deck rafters 24'' apart

I once read a deck construction instruction book that recommended 24 inch centers.

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Seems to me that the spacing would depend upon the decking material and the loading expected. For example if the decking was Trex or one of the other synthetics then 24" would be unthinkable. On the other hand if the decking were 2-by material then I wouldn't have any problem accepting it. If the decking were 5/4 material and the expected loading was light it would probably be fine too although it leaves less margin for the time when the material begins to deteriorate years down the road.

If it doesnt flex why worry

Must be a newer house or deck, they cut corners like crazy now.

I think that you may be referring to joists as rafters here, but either way, 24" OC is one of the standards for both. The spacing will affect what you can use to span the joists (or rafters).

wrote

Technically they are referred to as _ joists_ not rafters. Local codes vary, but around here the code is: 16" O.C. (on center) for 5/4" x material decking. When using 2" x material decking you can go 24" O.C. with the joists. This is for wood products, I'm not familiar with the man made products.