Roof pitch and covering
The steeper the roof, the faster water runs off: each covering has a lowest pitch at which it keeps the rain out.
The rules it illustrates
Each covering has its lowest pitch. Pitch a roof no lower than its covering allows (roughly 45° for thatch, 35° for plain tiles, 20–25° for slate, 15–20° for interlocking tiles, a few degrees for sheet metal) and steeper where snow lies long or rain is heavy.
The steep northern roof. Over the vaults of a northern church raise a timber roof of about 55–60°, its section close to an equilateral triangle, so that snow slides off and the slates or tiles overlap generously.
Where it appears in the book
In the chapters The Roof.