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  1. #1
    Join Date
    Jan 2008
    Location
    Newcastle
    Posts
    10

    Default Pergola - Free standing - 6mtr spans

    Hi everyone,

    I am new to this forum and after reading throught literally hundreds of posts I am unable to find a match to the work I am wanting to carry out.

    I am wanting to build a freestanding pergola, 6m x 6m, with a skillion style roof with a slight slope from front to back supported on only 4 posts (the two front posts at a height of 2.8m and the two rear posts at 2.3m) which are going to be 200x200mm hardwood.

    The roof will be colourbond, and the underneath of the roof will be fully enclosed with fibro sheeting. The whole structure will cover a timer deck which is going to be built over a new concrete slab.

    The biggest problem I can see with my wishfull design is going to be the 6m spans. I really want to only use 4 posts if at all possible, and this may require me to look at alternatives other than timber. The size of the structural beams I was wanting to use were either 300x50 or 300x75 in either pine or oregon.

    The design I have come up with only uses beams with no rafters. Is this design acheivable considering the weight of the roof and the spans?

    The design uses all the same sized beams, 4 sides supported in each corner by 200x200 posts. Then 5 beams running left to right 1mtr appart, these 5 beams will be seperated by 1 mtr spreaders beams running front to back making a cris cross pattern when viewed from above. I tried to add some drawings but had no luck sorry hope this is descriptive enough.

    Thanks
    Brett

  2. #2
    Join Date
    Oct 2007
    Location
    Seven Hills, NSW
    Posts
    159

    Default

    The 5 beams are your rafters in a way. What you have created is a grid pattern and so will not be fully covered in span tables. But referring to the tables for guidance may give you a starting point. The beams that are 6m long will have to be quite large and just check on the maximum span of the roofing and the lining. Also, the structure will be susceptible to wind loading. So I will need to know the details of your location (general), the local topography (on a hill?) and the surrounding structures.

    Based on 40ms-1 wind speed for ~ N2

    On my rough calculations you will need a larger beam size. A 300x75-F7 is OK for strength but will deflect over 30mm under dead loads. I think you are getting into the realms of steel or larger timber beams (400 or larger)

  3. #3
    Join Date
    Aug 2004
    Location
    Perth WA
    Posts
    355

    Default

    Do pergolas have covered roofs and lined ceilings? I thought they just had a a few sticks across them?

    6m spans unless massive steel beams or webbed steel trusses are used it won't stay straight for long. If using timber then it won't be long before it becomes one of those sagging messes that one sees.

  4. #4
    Join Date
    Nov 2003
    Location
    Australia and France
    Posts
    2,869

    Default

    You may just have the odd bracing issue too, oh, and a bit of uplift.

    How big did you say those reinforced concrete columns were??

    P

  5. #5
    Join Date
    Aug 2007
    Location
    Noosa Heads
    Posts
    446

    Default

    360 x 63 LVL hyspan will do for your rafters @ 900 centres. I dont know what you are going to use as beams between the posts. Your 4 posts will have be designed as Canterlever columns so they need to be sunk 1200 mm into the ground.
    Last edited by Dirty Doogie; 14th January 2008 at 03:52 PM. Reason: hmm

  6. #6
    Join Date
    Nov 2006
    Location
    SE suburbs, Melbourne
    Age
    60
    Posts
    85

    Default

    Re Rod1949's comment, I think the correct name is "gazebo" (has a roof) rather than pergola (no actual roof cover)?

  7. #7
    Join Date
    Jul 2003
    Location
    Gorokan Central Coast NSW
    Age
    80
    Posts
    941

    Default

    I'd be inclined to go for a low pitch hipped roof using trusses. Having just a while ago bought 2 - 6.5M x 263 x 65 LVL beams you might agree with me when you check the price of your beams.

  8. #8
    Join Date
    May 2003
    Location
    Kuranda, paradise, North Qld
    Age
    63
    Posts
    2,026

    Default

    You can span 6M with a 150mm metal C purlin. You'd need to get it engineered but you could use a couple of them (or possibly 200mm) at the ends and then run 150s as rafters. If you space them closely enough you won't need ceiling battens.

    Mick
    "If you need a machine today and don't buy it,

    tomorrow you will have paid for it and not have it."

    - Henry Ford 1938

  9. #9
    Join Date
    Jan 2008
    Location
    Newcastle
    Posts
    10

    Default

    Hi all,

    Thanks for all the reply guys, looks like I am going to have to redesign my "gazebo" as timber looks to have too many limitations, and I was hoping to avoid using metal if I could.

    Maybe I will go back to my original idea, and that suggested by termite and go for a 6x6 hipped roof using prefab trusses.

    Thanks again for all the replies.

    Brett.

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