Joist spacing 12" or 16" OC?

Old 05-29-20, 02:06 PM
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Joist spacing 12" or 16" OC?

Hi there,

I am building a deck with 20 ft long composite decking boards, likely from Trex or Fiberon, perpendicular on the joists. From web,site, the minimum joist spacing is 16". One contractor did my friend's deck at 12" OC, and one contractor says that is waste of money and added more weight on the beam.

I am building my own so money or time is not major concern. The pros of using 12" are:
1. Stable
2. Less wavy. A bit questionable since I may have to make all joists level before putting on the boards. Planning and keeping 21 joists level is 30%+ harder than on 16 joists.

The cons are:
1. Put more weight, about 5 joists more, on the beam, which is 2 ply of 2X10X10fts, spliced on the center of post.

Which way, 12" or 16" OC, is better?

Talking about installing and preventing wavy, is 20 ft long composite decking board harder to deal with?

Old 05-29-20, 04:28 PM
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The way I see it, 16" centers is the standard for all homes. Why should a deck be different?
Old 05-29-20, 04:41 PM
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Congratulations, you have made an excellent long term decision that you will never regret.

Too many people look at the now cost and do not take into account the many frustrating hours of repairing and staining PT decks.

Here is a recent post on my now 11 year old composite deck, just recently cleaned, never repaired, never stained, how can you argue!!

11 Year Old Composite Deck

Sorry, I get off track, anyway, some composite manufactures say 12 OR 16, I have only used Timbertech, this deck is the 5/4 solid boards with joists @ 16", rock solid,

Maybe if your using a really thin, cheap decking you would need 12"!

And sorry, there is no plastic feel, and we love being on that deck in bare feet without ever having a fear of splinters.
Old 05-29-20, 08:02 PM
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The contractor said the same thing, since house is built at 16" OC, why not deck. :-) But I guess it makes sense to be different. Trex's board is about 1" thick.

Thanks for your inputs.
Old 05-29-20, 08:29 PM
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If you are using Trex, their installation instructions recommend 16" on center for residential and 12" on center for commercial. Also if the decking will be applied at a 45 degree angle to the joists, the spacing is reduced by 4". So 12" residential and 8" commercial.

The best thing you can do to ensure your deck is built (and stays) flat is to reduce the span of your joists by using more beams. Deck span tables are created to show the maximum limits for safety. They are not recommendations to use the smallest dimension lumber possible or to go with the longest spans possible. Common sense would dictate that if you want a solid deck with no waviness as you sight across it, you could beef up the size of the joists and increase the number of beams under it.
Old 05-30-20, 08:58 AM
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Thanks, XSleeper for your input. That makes sense. I've learned a lot by talking to experts like you and other people in this forum.

I am building a typical 20' x 12' deck, with ledger board on house rim joist and a spliced beam on 3 posts connected to concrete piers, 10' away from the house. I am using 2x8x12ft southern pine as the joists, (2) 2x10x10ft southern pine as the beam, all are PT and GC. The joist span is 10' and joist hangout 2'. Beam span is 8' and hangout 2'.

I may not change the joist size to 2x10, may not add another beam under, btw digging and pouring concrete is no fun, :-), and was wondering if using 12" OC would make deck remarkably more solid, and worth the effort and time. From the other side, I wanted to hear from you guys, especially those experienced, if 16" OC is solid enough for 1" thick composite decking board or not.



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