Deck beam cantilever

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Old 10-18-16, 01:32 PM
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Deck beam cantilever

Hi guys,
I am building a deck as I go and I am confused about the beam cantilever rules. I keep reading that the cantilever of the beam can't be more than 1/4 of the span of the posts.

I can't figure out anywhere if that is the span from one end post to the other end post or if it is from the end post to the very next post.

I am building a deck 18x20.

So my beams are going to be the 20' length and my Joists are gonna run the 18' length.

If it is end post to end post then I will be able to have a 5' cantilever I think.

All I need is a 2 foot cantilever because I planned poorly and installed my posts before I thought about the cantilever issues...

My posts are about 5'4'' apart o.c.

That leaves the 2' on each side for cantilever.

Any idea if this 2' cantilever is going to be too much?

Thanks for your help,
Dave
 
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Old 10-18-16, 01:41 PM
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It isn't just how far the cantilever hangs out, a lot depends on what size joist you are using.
 
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Old 10-18-16, 02:22 PM
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I was going to use 2x6 Joists. I am going from a ledger board on the house to 8' out to the first beam then 8 more feet out to second beam and 2 foot joist cantilever with Joists 16''oc and I planned on using (2)2x6 for the Beams
 
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Old 10-18-16, 02:50 PM
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Since you are asking about the "beam cantilever".... Span = center of one post to center of next adjacent post... 64" x .25 = max 16" cantilever.

They may give you an exception to the rule if the beam is sized large enough... but with your posts so close I suppose its small.
 
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Old 10-18-16, 02:57 PM
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If i were to put an angle support from the post to the beam would that help me get out to the 2 foot mark?
 
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Old 10-18-16, 03:12 PM
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That's a question for your building inspector. We can't say what he will allow.
 
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Old 10-18-16, 03:21 PM
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I know but I'm just wondering if it would be structurally solid and not Bouncy.

Thanks
 
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Old 10-18-16, 03:36 PM
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Hate to be the bearer of bad news but with 2x6 joists I guarantee it will be bouncy. Your joists are undersized for the span and you can't have a 2 ft joist cantilever beyond your beam. No way the deck you are planning would pass an inspection, it's not to code.

https://www.google.com/url?sa=t&sour...iOTibU6h5d3N7Q
 
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Old 10-18-16, 07:58 PM
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First of all, your beams should probably be double 2x8 or maybe even double 2x10. Use one of the many online calculators to determine this. Note, however, that beam size and joist size go together.

If you have one row of posts (and beams) at 8' and another at 16', then your joist span is 8'. For 2x6 SYP, the maximum span at 16" OC is 9', so this is fine. Additionally, at an 8' span, you can cantilever two feet, so you are fine doing an 8' joist span and a 2' cantilever provided your beams are sized correctly.

What I would do if I were you is this:

Notched 6x6 posts and double 2x10 beams at:
1) 8' from the house, at 2', 10' and 18', and
2) 16' from the house, at 2', 10' and 18'.

Your joists at that point can be 16" OC and at 2x6 will span that without being bouncy.
 
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Old 10-18-16, 08:31 PM
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See page 10 for a post to beam description. http://www.awc.org/pdf/codes-standar...Guide-1405.pdf

Thanks Brant, I bookmarked the one you referred to as well.
 
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Old 10-18-16, 08:39 PM
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IRC table R507.5 says a 9' joist span is allowable only when there is no cantilever. With cantilever, the maximum span is 6' 8", (less than his 8' span) and the maximum joist cantilever in that case would be (span /4) or 1' 8". (2012 IRC says max. 1' 1" joist cantilever with 2x6 joists)

Beam size and joist size do not "go together" when you have a joist over beam cantilever.

No idea if this deck is permitted or will be inspected, and if it is we dont know what code the inspector goes by, (2009, 2012, 2015) but even if it's not inspected now- it will be someday when the house is sold.
 
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Old 10-19-16, 08:04 AM
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I just built a flush beam deck, so I was using those tables. I didn't know there was such a stark difference between joist spans with and without cantilever. I know there are additional stresses with cantilevers, but I did not think it was that significant.

That said, beam size and joist span are not independent. The way I understand deck building is that you first start with the intended deck size and then determine your joist span, joist orientation, and post location. These are interdependent as the size of the joist (not to mention the grade of the lumber) determines the distance it can span. If you have a limiting factor (low decks, for example, might prevent the use of wider joists), then you design your post structure around your joist limitations. Otherwise, you might simply engineer an efficient combination that lets you maintain structural integrity while minimizing materials and labor.

Following that, beams are sized based on the distance they span as well as the load they bear. This itself is a function of the joist length and the joist span. You can't just build a deck and beams independently as a certain deck structure will require a certain minimum post and beam structure while a certain post and beam structure can support a maximum deck structure.

I would also like to add that it's extraordinarily confusing and counterintuitive to try to make sense of the various guides against the IRC. A municipality might have a simplified guide - cantilever is 1/4 the span, no difference between cantilevered and non-cantilevered decks - while the IRC is significantly more complicated. This is just as bad as the NEC.
 
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Old 10-24-16, 09:52 AM
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So I didn't plan this project to well as you guys can tell. I am canceling this project and going with a stone/paver patio .

Thanks for your help
 
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