It also depends on your building code. where I live, you can't go more
then 12 feet in any direction without applying for a building permit, so
the sheds are limited to be no larger than 12x12 if you don't want to
apply for a permit and deal with the footing issues.

James wrote:

I am neither a builder or carpenter. I saw a shed today at Home Depot,
that measured 16 x 16. It had a loft second story to it for additional
storage.

There are no internal supporting posts. The joists were 2 x 10, and
spanned the full 16 ft width of the shed.

I was wondering if a person could use 2 x 12s, and span up to 24 feet
without supporting posts? I doubt it. Yet, I walked up into the loft 2nd
level and jumped on the floor (I weigh 200 lbs), and the floor felt
"pretty" solid. Not rock solid , of course. Of course, going to 24 feet
would be 8 more feet of span, and could make a big difference

Bottom line, how far can you span with 2 x 10's, and how far with 2 x
12's ? By the way, these joists were 12 inches apart.

Thanks for any comments !!

--James--