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Method of fixing/supporting flat roof joists

Discussion in 'Roofing and Guttering' started by walkingman, 26 Aug 2012.

  1. walkingman

    walkingman

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    I am installing a new flat roof in my new garden building. Due to height restrictions I need to use some method of hanging the roof joists between the top of the walls. There will be 11 joists each spanning 3.6m and at 40mm centres. I plan on using 175x50 joists. The wall are 140mm single skin concrete block.

    From my research I notice there are a number of methods of doing this,
    1 Joists built into walls.
    2 Supported by steel beams.
    3 Supported by joist hangers built into walls.
    4 Supported by timber bearers and a supporting fillet.
    5 Supported by timber bearer and framing anchor.
    6 As part of a frame held together by framing anchors and joist hangers.

    I am leaning towards 3 or 4/5 as being the simplest options. 1 is a non starter. Also, given that the building is single skin and the walls are going to get damp I want to minimise contact between wall and timbers.

    Obviously 3 specifies joist hangers built in to the wall. At the top of the wall this won't be possible. Is using hangers directly bolted to the wall a viable option?

    If using timber bearers how would they be secured to the wall and of what size would they be?

    All this will be holding up a bog standard felt flat roof with chippings.

    Ease of construction and cost are paramount here. I would be jolly grateful for any input. This is the last big headache of my build. Get this sorted and it's plain sailing - I think. Well maybe.
     
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  3. chickenlips

    chickenlips

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    Joist hangers can be used, but you need to use Safety fast hangers (Simpson Strong Tie) or Rapid build (ITW Cullen) hangers. These are designed to work with no blockwork/brickwork above.

    Normal masonry hangers require 675mm of cured blockwork above to work. There is also the return style option in standard masonry hangers which still require blockwork above, so dont be fooled by them.
     
  4. walkingman

    walkingman

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    Many thanks chickenlips.
    In the end I opted for No.4 as it seemed the easiest option. The 7x2 timber bearers are held up by 24 M12 through bolts and the joists are notched over a 3x2 fillet and skew nailed into that and the bearers. It all seems pretty solid :)
     
  5. aztectt2000

    aztectt2000

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    Hi - I've just this second posted a similar question. Can you explain what option 4 is/how you did it? I don't quite understand :)
     
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  7. walkingman

    walkingman

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    Hi Aztec,
    The method I used involved two 7"x2" timber bearers bolted to the longest walls opposite each other such that the top edge of the timber is level with the top of the blockwork. Then 3x2 timber fillets screwed to the front of the bearers such that the bottom of the fillet is level with the bottom edge of the bearer. Then using 7x2 joists (This was for a 3.5m span), cut a 3x2 notch at each end and sit them on the fillets flush to the bearers. Skew nail into both bearers and fillet. You can then fit your firrings on top of the joists running them over the top of the walls at either end.
    The only reason I did this was to keep the building below 2.5 metres for planning purposes. If you don't have that restriction I wouldn't do it that way. I used M12 through bolts to attach the bearers to the block walls, using 12 bolts either side over a 5 metre bearer length.
    If I'd left it at that, even with just a reasonably light felt flat roof I'd be worried right now. I had to place the bolts closer to the top of the blocks than I should have (10 -12cm rather than the 18cm advised) as it is I had to cut notches in the timber fillets to fit around the protuding bolts.
    However, I built an inner timber frame about a inch inside the block wall which which rests directly on the foundations adding additional support to the bearers/fillets.
    Hope I've made things clearer. If not just ask. :)
     
  8. ^woody^

    ^woody^

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    With flat roofs, the easiest (and best) option is to sit the joists on the wall.

    This not only saves material and labour costs, but allows a good edge detail with the joists carrying across the wall to finish the roof cover and allow fascia fixing etc

    Bolting a bearer plate, is not ideal as if there is any movement in the roof, it will dislodge the top course of masonry as there is little weight above it
     
  9. aztectt2000

    aztectt2000

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    Thanks WalkingMan and Woody.

    Yes I understand now what you did, and it sounds good, however my current situation looks like reinforced joist hangers will be the best bet. If I could sit the joists on the wall I would, but that would take me over the 2.5m height (due to needing 9x2s for a 5m span) and I don't really have the option to lower either the wall or floor height.
     
  10. chickenlips

    chickenlips

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    If your struggling on height then maybe look at engineered open web joists, there are a number of end details which can be used that will still have the joist sitting on the blockwork rather than a beam/ledger.

    Yes they are a little more costly, but you should be able to go down to 8" on that span.
     
  11. DIYnot Local

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