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Timber sizes and spacing for a monopitch roof on timber outbuilding

Discussion in 'Building' started by MrPlank, 14 Jul 2019.

  1. MrPlank

    MrPlank

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    Hi Guys

    I am building a garden room in timber and wanted to ask what size timbers I need to use for the roof. The span it will need to cross is 4 metres, the actual size is 4m x 7m, the monopitch of the roof will be 4 degrees. Will using smaller spacing mean I can use slightly smaller sizes or is this irrelevant. I intended to use 400mm spacing. I had planned on using 145x 47mm but recognise that this is not going to be good big enough, the roof covering will be cladco box profile. As always any advice greatly appreciated and I hope my wording makes sense.

    Thanks
     
  2. Notch7

    Notch7

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    170 x 47 C24 is really the min.

    145 x 47 would sag a bit over 4metres, putting in more joists wont help with the sag, but it would somewhat increase load capacity, say for snow loading. Maybe 145 x 47 would be ok for box profile, its not particularly heavy.

    If the external building size is 4 metres, the clear span will be less, say 3.8m
     
  3. MrPlank

    MrPlank

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    Hi Notch7

    Thank you for prompt reply yes the clear span is 3.8 metres, the roof will have an overhang at the front of the building, the length of timber I intend to use is 4.8 allowing around 50cm ish over hang at the front. will 170x47 still be sufficient or is this detail not really rellevant.

    Thanks again
     
  4. Notch7

    Notch7

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    no the overhang is irrelevant -what counts is the clear span.
     
  5. tony1851

    tony1851

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    More joists would reduce the deflection of the individual joists because each joist will then be supporting less load, which proportionately reduces deflection.

    Additionally, deflection is only a 'serviceability' issue and not a structural problem.

    If there is no plaster ceiling, and if the steel sheets are full-length, the deflection may be of little consequence, 6" x 2" @ 300-350c/s would be fine.
     
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  6. MrPlank

    MrPlank

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    Hi guys

    Thanks for the responses, the box profile sheeting will be full length and the internal ceiling will not be plastered, I intend to finish it in 10mm ish softwood cladding.
     
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