So this is my first ever shed construction, about 20' x 10' by 8' high at the front, built from 4 x 2 s. Slope is about 1 in 9.5 or 6 degrees.

I plan to use 4 x 2s on the edge for the roof rafters, and sheet with metal cladding. At a 2' spacing, and with a 10 foot span, 4 x2s seem strong enough with not much sagging. I haven't decided yet what to use for the purloins, maybe 2 x 1 1/2 or 3 x 2 which would give better support for walking on the roof if any maintenance needs to be done, plus less likely to be split by the Tek screws.

So some questions:

When I rest the upper top plate on the lower one, it doesn't sit properly because the top plate of the front wall is level (see 4th and 5th photos). Should I worry about that or just clamp everything down and nail? I could perhaps mitre the top of the stud at the back wall so that the plate seats better.

For a 4 x 2 what's the recommended depth of the heel cut of the birds mouth? I don't want to weaken the overhang of the rafters if I cut in too much. I've seen figures of 1/3 or 1/4 of the width of the timber mentioned as a rule of thumb. Other suggestions are to make the seat cut the width of the top plate ( 3 3/4" in my case). I tried this on a test piece of timber and the heel cut is quite short for the slope of the roof, about 3/4 inch.

When attaching the box profile sheets, I'm going to space the purloins about 3 feet apart. So do I need a purloin at the front and back of the shed in line with the top plate, or would the sheets bend down to the level of noggings which would be just above the top plate? (do I need noggings here or just rely on a board nailed on to the overhanging ends of the rafters?)