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Roof question

  • 26-06-2020 7:54am
    #1
    AVFC.Stephen
    Registered Users Posts: 293 ✭✭


    Hi all.
    I'm planning to build a workshop beside the house. The width will be 2.9m which is the way the roof rafters will run. The roof will be osb vapour and mineral felt.. (flat roof)

    My question is. With such a light roof over the span of 2.9m would I need anything more than 4x2? My original plan was 6x2 but when I saw what is in the roof I thought 6x2 would be overkill


Comments

  • #2
    ted1
    Registered Users, Registered Users 2 Posts: 23,743 ✭✭✭✭ted1


    AVFC.Stephen wrote: »
    Hi all.
    I'm planning to build a workshop beside the house. The width will be 2.9m which is the way the roof rafters will run. The roof will be osb vapour and mineral felt.. (flat roof)

    My question is. With such a light roof over the span of 2.9m would I need anything more than 4x2? My original plan was 6x2 but when I saw what is in the roof I thought 6x2 would be overkill

    I’m a huge fan of overkill. What’s the price difference?


  • #3
    TheBoyConor
    Closed Accounts Posts: 3,292 ✭✭✭TheBoyConor


    If you look at something like the homebond house building manual there are span tables in there that you can use to select a suitable joist size.


  • #4
    jack of all
    Registered Users, Registered Users 2 Posts: 2,376 ✭✭✭jack of all


    The old rule of thumb for floor joists was "half the span in feet plus 2 inches"- in your case (and a flat roof is different again) this would suggest something closer to 7" or 175mm deep. I had a look at the Homebond manual and a C16 44x 175mm (7x 2") at 400mm c/cs is what you'd need to consider, which takes account of snow load etc. I seen a few flat roofs that were built using undersized joists- they won't collapse or anything but the joists sag and unless you have significant falls built in this leads to water ponding and no way to get it off. A sure fire way to dramatically shorten the life of whatever membrane you put on it afterwards.


  • #5
    AVFC.Stephen
    Registered Users Posts: 293 ✭✭AVFC.Stephen


    jack of all wrote: »
    The old rule of thumb for floor joists was "half the span in feet plus 2 inches"- in your case (and a flat roof is different again) this would suggest something closer to 7" or 175mm deep. I had a look at the Homebond manual and a C16 44x 175mm (7x 2") at 400mm c/cs is what you'd need to consider, which takes account of snow load etc. I seen a few flat roofs that were built using undersized joists- they won't collapse or anything but the joists sag and unless you have significant falls built in this leads to water ponding and no way to get it off. A sure fire way to dramatically shorten the life of whatever membrane you put on it afterwards.

    Wow 6x2 is what I would of thought as a max... the fall i was thinking could be as little as 1 inch. But now thinking about it if the structure moves. Maybe I need to consider more? Gutters will be in place also directed to the drain


  • #6
    jack of all
    Registered Users, Registered Users 2 Posts: 2,376 ✭✭✭jack of all


    Looking at the same table a C14 44x150 (6x 2") at 400mm c/cs will span up to 2.4m, a C16 up to 2.51m. Even tightening up the centres to 300mm wont work with a 6x 2".


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  • #7
    muffler
    Registered Users, Registered Users 2 Posts: 46,408 ✭✭✭✭muffler


    jack of all wrote: »
    The old rule of thumb for floor joists was "half the span in feet plus 2 inches"- in your case (and a flat roof is different again) this would suggest something closer to 7" or 175mm deep. I had a look at the Homebond manual and a C16 44x 175mm (7x 2") at 400mm c/cs is what you'd need to consider, which takes account of snow load etc. I seen a few flat roofs that were built using undersized joists- they won't collapse or anything but the joists sag and unless you have significant falls built in this leads to water ponding and no way to get it off. A sure fire way to dramatically shorten the life of whatever membrane you put on it afterwards.
    Very appropriate user name :)

    It's actually half the span plus 1


  • #8
    whizbang
    Registered Users, Registered Users 2 Posts: 1,959 ✭✭✭whizbang


    How are you going to fit the roof ..
    Would you trust your life, plus the weight of a roll of felt etc, on an almost 3m length of 2 x 4 ??

    I wouldn't let the dog up on that.


  • #9
    AVFC.Stephen
    Registered Users Posts: 293 ✭✭AVFC.Stephen


    whizbang wrote: »
    How are you going to fit the roof ..
    Would you trust your life, plus the weight of a roll of felt etc, on an almost 3m length of 2 x 4 ??

    I wouldn't let the dog up on that.
    Its quite strong... I wouldnt be holding parties on it but still strong enough...


  • #10
    GinSoaked
    Closed Accounts Posts: 1,049 ✭✭✭GinSoaked


    Just been and double checked "one I made earlier" and for a 3.4m gap I used 2 x 5 at 16 inch spaces. Now I would have preferred to use 2 x 6 but with gutters head room and the fall I had to fit in what I could. Certainly no problem with two people on the roof with several rolls of felt.

    So from actual experience with a very shallow pitch on a flat root I'd be happy enough with 2 x 5 for the OP's project


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