Number of posts and beams needed

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  #1  
Old 05-28-03, 01:26 PM
swmartin
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Question Number of posts and beams needed

I am planing to build a 10'x16' free-standing deck with a total height of 17" or 24". I'll be using concrete footings 18" into the ground (per local code) with a 12" diameter and 4x4 posts. For the beams, I was thinking of sandwiching 2 2x8s with 1/2" spacers. Joists will be 2x8s.

Ideally, I'd like to use two 9' 9" beams with three posts per beam, one at 4' and another at 12'. I have a feeling that this is not enough support. Should I instead a) use three (or four) 10' beams or b) run two (or three 16' beams with four posts per beam or c) something else all together?

As you can probably tell, this is my first deck, and I want to be as cautious as possible.

Thanks,

Sam
 
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  #2  
Old 05-28-03, 02:23 PM
petrsjo
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Sam,

You're initial design is pretty good. I would just tweak it slightly. The 4 foot cantilever of the joists over the beams is a bit long. I would try to cut that down to 3 feet or under by putting the beams at the 3 foot and 13 foot marks. That gives you a 10 foot span on the joists in between the beams. That should be fine, especially if you space the joists at 12" o.c. 2x8's will span 12+ feet. I have 2 x 8's spanning 9 feet at 12" o.c. and I see almost no bounce.

If you really want to make the thing rock solid, then you could add a center beam and cut your joist cantilevers to under 2 feet.

Always check your local building codes......

Good luck.

John P.
 
  #3  
Old 05-28-03, 11:08 PM
L
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Sam,

Which way do you want the deck boards to run? In the 10' direction, or in the 16' direction?

If i were building this deck, I would probably run with 16' deck boards, which means my beams would also run in the 16' direction. Those beams would be spaced at 2' and 8' from the edge of the deck, and be each would be supported by 4 posts -- 1 a foot in from each end and the other 2 spaced a 4' 8".

If you choose to run the deck boards in the 10' direction, you would need 3 beams -- 2 outer ones 2' in from the edge of the deck, and a center one. Support each of those beams with 3 posts -- one a foot in from each end, and a center one.

2X8 joists, at 16" O.C. will span between the beams just fine, no matter which direction you choose to build the deck. However, IF you are going to run the deck boards diagonally, cut the joist spacing to 12" O.C.
 
  #4  
Old 05-29-03, 02:14 PM
swmartin
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John and Lefty:

Thanks for your advice. I ran my plans by a friend who is a structural engineer last night, and we decided on using three beams (at the 3' 8' and 13' marks) with three posts per beam.

When we looked it up in COBA, it said that hanging joists at 16 oc would be more than adequate for running #2 5/4x16 spf decking diagonally. In your experience, would this be noticeably spongier than 12 oc?

Thanks again,

Sam
 
  #5  
Old 05-29-03, 03:24 PM
L
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5/4 decking run diagonally needs the joists at 12" O.C. -- At least that's what I ALWAYS do for my customers.

SPF for decking? It'll work, but you sure are creating a lot of work for yourself!
 
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