I am building a car port which has a "shed roof". That means
that the roof surface only slants in one direction.

the roof joists sit on top of two long beams, one higher than the other (due to the shed roof design) . Thus the forces exerted
by a single joist on each beam is a function of the ratio of the distances from where the load is applied to these beams. The
forces are not a function of the angle of the roof joists as near as
I can tell.

intuitively, it seems to me that there is a sliding force that is
parallel to the roof joist. Thus in my imagination a snow load will
want to push the roof joist in the direction of the roof joist angle.

however, when I take a sum of moments I end up with an equation
which is simply sliding force = load/(2*tan(angle)). I assumed that
the load was applied in the middle of the roof joist.

this answer has to be wrong as the smaller the angle the greater
the sliding force can be on some imaginary object.

I can't believe that I can't solve this problem.  What is wrong with
my thinking.  I would rather you help me develop a clearer way of
thinking than just give me the answer. Perhaps ask me questions
which might lead me to think in a different manner.

If you could reply by email it is a bit easier for me to catch the answer as I am not sure I will ever be able to find this site again!

Bil

this answer implies that