I need help with the deck load calculation of our new deck.
The deck will be 12’x32′ in size. It will be 8′ off the ground. I will use 24″ Bigfoot footings under the deck post. The floor joist are going to be pt 2″x10″s 16″ oc. I will be using 1″x6″ Ipe decking which will run parallel to the house. The deck will have a 4/12 pitch roof made of 2″x6″ framing, 1/2″ plywood and standing seam metal roofing.
My question is: how many deck posts do I need and what size do they have to be? I was going to use 6″x6″ or 8″x8″ pt posts and wrap the in cedar.
Martin
Replies
I recommend using 6x6 Fir post rather then ACQ (P.T) because on 8' it will tend to bend and twist. The footing I place a 1/2" anchor bolt before concrete cures. You don't have do it that way, you can drill and anchor them after the concrete cures. I use Simpsons ABA66-Z (Z means Z-MAX for exterior use)) for the post plates. If you are installing top-load application (3- 2x12 on top of post and 2x10 resting on the beam) 9'or 10' between the post it will be fine. If you are doing side-load application (6x6 notched around the 2-2x10 rim joist) then 7' or 8' between each post will be fine. Hope this helps.
Here is a site with lots of info including load info:
http://www.decks.com/
I don't think you will need 8x8's. this article exlpains post height to load calc.
http://www.decks.com/article29.aspx
Deck support posts: On my covered deck similar to yours (12' deep x 82' long with 4 pitch roof), I'm putting deck posts about every 5-6' [using 2-2x10 deck header beam http://www.decks.com/article37.aspx I don't think you can go much past 6-8' with doubled 2x10, but less distance is better].
Roof support: The posts for the roof are going to be 6x6 12-10' apart. supported by 2- 11 7/8" LVL's [my advice is to place the roof support posts where you want them to fall (i.e. not in front of windows ...)]. Roof support posts bear directly down on deck support posts -> to oversized footings that will support the roof loads too.
Roof: I'm using 2x8 for the roof rafters [as I judge 2x6 to be a bit flimsy] and 5/8" ply for the roof deck [same reason].
Roger <><
one more thought. ...
Part of my deck will be 10' off the ground, so those footings go down 4' +.
I'm using 6x6 treated to .60 rather than the usual .40 for ground contact (that will cost about $42 for a 14' 6x6 (usually around $32 for .40).
I like that arrangement as there will be less vibration/swaying due to long post lengths.
I typed a long explanation and response and then the just after using the spell checker the window went !!!poof!!! and was gone. Gotta love computers.
None the less I also made a pic for you that explains tributary areas and loading on posts for deck floor systems. I assumed 40 PSF - for floor loading only. I also assumed double 2x10 girders. To take into account roof loads too you might want to just double the #s shown on the picture. Really though I have no idea what the snow load is on BC. BTW - a 6x6 post supports about 12,000#s.
Edited 4/6/2007 11:19 pm ET by Matt
Edited 4/6/2007 11:31 pm ET by Matt
Let's try the drawing again...
one more time.
Edited 4/6/2007 11:30 pm ET by Matt
PS - in the pic the reason for the 8' post spacing is because of the double 2x10 girder I selected. This could be a flush girder or a drop girder. Also I'd double the 2 end joists. This may not be essential, but inspectors in our area require it.
Also regarding roof loading on the posts I assumed a shed style roof - you didn't really specify but I guess it would have to be with a 4:12 pitch on a 12'x32' footprint. The posts supporting the roof could be 4x4s and would be installed directly on top of the floor support posts.
Edited 4/6/2007 11:54 pm ET by Matt
The biggest variable in your scenario is the one you leave out - the size of the girder beam that the joists will tie to.
Cannot calc load on post without knowing span that can be handled by it.
Seat of pants guesswork is five 6x6 post with tripled 2x10 girt spanning about 8'
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Edited 4/7/2007 7:14 am ET by Piffin
Going back to the deck design, I was very busy for a while...(the kids were sick)What is your advise on the girder beam? Should I notch the 6x6 post on either side and put 2x12's in it. To fill the "sandwich" I would put 2x12 between the post. That way I could run the 6x6 all the way up to support the roof. The floor joist would be attached to the girder beam with joist hangers.MartinPS: thanks everybody for your help
Well, I already gave a guess at the girt beam Q once.If you notch that much out of a 6x6 you don't have much leeft so theere is little gain in trying to run it al the way up.Several questions have been asked of you in this thread that need answers but the overall will depend on you local inspectors and their requirements for engineering on this.The fact that you plan only 2x6 roof rafters to span 12' means to me that you are in a southern climate with little or no snow load needs, or that you have not checked that out locally either.You are asking specific advice without providing specific local requirements.
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I would suggest running the decking at 45 degrees. This will give a MUCH stiffer deck than running at 90 to the joists, looks better too.
How in the world would it do that?
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Lots of triangles because of the 45 degrees. Known a few "90" degree decks and they have a "swing" to them. Built the same size and rock solid.
Here's a calculation for you: E = mc²
That is pretty much everything you need to know.
But, there is something we need to know. What size member is desired to support the rafters of your roof, and what size member is desired for supporting the floor joists?
Furthermore, what kind of snow load, if any, do you get where you are building? We need to know something called the "ground snow load."
It also might help to know the bearing pressure your soil can handle. Where I live a single 24" bigfoot can handle the weight of about three passenger cars.
It appears that your method is not covered by the prescriptive code. I think you need an engineer.
Here, almost everything requires an engineers stamp. Except porches unless they have unusual circumstances like > 16' high. Granted construction methods vary somewhat regionally, but based on the sketchy info he has provided, what he is proposing is common practice.
>>
It appears that your method is not covered by the prescriptive code. <<
Can you be more specific?