James,

I plan to build a small garage, 20 feet by 30 feet. I want to be
able to span the 20 foot width, so that I don't have to use interior
posts.

A few years back we built a 24'x28' garage. I wanted a completely open
lower level, and a fully usable attic (i.e. No truss webs). I never
expected to put any real weight in the attic.

I used regular #2 Douglas Fir, 2x12 joists at 16" OC to span the 24'
width of the garage. I added solid 2x12 blocking every 8 feet along the
span. I used 2x6 rafters at an 8/12 pitch and ended up with about 7 feet
of headroom running down the center of the attic.

Yes, with a 24' span the attic floor is a bit bouncy, but not nearly as
much as I expected. If you stand in one place and bounce up and down you
can feel it, but it's not really detectable in normal walking. Still, I
wouldn't want to store an engine block or a library of books up there.

Mostly, we just use the attic to store items like long PVC pipes, empty
stereo/TV boxes, holiday decorations, etc. But, I do have a few heavier
items up there too.

I could have easily strengthened the floor by doubling up joists or going
to 12" spacing.

that is 16 feet wide, using 2 x 10 beams on 12 inch centers.

We used 2x10 douglas fir joists at 16" OC in some parts of our house.
That's close to the maximum span for doug fir, but the floors still feel
quite solid.

I know I could use an engineered beam, but I don't want that.

You could use wood I-joists and easily span 24' or more. They're light,
straight, and about the same price as 2x12's around here.

Alternatively, you may want to check with a truss company for "attic
trusses". These can easily span 24' and still have a usable room in the
middle of the attic. I decided against these because of cost, and because
I was mostly working alone. It would have been very difficult to install
trusses by myself.

Take care,

Anthony