The best cc spacing for shed floor construction ( that's center to center!) is actually 12".
Economically speaking, you could be less then this but then cost would be prohibitive and 12" on center is perfectly fine for cc spacing of joists for a shed.
Most typically floor joists are 16" on center for residential floor construction, but we are just dealing with a small footprint here, and having your floor joists 12" on center for added strength is only going to cost you few bucks more. In the long run, you are going to get much more added strength for your floor and less sagging.
Lets say for example, using the picture above. Originally you wanted to frame your shed floor with 16" cc spacing. You would have for a 10'x12' shed 10 floor joists. Lets also assume that you can get a 2x4x10' board at a cost of $3.50. This equals $35.00 for just your floor joists.
Now lets figure out for building the same 10'x12' floor with 12" cc spacing. This will figure out to be 13 floor joists. Using the same cost per 10' joist, the total cost here would be $45.50. A difference of $10.00!
If you don't think the extra $10.00 is worth it then by all means stick to your 16" spacing. But for my book, I would definitely go with the 12" on center spacing.
In the long run, you'll be glad you used the 12" cc spacing.
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