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Old 06-10-2013, 08:28 AM   #16
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deck joist span question


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You have strange logic. Im sure I dont understand it, especially the thing about headroom, unless you mean the headroom between the joists, which is ludicrous.

I think you are hung up on the concrete pad, when it is fairly easy to go through it. You cannot span 17.5 feet with dimensional lumber, I really dont care what the span tables say. Especially with the crap lumber they sell today.
I think the poster means that the headroom is for the useable space under the deck. With the beam parallel that would still be 8'-plus, which still seems fine.

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Old 06-10-2013, 08:49 AM   #17
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deck joist span question


how are you planning on attaching the beams at the dwelling? is this a free standing deck or will you attach the beams to the ledger?
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Old 06-10-2013, 10:56 AM   #18
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I think the poster means that the headroom is for the useable space under the deck. With the beam parallel that would still be 8'-plus, which still seems fine.
To my point. Some people get an idea in their head, and they cant get it out. Happens a lot on DIY forums. They ask for advice, then argue with the advisor. It would be much better if people started with a clean slate, and let the people with experience give them some ideas. In this case, there is no dimensional lumber that will span 17.5 feet and feel solid under foot IMHO, and running the joists parallel to the house flies in the face of common sense, but so be it. People have their reasons I guess.
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Old 06-10-2013, 05:38 PM   #19
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deck joist span question


I apologize if I've irritated anybody, since that's certainly not my intent. I'm just looking at the options, and this would be a good layout, given the existing pad under the deck. I'm willing to listen to reason.

As I mentioned, I've been looking at the deck building info here:

http://www.awc.org/Publications/DCA/DCA6/DCA6-09.pdf

They allow for joist spans up to 18' (see Tables 2 and 3) and there is plenty of info on freestanding decks (see p. 14 of the same document). The concern I had was that the beam/joist span table doesn't go far enough for my drawing.

In the publication cited above, they give the following formula for calculating post loads (where R is in pounds):

R = 50 * (Lj/2 + Lj/4) * (Lb/2 + Lb/4)

Here, Lj is the joist span and Lb is the beam span. On my middle beam I'd have Lb = 4 and, due to the joists from 2 sides, I'd have to use Lj=27.5, which gives R=3093.75. They say that a 4x4 beam can support 4000 pounds, and I'd be using 6x6 beams, so that would be OK. They also give formulas related to footing size/thickness, and I think I'd be OK on those too. So, it looks to me like it should work.

If you see a problem here, please let me know specifically what it is, and why it's a problem. Thanks.
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Old 06-10-2013, 06:14 PM   #20
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deck joist span question


You can do whatever you want to do, and this is simply my opinion, so it does not matter at all to me what you do, but when this is finised, you are going to run into a post every time you turn around.

Your plan shows 12 posts, what I recommended would require 4, 14 feet away from your home, and your deck would run longitudinally with your house, not perpendicular to it. You could probably even move the girder out to 16 foot, and cantilever 2 feet to get by the pad. if you use 2 x 12 Joists 12 inches on center.

Again, it is your deck, and I am sorry if I sounded irritated. I am not. I would only be irritated if I walked into one of your posts, and there is not much chance of that.
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Old 06-11-2013, 08:16 AM   #21
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deck joist span question


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Originally Posted by jagans View Post
You can do whatever you want to do, and this is simply my opinion, so it does not matter at all to me what you do, but when this is finised, you are going to run into a post every time you turn around.

Your plan shows 12 posts, what I recommended would require 4, 14 feet away from your home, and your deck would run longitudinally with your house, not perpendicular to it. You could probably even move the girder out to 16 foot, and cantilever 2 feet to get by the pad. if you use 2 x 12 Joists 12 inches on center.

Again, it is your deck, and I am sorry if I sounded irritated. I am not. I would only be irritated if I walked into one of your posts, and there is not much chance of that.
Here's a little more detailed view. There is a hillside on either side ("up" on the rhs and "down" on the lhs), so the left and right rows of posts are not really in the way of anything. Also, my thinking is that these posts would not present any more of an obstacle than, say, the walls of a garage (btw, the pad is garage-worthy, but we don't see the need to build one).

From the deck design document previously mentioned, using a 14' joist span with 2' overhang, and three 2x12's (douglas fir) for the beam, the maximum post spacing is 9' 1". So, it could not quite span all the way across the concrete pad, which I'd like to keep open, if at all possible. Finally, getting a post near the middle of that 17.5' span would be difficult since there are some obstacles there, so that would almost certainly require multiple posts. In fact, the existing deck has 5 posts in that area (in a very strange configuration), as a way to work around the obstacles.

Anyways, I just wanted to point out that there is some method to the madness. I'd tried to keep the original description simple, which might have made it all seem even stranger than it really is.

Thanks again for all of the feedback. Your responses have definitely gotten me to think about it a little more and given me some additional options.
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Old 06-11-2013, 03:57 PM   #22
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deck joist span question


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Here's a little more detailed view. There is a hillside on either side ("up" on the rhs and "down" on the lhs), so the left and right rows of posts are not really in the way of anything. Also, my thinking is that these posts would not present any more of an obstacle than, say, the walls of a garage (btw, the pad is garage-worthy, but we don't see the need to build one).

From the deck design document previously mentioned, using a 14' joist span with 2' overhang, and three 2x12's (douglas fir) for the beam, the maximum post spacing is 9' 1". So, it could not quite span all the way across the concrete pad, which I'd like to keep open, if at all possible. Finally, getting a post near the middle of that 17.5' span would be difficult since there are some obstacles there, so that would almost certainly require multiple posts. In fact, the existing deck has 5 posts in that area (in a very strange configuration), as a way to work around the obstacles.

Anyways, I just wanted to point out that there is some method to the madness. I'd tried to keep the original description simple, which might have made it all seem even stranger than it really is.

Thanks again for all of the feedback. Your responses have definitely gotten me to think about it a little more and given me some additional options.
Good luck with your project, and happy grilling.
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Old 06-11-2013, 04:54 PM   #23
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deck joist span question


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You cannot span 17.5 feet with dimensional lumber, I really dont care what the span tables say. Especially with the crap lumber they sell today.
Dimensional lumber is graded, and span tables specify grade and species. If a table says you can span 17.5', and you're using the species and grade (or better) specified in the table, you certainly can span that distance safely. Whether the deflection (bounce) will be to your liking...that's another story. Certainly, the closer you get to a span's upper limit, the more bounce you can expect. A good deck builder that's not working on the cheap will stay away from those upper limits.

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