I've built numerous post and beam houses
where we used 1-1/2 native pine T&G for
loft flooring on 4X10 joists 4 foot OC and
never had any owner complain about
springy behavior. 8/4 ash would be way
way stronger - I wouldn't hesitate at all
to use it as you describe.

The complaints I did get had to do with sound and dirt -
every step is heard perfectly
from the floor below, and eventually, dust/dirt
works its way into the T&G joints and when
the low humidity in winter has the flooring
shrunk to its narrowest width, the dirt finds
its way through and onto whatever's below

hope this helps

Are the Beams rated to support living space?

1 1/4"-1 1/2" is well more than enough. Don't overthink it. 2x6" t&g #2 spruce is used for this all the time at 1 1/2" finish.The Ash is considerably stiffer.
The dirt working thru is a real problem, though the Ash being kiln dried should stay much tighter.

How about a sub-floor first? That could be 4/4 or 5/4 T&G material. Then a layer of the typical laminate flooring 1/16" thick sheet cushioning material and finally a 5/4 or 6/4 top layer of ash flooring. If you ran the ash at 45 degrees to the sub-floor it would be even stronger.

The whole system would look good from below, have plenty of strength, solve the dirt drop issue and be quieter due to the sheet of cushioning material.

BH Davis

Wow, thanks to everyone for all the awesome information! The sagulator is exactly what I needed! I also like the idea of making a subfloor from the same material and running it at a 45 degree angle, I'll mention that to the homeowners and see what they think. Thanks again, much appreciated!

If you are going to run the sub-floor at 45° then take another look at the Sagulator. Your spans are going to be greater so thicker material will likely be in order for the sub-floor.

BH Davis