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Loft conversion floor Joists

Discussion in 'Floors, Stairs and Lofts' started by shedifice, 14 May 2019.

  1. shedifice

    shedifice

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    Hi,

    I am about to convert my loft. I have my own drawings and was planning to use telebeams. The span from wall plate to wall plate is 5.2m.

    ON first visit by the BC they suggested that I could do the job cheaper with wood if the live load was reduced in the loft. As the loft is small, the suggestion is that the load is reduced from the 1.5Kn to 1Kn. Also current ceiling joists are at 400, so the plan is floor joists at 200 centers to reduce the load per floor joist even more.

    So the recommendation from the BC who will be signing it off is 200 ctrs, 7" x 2" joists to support the floor and roof (dwarf wall)

    Any views on the solution? I am thinking that I am going to struggle fitting noggins between 200 centre joists and need some reassurance on the span vs joist size. I am all for saving the cash even if its a bit more effort.

    Again, this is coming from building control, not just a guess from me.

    Thanks for any input.

    Conrad.
     
  2. JobAndKnock

    JobAndKnock

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    You can install solid strutting (noggins are in walls) between joists at 200mm centres, however it requires an angle drill or an angle impact driver to do it (no earthly chance of through nailing in such a tight space) and your strutting will need to be offset by the thickness of the strutting on alternate joists
     
  3. catlad

    catlad

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    8x2" at 300 centres over such a short span would be enough
    in my opinion.
     
  4. JobAndKnock

    JobAndKnock

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    Would be a heck of a lot easier to install, too (y)
     
  5. tony1851

    tony1851

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    The inspector has shown some common sense in allowing that a loft floor never gets loaded to the full 1.5kN/m2 because of the low headroom towards the eaves.
    However, if the floor is also supporting dwarf walls - and depending where along the span the walls come - the deflection could well be beyond the normally accepted limit for domestic floors. 7 x 2 is not much for a 5.2m span. It won't collapse, but might bounce when you walk along it, and maybe cause some cracking in the ceiling below.
    As Cat advised, consider 8" instead. If you could glue and screw the boarding down, that would also improve rigidity.
     
  6. shedifice

    shedifice

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    Thanks all for the feedback. Its the spring that I and a bit concerned over. the 7x2 are already losing me 1" over the aluminium beams and at 8x2 it would be 2". at 6'2 I am after every inch I can get!

    I can glue and screw the flooring. Rafters are at 400mm so 300 would go a bit odd.

    Good to have a common sense inspector and he is really knowledgable which is good.
     
  7. Leofric

    Leofric

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    Seems surprising that Building Control are specifying what you should do , they usually require a structural engineer's input for structural alterations :!:
     
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  8. tony1851

    tony1851

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    If you doubled up the 7x2s by screwing them together in pairs and fixing @ 400 c/s, they would be slightly stiffer than having them singly @ 200 c/s.
    (With timber, 2 identical pieces screwed together is about 10% stronger in bending stress and will also give slightly less deflection)
     
  9. shedifice

    shedifice

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    Leofric,

    He used to be an SE. I have fully designed plans with calcs etc but he is being helpful in looking to see if I can save money. Pretty awesome I thought!
     
  10. shedifice

    shedifice

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    Tony,

    Is the increased stiffness due to reduced twist? The area is not different so the moment of inertia would be the same.

    L.
     
  11. tony1851

    tony1851

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    Timber is a natural material which varies in strength, even joists cut from the same log.

    All timbers are 'strength-graded', taking into account defects such as fissures, knots and slope of grain. A timber joist with a small knot near the bottom will have an in-built defect, which will slightly reduce its strength, depending where along the length the knot is. Conversely, the next joist to be checked might have no knots in critical conditions and will be very slightly stronger, even if in the same strength range.

    Fix the two pieces firmly together and the slightly-increased strength of the latter piece will compensate a little for the weakness of the first piece - hence the combined beam will be slightly stronger.

    In general, for two combined joists, the bending strength is increased by about 10%, and the modulus of elasticity by a factor of 1.14. The moment of inertia is irrelevant in this instance. The slightly increased modulus of elasticity will give a corresponding reduction in deflection.
     
  12. shedifice

    shedifice

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    Fantastic answer Tony, clear and quite obvious when you think about it. I am a mech so used to metal that should not have voids~:)

    Cheers.

    l.
     
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