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increasing joists sizes for building on a flat roof

Discussion in 'Building' started by nicola armstrong, 15 Jan 2018.

  1. nicola armstrong

    nicola armstrong

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    Hi, I am building a small extension on top of an existing kitchen (flat GRP roof) and have been advised by the structural engineer that I should increase joists from 6 x 2 to 7 x 2 (span is 4m).

    The question is: do I need to remove the existing joists or can i just place new joists in the center of the gaps between the existing joists? (pic nba22 d shows the roof opened up).

    The kitchen ceiling is fixed to the existing joists so not removing the existing joists would avoid damaging the ceiling.

    And a second question to the same area ( see pic nba22 e ) - i need to build on top of the parapet wall but there is a conservatory nest to it. Am I able to just remove the coping and internal side flashing and build block/block above?
     

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  3. John D v2.0

    John D v2.0

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    You don't need to remove the old joist and that's how you can convert a loft without touching the ceilings belle, but I'd imagine the se has specified solid blocking or herringbone struts, so you'll struggle to get those in.
     
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  4. noseall

    noseall

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    You need to fit the joists so that the adjacent upstairs existing floors align through with new, irrespective. If this means that your new joists fall between the old, then so be it. This will mean added fun for the electrics and plumbing though, both in removing any old stuff and installing new.

    However, if the exiting joists have been laid on a timber wall plate, you will need to remove them in any case.
     
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  5. noseall

    noseall

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    Yes, as long as you maintain the clear cavity with either clear air space (remove any bridging masonry) or fit a CT if the existing part is solid.

    I presume you have had a foundations suitability test hole dug?
     
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  6. tony1851

    tony1851

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    7" x 2" on that span might be a bit bouncy.
     
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  8. 4m span with 400 centres should need 8x2, but as Noesall says, what's the alignment with the existing floor levels, as this may not be an issue once you've worked this out. But you may need to take off the top course of bricks, and then re-instate the wall plate. If you start dropping them in the middle of the existing joists, then the outermost ones won't be providing support by the walls, and will throw the centres out, so might be best to bite the bullet, and replace them properly as you go.
     
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  9. nicola armstrong

    nicola armstrong

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    ok thanks for the info.

    so, something like this (from your comments and talking to the builder):

    1. Prop existing joists from the kitchen.
    2. Cut the existing joists (giving allowance to connect them to joist hangers later)
    3. Cut out wall plate between joists (wall plate is sitting on the internal wall).
    3. Build up the wall and fit build-in joist hangers. Block over the existing joists so there is no pressure on the wall plate.
    4. Fit existing joists and new joists onto the build in wall hangers.
    5. Fit a dpc membrane with vents to stop water coming into contact with the timber.

    From some of the comments:

    The existing flat roof is 100mm lower than the internal floor of the house so we have room to achieve uniform floor levels.

    Trial holes were done and BC inspector ok'd it.
     
  10. So you need to work backwards from the existing floor to find out where to set the new joists. Take off the floor thickness, then the joist thickness, and then see where that leaves you. Sorry, didn't pick up on the existing ceiling, so yes, try and save it, it'll be much cheaper if you can. You could cut the ends off the current joists so they don't extend beyond the inner skin. Then put the new joists in between the existing joists, and then at the end of the room as well.

    Why should water be coming in contact with the joists.
     
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  11. nicola armstrong

    nicola armstrong

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    thanks again. sorted !!
     
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