Plywood: 12" vs. 16" OC

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Old 06-27-05, 01:06 PM
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Plywood: 12" vs. 16" OC

I have a large, old deck (26 x 13) built with 2x6 joists, 12" OC at 11-foot span. The decking is 3/4" plywood, carpeted. A little weathered, but basically sound. Not for everyone, but OK with us. I have a simple question.

Now that I'm rebuilding the house section that the deck is attached to, I'd like to make the deck less bouncy. No doubt, most of the current bounce is due to the 11-foot spans that the 2x6s are running. In re-joisting the deck, certainly 2x10s at 16" OC would be a distinct improvement in stiffness. The question is: Due to the 3/4" plywood, what would be stiffer on the 11-foot span -- 2x8s at 12" OC, or 2x10s at 16" OC?
 
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Old 06-30-05, 04:29 AM
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Speaking as an engineer, which is to say based on my knowledge of structural theory rather than experience particular to deck-building:

I believe joist spacing has to do with the type of decking rather than the cross section of the joist. If your decking is going to sag with a certain amount of spacing, it isn't going to matter if the joists are 2x4 or 2x12.

The size of your joist should be based on the span of your beams and the joist spacing as driven by the decking material. From the table in my book about deck building, with 12" spacing, up to 11'7" span is OK for 2x6. You can go up to 12'10" on a 2x8 with 16" spacing.

With plywood as your decking, I would think that in order to prevent sagging, the best thing you could do is make sure you use enough screws to attach the plywood to the boards. The fewer the number of screws, the more load each screw has to carry as it is resisting the wood being pulled on when there is weight on it... And the more load the plywood exerts at the screws, the greater the tendency for the holes to become elongated, which will result in the wood being able to sag under load.
 
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Old 07-03-05, 06:05 PM
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WillK's comments about joist spacing are right on. Joist spacing is important to reduce the amount your plywood flexes between joists. Provided you stick with 3/4" plywood or OSB, 16" centers will be fine. 12" centers would be overkill.

2x6's will span 11ft without failing, but you will notice deflection in the floor. 2x8's would be better, and 2x10's would be even better than that. If you want a stiff floor that won't shake or bounce, go with 2x10's, and glue and screw your plywood or OSB (preferably tongue and groove ply or OSB). I don't particularly like my chair to rock when someone heavy walks past.
 
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Old 07-03-05, 09:40 PM
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Leave your joists the way they are. They're fine. If the plywood is in good shape, leave it alone too. To eliminate the bounce, simply add a 3rd beam under the joists, 1/2 way between the exiting, cutting your joist span to 5-1/2'.
 
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