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  1. #1
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    Default Building Deck - Bearer / Joist

    Hello all,

    I am building a low-level deck out the back of my house and I'm getting mixed signals from various websites as to the thickness of bearers and joists for the deck. Maybe someone can give me some advice.

    Currently the plan is:

    Bearers: 90mm x 90mm
    These are approx. 3.5m long and will have posts embedded in concrete at each end and one in the middle (so a span of about 1.5m between posts)

    Joists: 90mm x 45mm
    To minimise height these will be cut into pieces and rest on joist holders between the bearers rather than sitting on top of the bearers

    Bearers and joists will be of treated pine, decking will be Merbau (hardwood).

    Can anyone see anything wrong with the thicknesses of these bearers and joists? Various plans seem to suggest that they should be thicker than that but others seem to say that they are ok. I can't find anywhere that mentions if having joists between the bearers rather than on them makes a difference.

    This is my first deck and its out the back of my own house so I don't want to get it wrong

    Thanks,
    Easy
    Last edited by Easy; 25th Sep 2008 at 12:53 PM. Reason: left out a coupla words

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  3. #2
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    What's the span of the Joists? I would say go to 140x45 on the joists - 90mm TP will flex a bit.

    The deck I just did has bearers being supported every 2m, and bearer size was about 140x45 (I think). Joists are 140x45, with spans of 2.1m.

    cheers,
    Dave
    ...but together with the coffee civility flowed back into him
    Patrick O'Brian, Treason's Harbour

  4. #3
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    Cheers,

    Not really possible to have joists 145 x 45 if the bearers themselves are only 90 x 90 and the joists are going between them? Or am I missing something?

    Maybe I should dig down a bit and make the bearers themselves bigger.

  5. #4
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    missed that one Make the bearers 140 too.

    Don't have huge experience in this regard (beyond my own deck). Maybe others will have a more experienced opinion.

    Cheers,
    Dave
    ...but together with the coffee civility flowed back into him
    Patrick O'Brian, Treason's Harbour

  6. #5
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    90x90 is OK for bearers and 90x45 for joists is probably OK too but you don't say what the joist span is.

    However, I would either go up to 140x75 bearers and 140x45 joists or add some extra posts because of the bounce you'll probably get. I reckon it's better to over-engineer decks because you're likely to have a lot of people standing on it at BBQs etc. If you added another post and keep your joist span down, you will get away with the smaller sections.

    There are span tables that give the minimum sizes for things. Of course it depends on the type of timber you are using. If you give me your joist span, I'll look it up in my copy of the span tables.
    "I don't practice what I preach because I'm not the kind of person I'm preaching to."

  7. #6
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    Ah! Yes! Joist span is about 1.5m.

  8. #7
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    Try this PDF from Queensland timber about timber Decks close to the ground...
    http://www.bretts.com.au/sound-proof...ntheGround.pdf

    On spans of 1.5m for joists and bearers, it recommends 120x70 bearers and 120x45 joists.

    90mm will probably work, but the bounce will drive you nuts. In my case, I've used compressed cement sheets and tiles on the top, so bounce like that would destroy the tiles.

    cheers,
    Dave
    ...but together with the coffee civility flowed back into him
    Patrick O'Brian, Treason's Harbour

  9. #8
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    OK, according to my span tables you're not going to do that with a 90x45 joist. Because you are cutting them in between the bearers, you are creating a series of single span joists, and single span generally requires a larger section (in fact even continuous span can't be done in 90x45 over that span unless you go up to F7 hardwood).

    If you go up to 125x75 in F5 for the bearers and 125x38 F5 or F7 for the joists, you'll be well within the specs. To be extra sure, you can make your joists 125x50 F7. That will span up to 2100mm.

    In case you don't know, F7 is the structural grade. There's a new system that is used now which has an MGP number. I'm not familiar with it but if you tell them at the timber yard that you want F7, they should be able to give you the right stuff.
    "I don't practice what I preach because I'm not the kind of person I'm preaching to."

  10. #9
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    Very useful! Thanks. I have looked at a couple of tables like that before but they seemed to vary widely and were in bloody inches which drives me nuts.

    This makes it a lot clearer. I think I'll dig down a little and get thicker bearers and joists.

    Cheers!
    Tim

  11. #10
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    Easy, If height is an issue then stick with 90X90 and 90X45 F7 TP. Have the bearers span no more than 1600mm (1400 for single span), space them no more than 1300mm and joists at 600 centres. The good thing with 90X90 F7 is its H4 grade so you can let it touch the ground, so you can get the deck real low. Don't cut the joists between the bearers, it creates too much movement and too much extra work, not to mention the cost of a truckload of joist hangers.

    If you go 120X45 and 120X35 F7 as suggested above. Get prices on doing either first as 120mm is not a commonly used size and you may pay a premium for it.

    Also some yards now sell TP as F5. If you get F5 then drop your spans and spacings to 1200mm.

    Cheers.

  12. #11
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    If you use F5 you will need to use 2/120x45 for the bearers and 120x35 for the joists at 450 centres.
    Note that these are the minimum sizes and as a result you will have a fair amount of bounce in the deck-not a good feeling under foot.
    You really should spend the extra and up the member sizes to 140mm.
    I have never specified minimum sizes when designing a deck and usually go up 2 sizes for a good solid feel.

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