When I lived in Florida, I built a pole barn which would be somewhat
similar to what you're doing. I think you might want to consider a
similar design.

First, you can eliminate the heavy and expensive 4x10's. Don't worry
about running beams along the 25 length to support the ends of the
rafters. However, I'd up the poles to 4x6; 4x4 is too flimsy. Orient
them so that the long side (the 6" side) is parallel to the short side
of the slab (the 15' side). If I understand what you've described, you
should have six poles total, three at 8' on one side and three at 9' on
the other. Using 2x10's, make six rafters about 18' long (It's only a
little over 15' between the poles, but this will give you some extra
overhang to keep rain out). These rafters get attached in pairs to the
poles with 1/2" threaded rod and nice big washers, so that they form a
rafter-post-rafter sandwich. When you're done, you should have three
separate free-standing structures consisting of a pair of posts 15'
apart and a pair of rafters attached to either side of the posts. Add
some 2x4 blocking in between the 2x10s every foot or so along the
length of the rafters.

At this point, you connect the three separate sets of rafters with 2x6
purlins on edge at 24" on center, spanning about 12' from one set of
rafters over to the next. I'd use 2x6x14' purlins so that you get some
overhang on the ends of the roof. In any event, you'll want the total
length to be an even multiple of 2' since the corregated or 5-V roofing
is usually 2' wide. For this roof pitch, the purlins are basically
joists. They are 2x6's on edge running parallel to the long side of
your slab, perpendicular to the rafters. Connect them with H1
Hurricane ties like these:
http://www.strongtie.com/products/co...H.html#gallery
You should be able to find them right at the borg.

Speaking of hurricanes and high winds which you could certainly get in
south Texas, I would recommend using screws for the metal roofing, not
nails.

My barn made it through all the hurricanes last year (and this year
too, though I no longer live there) with no problems at all. During
Frances I had to go out to feed the horses in 70+ mph winds; the wind
was howling through the barn but the roof didn't make so much as a
creak. Meanwhile, back inside my house it sounded like the roof was
going to blow off at any second.

I've put a few pictures online which show how I built the roof so you
can see what I mean. My construction had a steeper roof pitch, so I
have collar beams as well as rafters, but it should still work for you.
The engineer I spoke to said I could get away with 2x8 rafters for my
13' span, but I'd recommend the 2x10's for yours.

The pics are at http://www.flickr.com/photos/jcaron2/search/tags:barn/

Josh

P.S. I drew up plans for the barn. I can probably track them down and
send them to you if you want.