300kg roof lantern, on 6" roof joists over 4m span?

Discussion in 'Carpenters' Talk' started by Ed209, May 22, 2020 at 7:34 PM.

  1. Ed209

    Ed209 Member

    Hi all,

    I am in the middle of building an orangery and thoughts are now turning to the roof.

    I would like a 2m x 3m roof lantern, but because of this being attached to a bungalow, I am limited with roof/ceiling height and can only fit in 6" joists.

    I've seen span tables where just the span is too much for 6" joists, let alone adding a 300kg roof lantern to it, so should I just forget the idea? Will doubling the up the joists make much difference? upload_2020-5-22_19-30-4.png upload_2020-5-22_19-30-33.png
  2. Jord86

    Jord86 Screwfix Select

    Yes doubling up would make a difference, tripling up even more so. Look at using JJI/I beam joists, they can span much larger openings and bear greater loads if designed to do so.
  3. Severntrent

    Severntrent Screwfix Select

    Yes doubling up would make a difference but not enough to make it structurally OK . Perhaps look at incorporating an steel RSJ within the structure.
    Last edited: May 22, 2020 at 9:45 PM
  4. wiggy

    wiggy Screwfix Select

    Triple 6x2 either side will easily span the 4m with 300+kg.
  5. Severntrent

    Severntrent Screwfix Select

    Strength wise yes, deflection wise just out of tolerance (based on quick calcs and C24 wood) No doubt SE will sort
  6. Mike58

    Mike58 Screwfix Select

    That really is an SE task ...
    One thought though. Can you use 150x75 or 150x100 steels rather than timber?
  7. Jord86

    Jord86 Screwfix Select

    I'd imagine if height is an issue, then steels may introduce cold spots and condensation issues, as it would be difficult to add insulation beneath without lowering the headroom more.
  8. Ed209

    Ed209 Member

    Thanks guys.

    Really it's 3.7m if you take off the 300mm cavity wall. What sort of deflection are we talking with triple 6x2s?

    Can anyone link me to the span table you are reffering to please?
  9. Ed209

    Ed209 Member

    And yes, I'm already at a ceiling height of just 2010mm, so can't afford to lose anymore headroom for insulating steels.
  10. Nev Hope

    Nev Hope Member

    Nice to see people progress in the trade, why only 2 weeks ago the OP didn’t know how to put a floor slab in a conservatory ;)
  11. Ed209

    Ed209 Member

    img_2171 (1)4mp.jpg Oh we're well beyond that now; internal block skin went up today.:) Advice from this forum is a big help.

    Got to have something to keep me busy when furloughed ;-)

    I'll post a better pic tomorrow.
    Last edited: May 23, 2020 at 10:45 PM
    chillimonster likes this.
  12. Severntrent

    Severntrent Screwfix Select

    Here is the span table but it is little use in your situation as the table is for a uniform imposed and dead loads over a flat roof with joists set at standard centres which will maintain its structural integrity and deflection within allowable limits, your proposed lantern exerts its own weight together with the associated roofing imposed and dead loads over standalone joists (1,2 or 3??) and hence structural strength and deflection requires to be calculated specific to the joists in question.
    IMG_20200523_224233330 (2).jpg
  13. Ed209

    Ed209 Member

    I see, thanks.

    An idea I have had is since I will need firrings anyway (50mm at their highest point), could I instead cut a taper into 195mm joists taking them down to 145mm at the end? This would give me an average joist depth of 170mm; a considerable improvement over the 145mm.
  14. Severntrent

    Severntrent Screwfix Select

    Thinking about it some more, if you convert the weight of the lantern to a loading of m2 of roof this would be would be 300/6 = 50kg = 0.5kn/m2. Assume this is the dead load of your roof (or use the actual roof structure dead load if higher than this value) and use 600mm spacing values from the table (this is because if you are assuming 3 supporting joists these would have been spaced at 600mm under the 3m long lantern) it can be seen that a 47 x 195 joist is acceptable as Wiggy suggested, but only just, at its top end of dead load values.
  15. Severntrent

    Severntrent Screwfix Select

    Whoa, now you getting into more complicated beam analysis with tapered sections. Assuming your beams are 145mm deep then you are outside the Table values, go with 170mm (???) and you can get back in using thicker sections but I think you need an SE to confirm re acceptability of a tapered beam (unless you are feeling lucky, punk :))
  16. Chippy mick

    Chippy mick Member

  17. Ed209

    Ed209 Member

    Ah yes. Well calculated; there is hope.

    I have also found a PVCU alternative which comes in over 100kg lighter:

  18. Severntrent

    Severntrent Screwfix Select

    Obviously tables show that 195 is OK but 145 isn't. Doing specific cacls show that a 145mm deep beam would suffice if your dead load value remained under 0.5kn/m2, back to the SE?
  19. Ed209

    Ed209 Member

    I don't have height clearance for anything deeper than 145mm, unless I ditch the firrings and taper the joists.

    So sticking with 145mm joists I'm thinking 2m x 3m PVCU Skypod at 200kg (100kg saving over aluminium), with tripled C24 45x145mm joists over 3.7m span. The lantern will spread it's load over 2m on these 3.7m spans.

    So each 3.7m span tripled joist will support 100kg distributed over 2m; I make that 0.17KN/m for each individual joist?
  20. Severntrent

    Severntrent Screwfix Select

    I agree (plus all the distributed dead loads and live loads associated with the roof area to the left and right of the triple joists)
    Ed209 likes this.

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