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Deck plan: trade-off btwn subframe timber size & no. of required posts/bearers

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  1. #1
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    Default Deck plan: trade-off btwn subframe timber size & no. of required posts/bearers

    Hi all

    This is my first post on this forum. I have tried to glean what Id like to know from previous posts but its more a best practise recommendation Im after. I hope this is in order with accepted forum protocol.

    I've recently had a deck & pergola plan done and have 2 specific concerns. Have attached plans (+ draft material list fwiw).

    Concern 1: Deck is 9.5m * 3.6m & there seems to be a lot of posts (21) and bearers (3 + 1 ledger) in this plan. Would it be preferable to increase the planned size of sub frame timber with the aim of decreasing the number of required posts & bearers?

    Concern 2: Ledger on a brick veneer wall that runs from 20cm above ground on the low side to 1.6m above ground on the high side. The high side of the ledger has no vertical support underneath it. Id read elsewhere (may have been an American forum) that this was not advised except on weatherboard houses.

    I raised the 2nd concern with the guy who did my plans and the gist of the reply was

    The practice of Dyna bolting a ledger beam to the brick wall for the use of supporting floor joists is an approved method by the Australian building commission and is the most used method

    His complete explanation seemed valid to me but Id like to know if I am able to use ... say 1 ledger, 1 middle bearer and 1 side bearer if the load on the ledger would then exceed some recommended threshold of sorts?

    Id appreciate any advice on these two questions please.

    Rgds Neil
    Attached Thumbnails Attached Thumbnails plan-pg2.jpg   plan-pg1.jpg  
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  2. #2
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    Personally would go 140 x 45 mm joists would have one mid bearer 1800 mm span.
    Ledger would do level with joists (same size) and use hangers, for deck that height will be much more secure.
    Not a fan of balustrade detail IMO not suitable for that height

  3. #3
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    Agree with cherub on the balustrade and the subfloor layout, however you could also run the bearers perpendicular to the house, eliminating the ledger. you would have a slight cantilever for the bearers so you didnt bore post holes close to the house. 2 posts per bearer, 4 or 5 bearers, increase the joist size to 140 x 45 or 150 x 45 LVL's. Honestly, some designers just love drawing lines on paper.

  4. #4
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    Quote Originally Posted by cherub65 View Post
    Personally would go 140 x 45 mm joists would have one mid bearer 1800 mm span.
    me too. his design will result in a forest of posts.

    However I would go with the ledger under the joists as drawn. I hate joist hangers.

  5. #5
    1K Club Member Pulse's Avatar
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    interesting plans, a few issues I can see:

    1. no timber grading - should specify F5 or MGP10 or something
    2. too many posts for sure
    3. ?box gutter at house - not specified or calculated, really bad design feature
    4. bearer only 45mm wide - needs to be doubled looking at span tables in AS1684

    Ledger is fine bolted to the house, but again, fixings not specified just "dynabolts"

    What I'd do:
    1. build a skillion roof as a fly roof overlapping the house roof
    2. Use MGP10 190x45 joists - can span 3600mm at 450mm spacing, using the ledger and one bearer (doubled) at the edge.
    3. Cantilever the joists - always looks good
    4. Use steel posts - better when loads are higher.

    Cheers
    Pulse

  6. #6
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    Thank you all for your comments and advice.

    Cherub may I ask what your specific concern is with the balustrade detail being unsuitable for the height please?

    Ringtail I'd like to stick with the decking running parallel with the wall of the house as I feel this'll flow better.

    Cherub/Jack - if I raise the ledger go with joist hangers it'll more likely cover the air vents than if the joists rested on top .... in order to get my deck flooring just on or below my house floor. So will probably stick with the 'on top' option.

    Pulse thanks for raising the other issues you did and for your suggestions. I'm trying to envisage what your skillion roof would look like. 'Skillion' 'fly' and 'overlap' all in one sentance was just too much for my brain to absorb after a long day. Tried googling for skillion images but there are seemingly many variants to be had.

    Thanks again everyone.

  7. #7
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    Quote Originally Posted by neilt100 View Post
    Thank you all for your comments and advice.
    Cherub may I ask what your specific concern is with the balustrade detail being unsuitable for the height please?
    As the baluster sits on the outside of the timber rail and is fixed normally by nails.
    This method was very popular in the 70's and is a constant maintenance job for us, we recommend replacing altogether.
    Better system is to use specific designed top and bottom rails which the baluster fits into
    Fedwood Timber - timber, wood, verandahs, brackets, balustrading, balustrades, handrails, posts, Verandah Brackets

    Think who ever designed this plan is stuck in the 70's, Would also redesign roof to not have the roof lines meet in such a way, it really is poor building design which will result in someone using poor building practices which will no doubt case problems for you in the future

  8. #8
    1K Club Member Pulse's Avatar
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    A fly roof sits partially above the house roof and falls either towards or away from the house, looks really modern and deals with the low clearance you have between the deck and the current roof structure. You can attach it to the existing roof with something like this which penetrates the roof and is sealed with a dektite. The fly roof frame is then attached to that.


    Cheers
    Pulse

  9. #9
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    Found some good pictures of fly over roofs should anybody be interested in what they look like.

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    Anthony

  10. #10
    Old Chippy - 4K Club Member
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    Quote Originally Posted by cherub65 View Post
    As the baluster sits on the outside of the timber rail and is fixed normally by nails.
    This method was very popular in the 70's and is a constant maintenance job for us, we recommend replacing altogether.
    Better system is to use specific designed top and bottom rails which the baluster fits into
    Fedwood Timber - timber, wood, verandahs, brackets, balustrading, balustrades, handrails, posts, Verandah Brackets

    Think who ever designed this plan is stuck in the 70's, Would also redesign roof to not have the roof lines meet in such a way, it really is poor building design which will result in someone using poor building practices which will no doubt case problems for you in the future
    Even using screws (which you must - not nails nowadays) it is a design rarely used (could be though if it was the look you wanted), but there are better and easier ways.

    BTW _ for DIYers especially, but for all decks I reckon going up a size or two on bearers is the way to go - fewer holes and little extra costs (in fact usually cheaper for fewer stirrups concrete and digging. But also close spaces up a bit too - BCA are minimum standards NOT best practice! And I hate bouncy decks!
    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.

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