Flat roof timber spans.

Discussion in 'Builders' Talk' started by christian, Jul 28, 2013.

  1. christian

    christian Guest

    Hi there,

    I've  started building a flat roofed timber framed garden office. It measures  3.6m x 3.6m and it's max height (without applying for planning  permission) is 2.5m.

    I  was hoping to use lengths of 4 x 2 spaced at 400mm for the roof, clad  with ply before the roof covering goes on but I'm starting to realise  that 4 x 2 over 3.6 isn't nearly strong enough, my problem is by using 6  x 2's I'm really compromising the height of the ceiling inside.

    The roof will have  60mm firings

    Does anyone have any suggestions?

    Can i use more 4x2 as sister joists?

    Or 5 x 2 perhaps?

    Will 5 x 3 help?

    Or, is there little option but to use 6 x 2?

    Thanks in advance for your advice,

  2. I don't have my span tables on me, but you can get extra length by increasing the timber size or decreasing the centres. but you are already going for the smallest centre sizes at 400mm so its just increasing the timber size that will dounless you want to put a binder in some way like underneath. 5x3 gets you more span, so does 5x2 but can't tell you how much without the tables. if you can get your hands on them, its trada tables your after
  3. chippie244

    chippie244 Super Member

    If you glue and screw ply top and bottom to create a torsion box you massivly increase the strength but I couldn't give you figures, is it for you or a client?
  4. joinerjohn1

    joinerjohn1 Screwfix Select

    LB, Trada span tables would suggest 170mm x 38 mm for a 3.6m span (7" by 1,3/4") at 400mm centres.  Nice suggestion by chippie there, but would need calcs for BC to be happy with it. ;)
  5. surfermick

    surfermick New Member

    as its just an office and not a garage would it not be possible to work around a central pillar, you could perhaps attach some shelving around it to use it as part of the office.
  6. Sean_ork

    Sean_ork Screwfix Select

    put in a shallow central I beam - this will half the span, skin the top and bottom with 9mm OSB, nail no more that every 100mm, joists notched and hung off the webbing

    you could end up spending a lot of money on timber if you start doubling up
  7. christian

    christian Guest

    Thanks for all of your suggestions guys.
    Yes, the unit is for me.

    I'm resigned to using the correct size timbers, I'd rather have a solid garden building with a straight roof and 2/3 inches less headroom than plenty of headroom and a dirty great puddle on the roof!! 

    Thanks chippie 244 for the clever suggestion, I'm tempted but it's a risk I guess.

    Out of interest, what thickness ply should I use if adopting this idea? (The roof is 18mm)

    Thanks and best wishes,

  8. chippie244

    chippie244 Super Member

    Anything from 6mm up works but as I said I can't give figures but for the joist to sag it has to stretch the glued and screwed ply.
  9. johncavill

    johncavill New Member

    3/4 ply screwed and glued to side of joists will strengthen a lot

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