# Deck beam cantilever



## Rick18071

Maybe someone can explain this to me?

Say I want to build a 12'x 6' deck with the joists spanning 6' from ledger to beam.

2015 R507.7 Beams shall be permitted to cantilever at each end up to one-fourth of the actual beam span.

According to this If I have a two 2x10 beam spanning 8' I can have a 2' beam cantilever on each end.

But if I really want to beef it up and use a three 2x12 beam and add another post between so I only have a 4' span I could only have a 1' beam cantilever.

So if I make the beam stronger with half the span I need to have a shorter cantilever?


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## JBI

Prescriptively, that seems to be the case. I'm sure a design professional could easily justify the longer cantilever for you. 
Prescriptive provisions are not 'absolute', they are a quick and easy (down and dirty?) way to avoid paying someone to design.


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## Rick18071

This section actually encourages to do the minimum allowable framing.

It seem s like it would make more sense if the code section said that the cantilever can be up to one-fourth of the actual beam length rather the actual beam span.


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## Francis Vineyard

As a reminder the (3) 2 x 12's having a 4 ft. span would be permitted to support a joist span over 15 ft. (#2 SYP)


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## JBI

Rick18071 said:


> It seem s like it would make more sense if the code section said that the cantilever can be up to one-fourth of the actual beam length rather the actual beam span.



The cantilever has a bigger impact on span than overall length. 
The Code is not really encouraging the minimum, it is intended to insure that not less than a minimum is used. The simple solution to your proposal would be to move the above-code piers farther out and reduce the cantilever to stay within prescriptive parameters. 
What you proposed would be kind of like putting a 12 cylinder motor in a golf cart.


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## FLSTF01

I would allow the beam to cantilever 1/4 of the allowable span of the beam, regardless of whether there are additional posts, presuming there were no splices at that additional post.


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## Rick18071

http://www.bing.com/videos/search?q...C4C46942FFCB238ABD92C4C46&fsscr=0&FORM=VDQVAP


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## FLSTF01

What does the above video have to do with deck beams?


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## JBI

FLSTF01 said:


> What does the above video have to do with deck beams?



That was in response to my earlier comment... LOL


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## Rick18071

Now I'm doing a plan review of another deck with the same problem. Just wondering how others take care of this problem? Should I make them indicate where the beams splice is so I don't need to count the extra posts? But what if the span is too long without the extra posts? What if the spices on the separate 2x's of the beam are staggered on different posts?

Just want to know how other plan reviewers deal with this.


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## FLSTF01

The allowable overhang 1/4 the distance to the first post.  There should be no splices between the first post in and the end of the overhang.


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## Rick18071

So if there is a second post is close to the first post creating a 8" actual beam spans the cantilever can only be 2" no mater how long the (3)2X12 beam is?


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## FLSTF01

Why would there be a post 8 inches away from the first post?  If the second post is there due to a splice being there, than I would say yes, only two inches.

What you are describing sounds like a screw up and i wouldn't go out of my way to try to accept something that will likely come back to haunt you.  Proper design and construction is not really all that tough.


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## Rick18071

The second post is also supporting a roof.


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## FLSTF01

1/4 the distance to the first splice.


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## Rick18071

So if a 40' long deck would have 2 beams that are made out of a  20' long (2) 2X8 it could overhang 5'. That doesn't seem right.


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## FLSTF01

To the first splice or post.  Experience in the field teaches these things.


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## JBI

The prescriptive provisions of the Codes are not the be all and end all of building design/construction. 
IF someone wants to avoid the cost of an engineered design, then they may follow the prescriptive path. 
IF they choose to deviate from that path then a design professional will need to determine (read as 'calculate') any elements that do not follow the prescriptive path. 
It's really not that complex a concept.


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## Rick18071

I was just wondering if this is a mistake in the code. Why can't the cantilever can be up to one-fourth of the actual beam length rather the actual beam span.


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## FLSTF01

Rick18071 said:


> I was just wondering if this is a mistake in the code. Why can't the cantilever can be up to one-fourth of the actual beam length rather the actual beam span.



Because the allowable cantilever is in-part based on the ability to support the load without deflection to the point of failure, usually at the top (tension side) of the member, directly above the beam.  It has been proven through years of rule-of-thumb building that 1/4 the allowable span is acceptable for a prescriptive approach.  Bigger (usually taller) beams span further distances between post and can therefore cantilever farther.


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