I need to size a 15 foot span header for a shed dormer. The shed dormer ends before the ridge. The snow load here is 65lbs and the pitch for the shed is 4/12. All of the common shed rafters will be resting on the header. Also I was wondering whether the load would be the same for the shed dormer plate.

In order to figure the amount of the load is on the header and/or the plate we'll need to know the other dimension. You've said the span is 15' what is the distance from header to plate?
To figure load at 65lbs snow load you'll need to know how many square feet is being supported by the header or the plate.
Also helpful is the spacing of the common shed rafters, how far apart are they?
Jim Rogers

Thank you for the help. The span is actually closer to 16 feet. The header to plate is 11 feet. The square footage of the shed roof is 176 feet. Spacing for rafters could be 2 1/2 feet on center to 3 1/2 on center.
-Collin

You also need to know the amount of roof above the header that the header has to carry. The real number you need is the area (in plan) of roof that this header needs to support. That's probably half the horizontal distance from the header to the plate, times the horizontal width of the dormer. Plus whatever amount you have between the header and the ridge that the header must carry. Are there purlins between the header and the ridge? CB.

I was coming up with header sizes that were quite large (9x15)and I would like my shed dormer to be as light as possible, so I spoke to the owner and we are going to carry the shed dormer up to the ridge.
Thank you very much for all of the replies. This forum is a great resource.
-Collin