Roofs with slates of intermingled thickness use at least three distinct variations in slate thickness; for example, one home might use ¼-inch, 3/8-inch and ½-inch slates.
A heavyslate roof uses slates at least 3/8-inch thick.
Thicker slates may be specified for a roof installation, but the thickest slates included in the order can’t exceed the minimum thickness by more than 1/8-inch. This means that if the order specifies ½-inch minimum thickness, the slates may vary from ½-inch to 5/8-inch.
In heavy slate roofs, slates up to 1 inch thick are not unusual, and these slates are heavy and rough. Slates this size may require framing that is heavier than what is typical for a roof, and you should be looking for signs of failure, such as rafter sagging, or cracking of interior or exterior wall coverings. Heavy slates seldom have cut butts.
This is from the Mastering Roof Insp. series