4.8 x 4.8 flat roof shed joist span questions

Discussion in 'Carpenters' Talk' started by EmmaHanid, Sep 28, 2021.

  1. EmmaHanid

    EmmaHanid New Member

    Hi all!

    This is my first post here and have tried to search for the answer in previous threads, but nothing quite answers my specific questions, so I'm starting my own thread. Hope that's OK!

    My dad and I are about to start building a workshop/shed in the garden and don't know how to go about the roof.
    I've given 4.8m x 4.8m as the dimensions, but that's just the max size of the concrete base we have.
    We're trying to keep costs down (partly due to not having much money anyway and this being the start of a business venture), but also because as we all know, timber is ridiculous at the moment.
    We're after a flat roof for simplicity and for planning reasons.
    We're either going to put a felt roof on it or corrugated metal sheets (as the old building next to it has this and it works ok).
    I'd also like to put two or three 12v solar panels on it if it's strong enough.
    I'd also like to insulate it (i've been given a load of rockwool, so was thinking of using that) and then lining it with thin ply rather than plasterboard or anything too heavy.

    What we'd like to know is the best sizes of joists to get and how to make it as strong as possible taking into account the best way to do it cost effectively.

    What do you think?
    If anyone has any useful thoughts or things we definitely shouldn't do, it would be great to hear them because I'm guessing there are quite a few very experienced people in this group and so having that knowledge would be amazing :)

    Thanks so much in advance and I hope to hear from some of you soon!!
     
  2. ejenner

    ejenner Active Member

    A lot of people are using 'space joists' now. I can't find any prices online because a lot of the companies want you to contact them for a quote but I think they might be cheaper than the equivalent solid timber.

    Space Joists | Griggs Timber Company
     
    EmmaHanid likes this.
  3. Jord86

    Jord86 Screwfix Select


    Considering that to span the distance you've quoted you'd want C24 8x2's but probably 9x2's at roughly £40-50 a 4.8m length at this moment in time, and at least nine full lengths not including stub ends or noggins so probably nearer 14 lengths, your hope of cost effective,strongest, best structure is sadly going to be anything but cost effective.
     
    EmmaHanid likes this.
  4. ejenner

    ejenner Active Member

    A potentially cheaper method would be to buy steel and weld a frame for the roof. I've done that before on a garden shed and you keep adding steel until it is strong enough. The drawback of this method is the welding skills required. I in-filled with sterling board which I'd painted with several coats of expensive paint.


    WP_20170326_19_20_36_Pro.jpg
     
    EmmaHanid likes this.
  5. EmmaHanid

    EmmaHanid New Member

    Thanks for your reply. Yes, cost effective in the most minimal sense of the word in these current times haha!

    I guess I was wondering if it could be done with some central crossways joist and then with joist hangers along it for shorter lengths... does that make sense? Is it possible?
     
  6. EmmaHanid

    EmmaHanid New Member

    Thanks for your reply. Unfortunately, we don't have the facilities to weld at the moment, but it's a good suggestion.
     
  7. Jord86

    Jord86 Screwfix Select

    Yes it's possible, but you won't really be saving any money by the time you triple up the central beam and buy a load of joist hangers. You could use an RSJ for the centre, but again the cost saving is negligible.
     
    EmmaHanid likes this.
  8. EmmaHanid

    EmmaHanid New Member

    Thanks!
    What would you recommend to be the equivalent strength then?
    3x what?
    and joist hangers with what in them?
    Just trying to get an idea in my head and you've been the most helpful source of info so far! Thanks :)
     
  9. Jord86

    Jord86 Screwfix Select

    Look up flat roof construction on google or Pinterest for an idea. The span being 4.8m is the governing factor, you either build a load bearing centre wall beneath inside the garage to halve the span and use smaller joists, or use larger joists to span the whole distance, or use a large beam then smaller joists but you'll need to pack them up to the top of the beam to keep the roof deck flush.
     
    chillimonster likes this.
  10. EmmaHanid

    EmmaHanid New Member

    Thanks. I can't seem to see anything like that on Pinterest though. I shall keep looking.
     
  11. ejenner

    ejenner Active Member

    One of our sheds has a telegraph pole as the central beam and the smaller timbers joined to that. There's a guy here selling them for £35 a pop: https://www.ebay.co.uk/itm/324179057856

    Bear in mind with any compromises that the strength of a roof, moreso with a flat roof must account for snow. 12-inches of snow across the entire roof weighs quite a bit!
     
  12. stevie22

    stevie22 Screwfix Select

    Can you tolerate a post in the middle? That could be a game changer.
     

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