Here's a span calculator for joists and rafters that ought to do what
you need: http://www.awc.org/calculators/span/calc/timbercalcstyle.asp




I've got a water tower on my property that stands off by itself and
I'm thinking of putting an 1100 gallon tank on it.

It has a foundation rated at 4000 psi (which is stronger than the mix
for any road in my state) and all loads bear directly. I haven't the
slightest idea of the framing sizes or how they align with the load.

I'm wondering if 2 x 8 joists will hold the load without there being a
problem with deflection.

The 2 x 8 's will bear on steel angle braces that are set who knows
how many inches apart, by god knows what kind of fasteners, but these
are fastened to 2 x 12 rim joists.

BTW - I have no idea how the holding tank engages the framing at the
footprint. It might be perimeter loaded or it might hit at eight
points distributed in some way that is not privy to me.

Thank you so much for helping me out with this.