Decking Floor Joist Size Question

I am building a deck 10' X 12' made from pressure treated lumber. The joists will be 16" on center spacing. I need to know what is the size of joist I need. I am planning to use 2X8X12' PT for the joists. Is the 2X8 big enough or should I go to 2X10X12'. Thanks for your response.

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Nov 18, 2008
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Joist Spacing and Span Distance
by: Editor - Rich Bergman

Joist spacing and span distances depend on the size of the joist you use, either 2x8 or 2x10.

More importantly they can vary depending on two things. First, how far apart the beams apart that support the joist are. Second, your joist spacing.

So 2x8 joists spaced at 12" centers will be able to span let's say 8' between supporting beams. Whereas 2x8s spaced at 16" centers might only be able to span 6' between beams. Complicating this further is the species of wood you use. Softer woods can not span as far.

If you visit the American Wood Council or do a search you will find engineering documents available that get into this question in great detail with tables and charts. It can get quite heady but is worth further exploration.

But to help you out I can give you a set up that should work very well based on many, many years of building experience.

Space your joists at 16" centers. Use 2x8x12 joists. Make three beams of 3-2x8x10. Space two beams 18" from either end of deck. Place the third beam dead center at 6' in the middle of the deck.

This arrangement will be more than enough - believe me.

Hope this helps you.

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