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Building Flat Roof

Discussion in 'Roofing and Guttering' started by Giant90, 19 Nov 2018.

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  1. Giant90

    Giant90

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    Hello All,

    I am building a workshop, the plan was for 8m x 5m which planning & building regulation granted permission. But would like to have more space and build it bigger.

    The new plans will be for 10m x 7m.
    I am stuck on the flat roof design, any advice is greatly appreciated.

    How should I design the flat roof? Use RSJ? Span distance? Suitable size joist? RSJ? Max Centres? etc.

    I did search the web and came up with I beam, JJI, Easy Joist etc but quotes came in at £1999+vat which is expensive for a workshop.

    Original Plan:
    Warm Flatroof - GRP>18mm OSB3>90mm Insulation>VCL>12mm Plywood deck>firring>75 x 225 joist>12.5mm plasterboard>3mm skim

    Thank you,
    Giant.
     
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  3. catlad

    catlad

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    If you are looking to make it cheaper then you will need
    a pillar in the middle.
     
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  4. Notch7

    Notch7

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    How about glulam beams?

    90 x 405 has a loading of 2.4t UDL over 7m length.

    2 glulams, 1 at 3.3m one at 6.6m. Then you could use 175 x 45 joists in between.

    A 7m 90 x 405 costs about £250+vat

    Im not a SE so above is just an idea, so its not professional advise.

    The snow load on a 70m sq roof is a lot!
     
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  5. noseall

    noseall

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    We are currently using Posi-Joists on a 7.1m clear span and they are big bu££ers! 120mm top and botton chords and 421mm deep. £2880 all in for 15 units.

    The blocks look tiny compared to the joists...…..
     
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  6. Notch7

    Notch7

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    Sparkies dream doing the first fix.
     
  7. noseall

    noseall

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    Correct. The void is huge. I tend to use them a lot and not for my benefit either.
     
  8. Giant90

    Giant90

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    Wow that is a big joist, If there is no other alternatives I'll see what the best price I can get the joist for.

    I did message a few SE but probably cheaper to use I beam jji etc.

    Thanks for the picture it gives a good idea on the size.

    Was considering low pitched roof but guess it will be as much.
     
  9. DIYnot Local

    DIYnot Local

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