I designed the shed myself, with polycarbonate roof to let in the light for painting. My son suggested having polycarbonate only on the back side of a pitched roof, so it couldn't be seen from the garden and a dormer window. The roof has cedar shingles and the dormer has a leaded light from a reclamation yard.
We laid 6x2 joists on an existing concrete base and on top is 18 mm flooring grade chipboard with 18 mm roof deck on top. Around the 4x2 timber frame are 12 mm brazil ply sheets, with 6" featherboard on the outside and a layer of 2" polystyrene, then T&G on the inside. Windows & doors are double glazed and the roof light will (eventually) have blinds made by a sailmaker. Apart from raising the timber frame, I did all of the work by myself and it took 8 months.