Shed Take Off: Calculating Fascia and Soffit 8x12 This shed will have 2x6 fascia and rafters. Rafters are 58 1/2" long and fascia are 13' long. You will need under sill under the drippage on all 4 sides. Under sill comes in 12. lengths. This shed will have a 6"overhang on all 4 sides and a piece of soffit will cover 12 sq feet. The fascia will be bent with a l" return (5 1/2 x l'= 6 1/2). A roll of coil stock=24" x 50' total sq feet (100sq). 1. How many sq feet of soffit is there?