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Overhang on pergola construction

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  1. #1
    dmx
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    Default Overhang on pergola construction

    Hi All
    Currently, i'm erecting the pergola with colorbond roof after all.
    the beam size : 190 x 45 ( 3 pcs of 3m length, total to be 9m)
    the rafter size : 140 x 45 at 5.4 length

    After did some 'search button', i still couldn't find thw answer for my questions :

    1. What is max overhang for beam (over the post)?
    2. for the rafter (over the beam)?

    based on AS1684.

    Thanks for the help

  2. #2
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    What type of timber do you have? Structurally there's a big difference between Pine, to LVL's, to F17 hardwood.

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    Lumberlubber Bleedin Thumb's Avatar
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    I wouldn't know if there is a standard, I would be reticent about going over 600mm though...just from gut feel and experience.
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  4. #4
    dmx
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    The timber is T/pine F5 140 x 45 at 4.5 m (corrected) span with 600 space between rafter.
    Because i need to lodge amended DA to council, and need to put overhang based on as1684.
    to : bleedin thumb : so base on ur experience, not over 600mm. means if i put 500mm, would be safe ? (in term of council approval)

    Thanks so much guys

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    Quote Originally Posted by dmx View Post
    Hi All
    Currently, i'm erecting the pergola with colorbond roof after all.
    the beam size : 190 x 45 ( 3 pcs of 3m length, total to be 9m)
    the rafter size : 140 x 45 at 5.4 length

    After did some 'search button', i still couldn't find thw answer for my questions :

    1. What is max overhang for beam (over the post)?
    2. for the rafter (over the beam)?

    based on AS1684.

    Thanks for the help
    The Rafter can overhang the beam by 1050-mm based on a 20-kg (1100-mm based on a 10-kg) mass per square meter. The overhang can't exceed 1/3 of the backspan.

    There is no provision for cantilever on rafter beams using AS1684. There are other ways to work it out although I'm not willing to post the result.

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    So you've got 140 x 45 F5 spanning 4.5m, plus the overhang? I would doubt very much that this pine is adequate, even if you didn't have an overhang.

    Do you mean the rafter is 4.5m in total length, but at midspan is sitting on top of your 190 x 45 main beam?
    If this is the case, I would agree with the figures Eastwing said - a bit over a metre is ok. Keep in mind though that a 1100 overhang won't fail, but it will be a bit bouncy, so to have it solid as well, you'd want to put the overhang to what bleedin thumb suggested.

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    Lumberlubber Bleedin Thumb's Avatar
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    Quote Originally Posted by Eastwing View Post

    There is no provision for cantilever on rafter beams using AS1684. There are other ways to work it out although I'm not willing to post the result.

    If you don't mind me asking...why not?
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    I'm happy to use my copy of the span tables, and share with people the information they contain. When something falls outside of the tables as what he want's to do does. Advise needs to come from an engineer, not a builder.

    As a building professional, I will only advise on things that are listed in the table.

    e.g.. the homeowner builds himself a deck at home and someone hurts themselves due to a non compliant handrail, there will be some trouble. If I built it or even gave professional, but wrong advice and someone gets hurt. I could be de-registered and face criminal charges. (Think that doesn't happen? Think again.)

  9. #9
    Lumberlubber Bleedin Thumb's Avatar
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    We are not talking about a deck, we are talking about a pergola

    The whole idea of this forum is to share knowledge....I can see where you are coming from but its just a matter of putting in a disclaimer in your post....the whole forum is crawling with disclaimers .....

    No one is giving professional advise here....its all just opinions .....people can do what the want with it......

    Just because you say you are a building professional doesn't mean didley squat....you could just be an over opinionated 17 year old school boy. not that I think you are...I'm just making a point.

    But my attitude is if you think that you have the answer don't be afraid to put your 2 bobs worth in. it.
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  10. #10
    dmx
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    Quote Originally Posted by Cooky View Post
    So you've got 140 x 45 F5 spanning 4.5m, plus the overhang? I would doubt very much that this pine is adequate, even if you didn't have an overhang.

    Do you mean the rafter is 4.5m in total length, but at midspan is sitting on top of your 190 x 45 main beam?
    If this is the case, I would agree with the figures Eastwing said - a bit over a metre is ok. Keep in mind though that a 1100 overhang won't fail, but it will be a bit bouncy, so to have it solid as well, you'd want to put the overhang to what bleedin thumb suggested.
    The rafter would attached at existing home facia at one point (of course , before that, the facia been strengthen through home's existing rafter) and also supported by 4 posts under the facia.
    And at the end of pergola's rafter (at 4.5m point) is sitting on top of 190 x 45 beam

    and at the middle of pergola's rafter would be braced to next rafter using 140 x 45 to avoid rafter bowing.

    is it adequate ?

    note : rafter size 140- x 45 choosed base on table from tabma following as1684
    http://tabma.com.au/documents/TABMA%...n%20Tables.pdf
    (single span, 600 spacing, F5)


    Quote Originally Posted by Eastwing View Post
    I'm happy to use my copy of the span tables, and share with people the information they contain. When something falls outside of the tables as what he want's to do does. Advise needs to come from an engineer, not a builder.

    As a building professional, I will only advise on things that are listed in the table.

    e.g.. the homeowner builds himself a deck at home and someone hurts themselves due to a non compliant handrail, there will be some trouble. If I built it or even gave professional, but wrong advice and someone gets hurt. I could be de-registered and face criminal charges. (Think that doesn't happen? Think again.)
    Please share your knowledge, Sir, for sake of us
    We don't even know ur real name, only your nick, how can we sue you ?

    Thanks again.

