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Old 10-24-2011, 04:36 PM   #1
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Span for 2x8 support beam


I've got a deck 30x14 off the second level and I want to build a deck the same size below it. The upper deck is framed with 2x8's. Joists are 14 foot in length 24" OC supported by 2x8's on both ends with a double 2x8 center beam. This center beam which runs the 30' length of patio is supported by 2-4x6 posts on each end and 3-4x4 posts. I want to know if I can add an additional 2x8 to the center beam and replace the 3-4x4's with one 4x6 at the center.

So the center beam will be atriple built up 2x8 with a Span of 15' from one 4x6 to the next.

Appreciate your help

Thanks Kevin
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Old 10-24-2011, 06:11 PM   #2
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Span for 2x8 support beam


Decks are generally designed by prescriptive codes. It should be a relatively simple matter to go to the building inspector and obtain a copy of the code, or if they use the International Residential Code you can check that on line. Getting design advice on an internet chat forum, where we don't know your location or specific code that you need to build to, is a weak plan.
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Old 10-24-2011, 07:58 PM   #3
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Span for 2x8 support beam


Simple answer is no... Not even (3) 2x12's will span that as a center beam configuration. In fact it sounds like there is a few undersized things on that deck. A quick sketch of what you have (just to make sure we are on the same page) with dimensions could help us out.
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Old 10-24-2011, 08:27 PM   #4
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Span for 2x8 support beam


if i were building these decks 24" o.c for the joists would be totally out of the question. 16" o.c is standard however since they are 14' joists i would probably up them to 2x10 or at least switch to 12" o.c and add 2 rows of blocking to stiffen up the floor. the beam would be 3 ply 2x10 supported by 4 6x6 posts.

the first thing that would be done is to configure the lower beam and then continue the posts up to the beam that supports the upper deck. 4x4 posts are out of the question for supporting the deck. locally 4x4 posts are only to be used for hand rail not to carry the deck
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Old 10-24-2011, 10:06 PM   #5
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Span for 2x8 support beam


Quote:
Originally Posted by wooworkbykirk View Post
if i were building these decks 24" o.c for the joists would be totally out of the question. 16" o.c is standard however since they are 14' joists i would probably up them to 2x10 or at least switch to 12" o.c and add 2 rows of blocking to stiffen up the floor. the beam would be 3 ply 2x10 supported by 4 6x6 posts.

the first thing that would be done is to configure the lower beam and then continue the posts up to the beam that supports the upper deck. 4x4 posts are out of the question for supporting the deck. locally 4x4 posts are only to be used for hand rail not to carry the deck
From what I gather he has a midspan beam cutting the effective joist span to 7' and even with 24" O.C. joists it does meet code... Would I built it that way? No.
There was a recent article in the professional deck builder magazine about 4x4 posts, reading it will open eyes up to how strong they really are, they are stronger than most think. We use 6x6 for aesthetic reasons, but if 4x4's make sense I would use them.

Like I said a quick diagram with dimensions will tell us all exactly what your talking about.
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Old 10-25-2011, 06:20 AM   #6
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Span for 2x8 support beam


[quote=robertcdf;756153] We use 6x6 for aesthetic reasons, but if 4x4's make sense I would use them.


Dont know about Colorado, but here 6x6 is code.

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

Bottom of page 7.

Last edited by 12penny; 10-25-2011 at 09:00 AM. Reason: correct page #
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Old 10-25-2011, 08:01 AM   #7
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Span for 2x8 support beam


[QUOTE=12penny;756296]
Quote:
Originally Posted by robertcdf View Post
We use 6x6 for aesthetic reasons, but if 4x4's make sense I would use them.


Dont know about Colorado, but here 6x6 is code.

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

Bottom of page 9.
Try reading this article (like I had said originally) http://www.deckmagazine.com/article/413.html I also know how to calculate loads and 4x4's can take a lot more than you think. Of all of the deck failures I have researched (not to say I have researched them all) I have NEVER seen one where the 4x4's failed.

BTW the gentlemen that wrote the article has books, numerous articles, as well as being a building inspector... Probably knows a little more than I do or you.

I actually live in an area where the building department has a brain and they know how to use it. They will calculate a load based upon MATH and not just a span table that is written to cover anyone's ass anywhere in the country.
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Old 10-25-2011, 09:17 AM   #8
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Span for 2x8 support beam


good article. Thanks. I dont have a problem with 4x4 posts. Local building depts like to see 6x6 and thats what I give them. Like you, I like the aesthetics of a 6x6 post and use them almost exclusively. No math required.
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