Building new the old way at Mount Vernon
Note the overlap of shingles on the ridge. Today we would cover the ridge with cut flaps from asphalt shingles or with metal flashing. I wonder how modern researchers figured how they did this in 1799, because no roofs would have survived from that time, certainly not the vulnerable overlap.
Effloresence is usually an indication of water penetrating through a wall: salts precipitate on the surface as the water carrying the salts evaporates. This is a new wall recently restored around Washington's central garden. I watched the mason work on this. He sure did take his time. Not paid by the job! In this case salts must be drawn out of the fresh mortar simply by rain. Both sides of the wall show the salt and it is all above grade.