Composite decking joist spacing

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Old 09-05-18, 08:21 AM
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Composite decking joist spacing

Hey gents, another project underway for me. I am building a 20' X 20' deck and will be using composite decking (azek or timbertech) I framed one half of the deck 16" OC but after stumbling across some reviews, it seems 12" oc is recommended to prevent bounce.

I plan to do 12" OC on the other half but what should I do about the 16" OC framing I already did? Starting to stress out a bit here.

I am planning on laterally bracing the 2X8's as well. Thinking about sistering 2x4's to the 2X8s to act as a landing strip and close the gap a bit, not sure if that would be a waste of money?
 
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Old 09-05-18, 08:44 AM
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If you are using the conventional 1x6 solid boards I have used the same on the past several decks all with 16" OC 2x8 joists.

Per similar question in the past I went out and stood in the center of the span and bounced and didnt feel any movement so I'm not sure why 12" OC would be needed.
 
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Old 09-05-18, 08:47 AM
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Thank you for the quick reply, I actually have a couple of pieces of azek decking im going to screw down tonight and test. Azek is a bit more narrow at 1 X 5.25"
 
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Old 09-05-18, 09:31 AM
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Short answer is that if you really need to be 12" OC, you will probably need to run a new joist between each currently existing pair, which. of course, would then put you at 8" OC.

What's the concrete - are you just putting posts onto a concrete patio?
 
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Old 09-05-18, 10:19 AM
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That is an old concrete patio, 5 posts (3 are on for the ledgerboard, which is already bolted to the foundation so its just for extra securement) are sitting on it with a bracket bolted into the concrete, the other 4 posts are 8" sonotubes with a 4x4 post and bracket on them.

Center post is a 6x6 holding up the 2 2X12's
 
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Old 09-05-18, 02:41 PM
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So i screwed down 3 spare board of azek, its about 85 out right now and if i stand directly in the middle, I feel a very slight bounce. I think I'll do the other side 12" OC but just leave this side 16 OC, I will put cross bracing between each joist as well.

I think it would be really overkill if i split the 16 OC and went in between them, not really worth it for an extra $175.
 
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Old 09-05-18, 04:25 PM
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Cross bracing or blocking will result in a big improvement..
 
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Old 09-06-18, 07:19 AM
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I plan on cross bracing, but i dont know if i should do the other side 12 OC or keep everything 16 OC
 
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Old 09-06-18, 08:42 AM
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If it were mine, I'd say "well shoot" and change it over to 12" o.c. You might wreck a few hangers, but that's not a big cost item. To put it into perspective, think what you're saving by doing it DIY vs. hiring it!

Is this going to be inspected? The inspector will likely know what the joist spacing requirement is for your decking.
 
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Old 09-06-18, 09:30 AM
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Yes, and it is to code, 16" OC, but the reason some people do 12" is to prevent a small amount of flex.
 
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Old 09-06-18, 12:23 PM
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I guess I was responding to 40+ years of indoctrination where we were not allowed to do something contrary to manufacturer's instructions. I always thought that 12" was a code thing. Today has been a good day because I learned something!
 
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Old 09-10-18, 06:40 PM
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is that a raccoon tail in the bottom of picture?


12"OC is needed if you run decking diagonally, otherwise most decks are 16" OC with composite and no problems.
Even the original trex from like 1999 is 16" OC and new composite is probably much sturdier.


sidenote,
I woulda just made that center double beam 10" lower and put the joist tails over top of it and sister them, instead now you need joists hangers all over.

8" sono tubes in Mass? 12" diameter required in Jersey, sometimes even 14" (or more if hot tub and 4' deep).

And no posts on slab in Jersey have to jack hammer out and go 3'.
the front posts are below frost line and won't heave (probably not even with only 8" diameter especially if bottom of hole is flared/belled out) but maybe that slab can frost heave a tad bit and then screw up the ledger connection (water entry etc, failure in worst case scenario) but I doubt that would happen but could depending on the soil and slab thickness.
 
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