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joist sizes for loft conversions

Discussion in 'Building' started by Railjohn, 17 Dec 2011.

  1. Railjohn

    Railjohn

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    I am coverting a loft into a railway room,and I want to remove parts of 2 collar ties.I have had drawings done by a structural engineer,these show that the existing 19mm chipboard flooring has to be replaced by 25mm chipboard.Thejoists are 3 by 2 inches,and there is supporting walls making the span for the joists between4 and 10 feet,although the new chipboard only covers 9 -10 feet either side of the centre.The collar ties are held in place by new 4 by 2 uprights bolted tothe collars and to the rafters.Are the joists man enough?
     
  2. mointainwalker

    mointainwalker

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    Well if the SE has done calcs for the floor thickness ( ?) then surely he must have considered the load on the joists .
     
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  4. ^woody^

    ^woody^

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    I can't believe that an engineer has spec'd 3x2 floor joists or that building control have approved them

    Even with the lightest loading and the closest spacing (400mm) they still wont span more than 1.1m - less than 4'
     
  5. RonnyRaygun

    RonnyRaygun

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    Are the 3x2s existing?

    No, they're not man enough, anywhere near, to span 10 feet, half that if you're lucky.

    25mm chipboard is usually used where joist centres are 600mm.

    If your joists are going in at 600mm centres then you need 50x175 C16 to span 10 feet, which is what your SE should have specified, assuming standard domestic floor loads.
     
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