I welcome any criticism on my roof plans. I'm not a structural engineer,
I'm only making educated guesses, and I promise you won't hurt my feelings
with good suggestions! The lumber isn't bought yet, my plans aren't cast in
stone, and I'd rather get it right the first time than have to clean up the
mess after it caves in under a snow load.

I'm putting a plain gable roof on my shed. I plan to call around to ask
about prefab trusses, but I have my doubts about how cost-effective that
might be. If they're not stock and already waiting in the yard, I don't
think a custom run of 12 units will be economical. I may end up building my
own.

I've built smaller (10') trusses using glued and nailed wood gussets. On
that job I used 7/16" OSB with good results, but with the shed's 12' wall
span I wonder if I ought to use plywood...? The cost difference is trivial.

The roof has a 12' span with 12" tails, pitched at 4:12, trusses spaced 24"
OC. My truss plan is a simple open triangle with a one-piece collar
tie/ceiling joist. I'd like to retain a little attic storage, but I could
add a couple of vertical braces at 4' intervals without impinging on that.

Local code calls for a 30PSF snow load. Roofing is architectural asphalt
shingle, so 10PSF dead load should be plenty. When picking rafter material
and looking at span tables, it seemed like there were certain grades of 2x4
that would carry that. But I don't think my local places carry such good
lumber so I'll probably go with 2x6 instead (I'll check available types and
grades before deciding; most of the local supply is Douglas fir larch, but
not sure of grade on the 2x6 stuff).

Lastly, I'm thinking 19/32" OSB for the roof sheathing. I know a lot of
people will only use CDX, but I can't quite fathom why yet.

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I'm putting a plain gable roof on my shed. I plan to call around to
ask about prefab trusses, but I have my doubts about how
cost-effective that might be. If they're not stock and already
waiting in the yard, I don't think a custom run of 12 units will be
economical. I may end up building my own.
I've built smaller (10') trusses using glued and nailed wood gussets.
On that job I used 7/16" OSB with good results, but with the shed's
12' wall span I wonder if I ought to use plywood...? The cost
difference is trivial.
The roof has a 12' span with 12" tails, pitched at 4:12, trusses
spaced 24" OC. My truss plan is a simple open triangle with a
one-piece collar tie/ceiling joist. I'd like to retain a little attic
storage, but I could add a couple of vertical braces at 4' intervals
without impinging on that.
Local code calls for a 30PSF snow load. Roofing is architectural
asphalt shingle, so 10PSF dead load should be plenty. When picking
rafter material and looking at span tables, it seemed like there were
certain grades of 2x4 that would carry that. But I don't think my
local places carry such good lumber so I'll probably go with 2x6
instead (I'll check available types and grades before deciding; most
of the local supply is Douglas fir larch, but not sure of grade on the
2x6 stuff).
Lastly, I'm thinking 19/32" OSB for the roof sheathing. I know a lot
of people will only use CDX, but I can't quite fathom why yet.

If you're going to build your own "trusses", why not frame a conventional
roof? Ceiling joists, rafters, and ridge board? This will give you the
maximum storage space in the attic, and it's easier to lift single boards
than an assembled truss.

A 12' span is fairly minimal. The rafter span would only be half that, so
you have a couple of different options:

16" OC - 2x6 ridge board with 2x4 rafters and 2x6 ceiling joists.

24" OC - 2x8 ridge board with 2x6 rafters and 2x8 ceiling joists.

In both cases, the ridge board is NOT a structural load bearing ridge. The
structure relies on the ceiling joists to triangulate the roof and keep the
walls from spreading under roof load.

For the sake of comparison, I framed the roof for my 24' wide garage using
2x6 rafters and 2x12 ceiling joists at 16" OC, with a 2x8 ridge board. I
have an 8/12 pitch to maximize the space in the attic.

As for the roof sheathing, I used OSB for my garage roof and haven't had
any problems. But I read something about CDX being preferred for roofs, so
I used CDX when I sheathed the roof on our house. As long as you follow the
span ratings for the sheathing (OSB or CDX), I don't think it would make a
lot of difference. OSB can be slightly heavier though, something to
consider when you're hoisting sheets up to a roof. Of course, you can
always upgrade to a thicker sheet than needed, in which case any minor
differences in span ratings would not be a concern.

Have fun,

Anthony

you can find some shed plans at alt.binaries.e-book.technical
Plans I saw were posted in October...there may be others as well.