When you contemplate building get the woodwork on the ground at the earliest possible moment, and get a priming coat of paint, mostly linseed oil, applied at once. And as soon as the plates, ridge and bars are cut to their lengths, two more coats should be given. You can paint on the ground just ten times as fast as you can on the roof.
As we butt our glass, there is no need of any painting after the roof is up and the glass in. The ends of the bars are always given an extra daub of thick paint just before they are nailed up, and this should be done by a boy who hands them up to the carpenter.
I will mention here that no part of the woodwork where moisture can lie should be flat. All parts should have a bevel either in or out.