I'm going to have to pay a structural engineer to calculate it though.

    I guess thats one advantage of going for the bigger cavity is they may well accept as is.
     
  12. egon999

    egon999 New Member

    no need for calcs surely? cheaper to just offer to dig it bigger if the building control bloke wrings his hands when you suggest it. cheaper than paying an engineer?
     
  13. Parttimechippy

    Parttimechippy New Member

    I've only just appointed B/C so he hadn't even reviewed my plans when I spoke to him. But he did suggest any 'specialist foundation' involving the existing would need calcs.
    Its a load bearing wall taking 1st floor joists and a truss roof so I can see where he's coming from

    obv I'm of the school of throw enough concrete at it and it wont move
     
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  14. egon999

    egon999 New Member

    if the bloke has enough years behind him and you bombard him with a solid proven detail (then offer to make it bigger!!) he should be ok with it. run it past him. I don't think its a 'specialist' design. just a standard detail.
    if he insists on calcs then look for a different building control bloke with a bit more willingness to not rely on an engineer to justify every step. BCOs vary in experience and some are easier to work with than others.
     
  15. Parttimechippy

    Parttimechippy New Member

    this is the bloke at the council. We've already appointed them so too late to change. But I've dealt with him before, he's a reasonable guy and he runs the office so assume he is fairly experienced.
    We'll work something out. I'll let you know whats accepted!
     
  16. egon999

    egon999 New Member

    sounds promising. copy him my sketch by all means if it will help. fingers crossed. post up what happens
     
  17. stevie22

    stevie22 Active Member

    BC will be looking for minimum depth dependent on soil type and proximity of trees
     
  18. Parttimechippy

    Parttimechippy New Member

    theyve said 1m deep trench fill and 600 wide for the cavity walls (100mm brick 100 cavity 100mm block)
    so presuming the same for the party wall section
     
  19. rogerk101

    rogerk101 Active Member

    I've never understood why standard residential foundations in Britain don't seem to have steel in them, so need to be a minimum of 1m deep and 600mm wide ... that's a lot of concrete!
    I've lived and worked in many different countries around the world, and nowhere else uses as much concrete in foundations. Instead they include a 4 x 10mm steel rebar cage and considerably less than half the amount of concrete.
    Despite all the concrete used here, I still regularly hear people referring to 'settling cracks' and people just accept this. I've never heard of or seen settling cracks in houses in other countries ... nor would anyone even tolerate them.
     
  20. Parttimechippy

    Parttimechippy New Member

    Roger didnt see this before for some reason.
    I dont think using the existing would work as we are going up to a second storey so it becomes load bearing for joists and new roof. But worth exploring
     

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