Doubling Up Joists or 4x6's

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Old 12-19-13, 12:52 PM
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Doubling Up Joists or 4x6's

Here's a question for anyone knowledgeably on the topic. I'm finally getting around to rebuilding my tall backyard deck built in 1979 (34+ years old), and low and behold - the building codes in 1979 wouldn't stack up to today's standards. Amongst one of the problems. It's 24ft along the house (attached to a ledger) and goes out 12ft. My girder is 9 feet away from the house (proper). So in a prior post a few years back many agreed my joists are not spanned correctly - they are 2x6's and 24 inch OC. By today's standards if I am to retain the 24 inch spanning between joists, I need 2x10s. So that means another ledger underneath my existing ledger, and potentially some mods around the perimeter because Redwood facia is 2x6. But 12 inch OC with 2x6 joists would also meet code, but the problem is there as places in the ledger where there is a damn bolt there. So here's my questions...

Can I go with pattern of two 2x6 joists say 3 inches apart (such that I can have two 2x6 joist hangers side by side on the ledger) placed every 24 inches? Net-net is the same amount of joists if they were 12inch OC. Is this an acceptable pattern that a building inspector would honor?

Can I do the same as above but just put the joists side by side and hang the 4x6 joist hangers (again, every 24 inches apart)?

Optionally can I just go with 4x6 joists every 24 inches?

Any help appreciated.
 
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Old 12-19-13, 02:06 PM
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You say you have a "girder" 9' from the house. Is this a "beam"? What size is it and how is it supported across the 24'? If this is a beam, then your cantilever of 3' is pushing the envelope. Normally there is only allowable 2' cantilever.

You can't cheat inspectors or death, so unless you do the spacing of the joists in a uniform manner, he will kick it. Even 2x6's at 12"oc will only span 10'9".

You may want to go over the current codes to preclude any "gotchas" later on. Here is a reference link to it. http://www.awc.org/Publications/DCA/DCA6/DCA6-09.pdf
 
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Old 12-19-13, 02:43 PM
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Yes - I stated girder, but it is the beam (24 ft). It's a 4x6 supported by five 4x6 posts spaced 5.5 feet apart with 1 foot cantilever at each end of the beam.

I always thought the joist cantilever over the beam was up to 25% of the total joist size (12 ft in this case, hence 3 ft OK). Perhaps that was the rule in '79. You are saying 2x6s at 12 OC will only span 10'9'' but isn't that to the center of the beam, so technically if the beam is 10' from the house, I could do a 2' cantilever past the beam for my 12' joists. I'm not suggesting anything that isn't "uniform", just sets of double 2x6 joists (as opposed to single) but each "set" of 2 joists would be uniformly spaced 24 inches from the next set. It's bizarre, I can't find anything on the internet that speaks to this topic, but from a physics perspective I would think the load is identical as single joists spaced 12 inches apart. I'm not doing this, just trying to get ideas on what's possible before I head down to county planning before I hire a contractor.
 
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Old 12-20-13, 02:45 AM
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The overall strength of doubling joists at 24" vs. single joists at 12" is comparable. The fly in the ointment comes from the fact that doubling up joists creates a crevice between the two that will collect moisture, tiny critters that enjoy eating wood, mold, and all kinds of other problems. All of which promote premature failure.

And then there's the fact that your local building department/inspector may tell you that such is just plain not acceptable. You would do well to check with them first, before making any deck rebuilding decisions.

P.S. A true 4 x 6 is not exactly the same as doubled 2 x 6s. Section moduli are 17.65 vs. 15.13 C.I., respectively. Meaning a true 4 x 6 would be about 17% stronger.
 
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