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    A couple of questions about your scenario. The posts under that house fascia suggest that you are attaching to existing verandah ? If you are putting plans to council then you will be expected to collect and divert stormwater with guttering downpipes and S/W connection. Have you considered what extra load a gutter full of water will add to the ends of your rafters ? I doubt that your span tables have considered that.

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    Crikey. I had a look at those tables, and in my opinion they're very dicey. I believe they are not in accordance to AS 1170 (as an engineer I must consider more than just AS 1684) - which is the Australian Codes for any type of load you must design for. In all my years of specifying what timber to use, I have and never will let F5 go to those spans. But dmx this is not against you, you're trying to do the right thing - there's nothing personal!

    I did a few calcs, and I would say that only at 190x45 would the rafters be ok, either F5, F7 or MGP10 - they wouldn't break but they will deflect a bit when fully loaded - which will be when a few guys are on the roof doing maintenance or something. I'm under the impression that MGP10 is pretty good these days, straighter and more consistently graded than the F5 or F7, so I would just go these. Please correct me if I'm wrong..?

    Eastwing - unfortunately you have a fair attitude. It only takes one occasion to get smashed and you're forced to adopt the attitude of trust no one until they've earnt it. Oh well, I haven't been smashed yet so I'll keep on blabbing my opinions for now

    Dufus - you're right the span tables don't consider those loads. The loads they consider are a 'dead load' of 10kg every square metre, and it seems a nominal 'live load' (people normally) of 25kg per square metre (it is this live load that is not correctly designed for, and for anyone who knows AS1170 refer to Clause 3.5.1 and Table 3.2)

    Cheers

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    Oh another thing, there is actually provisions for cantilevers in AS1684. I found them yesterday when I was cross-checking those spans you provided dmx. (it turns out I also believe that AS1684 - 1999 version are dicey!) They're in a supplimentary document to AS1684. For your rafter size and layout, 1100 is allowed theoretically. Again I think this is way to long, but for the sake of proving to council that your adopted overhang is ok, you can refer to this span/overhang. If you want the reference details let me know.

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    when I did by car port I used Tilling timber LVL tables and designed it to house roofing specifications. I know its a little overkill, however if a tree where to land on it or freak wind loading occured, it will stay where I put it..at least that is the plan.

    Using LVL13 did not add too much to overall price and that has included 2.5 metre overhang at one point. That was based on 260 x 60? main beams.

    LVL span tables are available from various manufacturers.

    may be something to think about

    cheers

  15. #15
    dmx
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    Quote Originally Posted by Dufus View Post
    A couple of questions about your scenario. The posts under that house fascia suggest that you are attaching to existing verandah ? If you are putting plans to council then you will be expected to collect and divert stormwater with guttering downpipes and S/W connection. Have you considered what extra load a gutter full of water will add to the ends of your rafters ? I doubt that your span tables have considered that.
    yes and using the post bracket to verandah's concrete
    actually, I did submit the plan to council and been approved. But i do little amended (overhang).
    and yes, 140 x 45 in the plan for rafter and also the gutter.
    House' existing gutter still there and new gutter just for collect rainwater from pergola roof.

    [quote=Cooky;769513]Oh another thing, there is actually provisions for cantilevers in AS1684. I found them yesterday when I was cross-checking those spans you provided dmx. (it turns out I also believe that AS1684 - 1999 version are dicey!) They're in a supplimentary document to AS1684. For your rafter size and layout, 1100 is allowed theoretically. Again I think this is way to long, but for the sake of proving to council that your adopted overhang is ok, you can refer to this span/overhang. If you want the reference details let me know.[/quote]

    Means theoretically (based as1684), 140 x 45 should be ok for 4.5 span ?


    i think, i just go with 500 overhang. should be ok ?
    yes, can i please the ref details if you don't mind.
    Thanks in advance

    Quote Originally Posted by jimc View Post

    Using LVL13 did not add too much to overall price and that has included 2.5 metre overhang at one point.

    cheers
    2.5 m overhang ? wow that's long.


    Thanks again to All

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    dmx

    This is how you should look at whether the 140x45 will theoretically work or not. For the worst possible loading scenario, one guy reckons its 2 fat blokes standing on the roof, 2 metres apart, directly on 1 rafter. Another one says its 3 fat blokes, yet another says 2 fat blokes but one is giving the other a piggy back so it's one point load. So for all the different 'theoretical' scenarios you'll get different answers.

    My worst case scenario is based on AS1170 which is obviously worse than that of AS1684.

    As you're the one doing the design, it's your decision about which 'theoretical' answer is best for the situation. If you reckon the roof is hard to reach by kids, maintenance only ever requires one person, or if more than 3 people are huddled together on the same rafter then they deserve to fall off, then no worries!!! (Keep in mind that people can't just randomly make up scenarios, there are minimum critical scenarios as set by different codes that you can't go below. The different 'guys' above was illustrative, not literal)

    One thing I would definately advise though, go at least 2 rows of the nogging. The nogging doesn't just help with bowing, it can actually stop beams from failing too.

    One last thing, these spans and overhangs you're using are based on N1 or N2 wind areas. Make sure that's you!

    OK, for the reference, you can say something like this:

    For 10kg/m2 roof material, 600 rafter spacing, 140x45 F5, single span
    Table 29 of AS1684.2 N1/N2 Supplimentary 1 - 1999 states
    Maximum of 4600 span and 1100 overhang

    (yeah it says 4600 not 4500, I didn't do a typo)

    Unfortunately I think this is an old edition though, the new AS1684 was released in 2006 and normally the supps are superceded too. Oh well, when submitting to council just take off '- 1999' and see what happens. If you replace it with 2006 you risk those values being different

    Good luck

    Cooky

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    dmx
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    Thanks cooky !!!


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