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what sort of joists and bearers do i need?

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  1. #1
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    Default what sort of joists and bearers do i need?

    im building a deck outside my back door about 30cm off the ground.
    it will span 1.5m out from the door and be 4.5m long
    can i get a way with a single span joist from the back door to the end of the deck?
    what grading and size of pine would i need for this and it would need to be H3 or 4 rated as it will get wet.
    i will be using modwood decking.
    it will be coming off a block wall at the house side and i will have to put in some posts in the ground the other end

  2. #2
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  3. #3
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    ive looked at them, find a bit confusing as i dont use them every day. but i worked out this morning i should be good for m10 h3 120x45 joists at 450 centres so just asking it here to see if i was right.

  4. #4
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    The problem with that is that 120* isn't a common size in most timber yards, 140*45 is tho and the cost difference in H3 isn't great
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  5. #5
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    90x45 MGP10 will span 1500 with 4mm deflection so it will feel a bit soft.
    120x45 is the right timber but probably hard to find so you will need to go to 140x45 MGP10 or F7

    Not sure about the treatment level for Cairns, you guys have the most aggressive species of termite up there.
    H3 is designed for outdoor use to get wet, but due to your high and constant rainfall I would be looking for H4, because if the H3 cannot dry out it will rot out.

    If you want to use H3 make sure to protect the joist tops with something like protecktadeck, call up someone like Payless timbers in Cairns to confirm the correct timber to use.

    Most important thing is to have airflow under it, and protect the tops of the joists.
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  6. #6
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    probably go 90x45 will just stick a wedge half way under the joists in H4 timber so they dont bounce, got about 60mm clearance under them.
    yes protecktadeck good idea will go that as well.
    thanks.

  7. #7
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    Quote Originally Posted by wozzzzza View Post
    probably go 90x45 will just stick a wedge half way under the joists in H4 timber so they dont bounce, got about 60mm clearance under them.
    yes protecktadeck good idea will go that as well.
    thanks.
    A wedge ?, wedge won't work, the 4mm is the deflection of the timber over the 1500mm span.

    If you want to use 90x45 put another bearer at mid span, this will get rid of any bounce it can be smaller dimension that the main bearers as it's not really doing anything except getting rid of the deflection midspan.
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  8. #8
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    will see how it goes.

  9. #9
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    Quote Originally Posted by wozzzzza View Post
    will see how it goes.
    It should be fine, 90x45 is well within its limits at 1500
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  10. #10
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    Would cantilevering the 90*45 at the outer end work?
    1500/4 = 375mm so would spanning 1200 with a short cantilever be better/ stiffer?
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  11. #11
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    Quote Originally Posted by Moondog55 View Post
    Would cantilevering the 90*45 at the outer end work?
    1500/4 = 375mm so would spanning 1200 with a short cantilever be better/ stiffer?
    Yep, 150 overhang at both ends will reduce the flex to 2mm.
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  12. #12
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    yeah i had a thought of that as well.

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    Quote Originally Posted by wozzzzza View Post
    probably go 90x45 will just stick a wedge half way under the joists in H4 timber so they don't bounce.
    I think what he meant was if a support/pedestal was placed in the middle of a 1500mm 90x45 joist, it would technically just be spanning 750mm thus reducing the sag big time, right?

  14. #14
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    Quote Originally Posted by wozzzzza View Post
    im building a deck outside my back door about 30cm off the ground.
    it will span 1.5m out from the door and be 4.5m long
    can i get a way with a single span joist from the back door to the end of the deck?
    what grading and size of pine would i need for this and it would need to be H3 or 4 rated as it will get wet.
    i will be using modwood decking.
    it will be coming off a block wall at the house side and i will have to put in some posts in the ground the other end
    use this site https://www.blocklayer.com/deckcalculator.aspx
    Advice from me on this forum is general and for guidance based on information given by the member posing the question. Not to be used in place of professional advice from people appropriately qualified in the relevant field. All structural work must be approved and constructed to the BCA or other relevant standards by suitably licensed persons. The person doing the work and reading my advice accepts responsibility for ensuring the work done accords with the applicable law.

  15. #15
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    Quote Originally Posted by jarod View Post
    I think what he meant was if a support/pedestal was placed in the middle of a 1500mm 90x45 joist, it would technically just be spanning 750mm thus reducing the sag big time, right?
    So that would be called a bearer, not a wedge
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