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Shed/workshop roof help

Discussion in 'Roofing and Guttering' started by jotto, 5 Sep 2008.

  1. jotto

    jotto

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    am building a wooden shed/workshop and have got the main frame up. It measures about 5.1 x 6.0 m.

    I need to sort out the roof but am unsure what way to go, budget is very small!
    I wondered about either a flat roof, ie timber frame, ply or OSB3 and then felting it or a small slightly sloped roof.

    For the span of 5.1m what would you guys recommend. Im concerned about going up much further in height so want to keep any pitch to a minimum.

    Would a single pitch of say 0.5m height be ok?

    looking for some help and advice on how to construct it.
    Thanks.
     
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  3. chappers

    chappers

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    how high is your shed you can go upto 4 metres for a ridged roof or 3 metere for any other.
    In answer to your question 0.5 metres over 5.1 metres would be more than adequate.
     
  4. jotto

    jotto

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    The shed is about 2.5m high.

    I was thinking of making 11 trusses so that when using 8x4 ply or OSB3, they would be supported at the edges and in the center. There will not be much weight on the truss, just the board and some felt.

    How many supports and at what angles would be needed in the trusses to stop the 5.1m span from sagging? The frame of the shed is 3x2 and I was thinking of using this for the trusses also. Would this be ok?

    From my basic calculations, a 5.1 span with a 0.5 height will result in a 5.12 diagonal. does that sound correct?

    If I went with a flat roof ( or is this not recommended ) what size timber would I need to use across the span without worrying about any sag?

    TIA.
     
  5. ^woody^

    ^woody^

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    If you are felting this roof, then you are effectively constructing a flat roof, so you may as well construct one.

    For trusses at such a low pitch you will have problems with timber sizes, and supports.

    For a 5.1 span you will need 50 x 220 c16 timbers at 400 or 450 centres. Then furring strips to get your 1 degree slope - these will be about 100mm down to nothing if running across the shortest span of 5.1m. So the depth of the roof at the highest end will be about 350mm
     
  6. jotto

    jotto

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    Thanks for that.
    2 questions. What is c16? Im guessing its a specification of some sort. Just wondered what it was in lay terms. Also, could you explain the furring strips for me as well. Are they strips of wood laid on the timbers 100mm thick at one end running down to nothing at the other end, in essence making a slope?


    googled..
    c16 is stress graded timber.
    "Firring" is a U.K. term for wood strips which are usually 50mm wide, tapered and fixed above wood roof joists to provide drainage falls below roof boarding.

    Also just found and downloaded a set of trada span tables, I see where you get the measurements from. Many thanks.

    What sort of price would 76.5m of 220x50 c16 run to as a matter of interest, ball park?
     
  7. noseall

    noseall

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    me and woodster still call 'em furrings but you may have to call 'em diminishing strips when ordering or buying. ;)
     
  8. jotto

    jotto

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    Will ask around at the local roofing merchants today.

    What sort of cost am I going to be looking at for this project?
     
  9. chappers

    chappers

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    Me too any timber yard worth their salt will know what you mean, I'm sure if i went into my local timber yard and asked for diminishing strips they would wonder what the hell i was on about.
     
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  11. jotto

    jotto

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    OK, this may sound like a really crap way of doing it but is there any reason why I cant just raise one side of the shed up by 100mm and then put the joists on?
    I know I would need to trim the ends a little, but over this span, the angle at the ends will be not be much at all.
     
  12. noseall

    noseall

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    there is no reason at all why you can't pitch the whole roof. many 1960's garages were done this way.

    it does mean the ceiling will appear sloped , but i would encourage you to cut the correct angle to the joist ends or it will look very odd.
     
  13. ^woody^

    ^woody^

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    Apart from the sloping ceiling, which may not really be a issue, it will mean that you have extra work in cutting the ends vertical to fix the fascia .... and possibly doing a small birdsmouth for the seat?. Also the fall will have to be in the direction of the joists.

    But yes, if you trim all the joists down on the floor, then it may be worthwhile and easier sloping them

    Or cut your own furrings out of some 4x2 or 5x2 with a circular saw

    50 x 225 is about £3.50/m around these parts.

    Also allow for nogins at 1/3 span centres. And strap the wall plate or joists ends every 2m
     
  14. jotto

    jotto

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    Thanks. Should be easy enough. Another n00b question, sorry...what do you mean by strap the wall plate or joist ends every 2m?
     
  15. jotto

    jotto

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    Oh, and finally...

    Should I sit the joists on top of the walls or hang them from the inside and what are the correct fixtures I need to be looking at?
     
  16. ^woody^

    ^woody^

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    Yes, joists normally over hang and the fascia is fitted to the ends. This forms a drip to protect the walls below.

    You could either block up between the joists or just fit some noggins. Then you would fit some 1200mm long twisted vertical straps every 2m or say every 5 joists of at 400 centres, and this will hold the roof down.

    You would not use hangers as this would be a weaker connection of the roof and walls as the load is only on the top course which could easily be moved outwards by the roof - and also there is the cost of unnecessary hangers!
     
  17. jotto

    jotto

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    Thanks for that.

    Finall got around to buying 16 lengths of 225x50 C16 timber, 5.4 m long. Will mount at 400 centers.

    Am using screws, brackets and wooden blocks to secure the timbers to the frame top and will buy the twisted ties as suggested.

    Am looking forward to getting this thing finished...been dragging on for ages now!
     
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