To be safe I would check with your local code enforcement office, it the shed will be considered a permanent structure. If it were mind I would use 2x6's, due to the possible snow load you may have in the winter
Also depends upon how high the peak is - slope of the roof
2x4's would do it
2x6's would insure long time success against that freak heavy snow storm
But a steep roof would also work at preventing snow load
"16' A frame/2 = 8' span - 8" walls = 7' 4" actual span ??" Span of rafter is measured from outside wall to center line. 16'/2 = 8' 8'. The only time to measure from something is (UBC 2320.12.7)Rafter span can be measured from purlin support Under IRC, 2x4 spans 7'10", not 8'. Under UBC, can not use any 2x4's for rafters. Hence, I gave two codes, as I don't know which one his local jurisdiction is under.
A forum community dedicated to Do it yourself-ers and home improvement enthusiasts. Come join the discussion about tools, projects, builds, styles, scales, reviews, accessories, classifieds, and more! Helping You to Do It Yourself!