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.
I see you are on your toes Kenton.
I was a teacher of the course and was just pulling your leg about where it is located.
However you are right about the origin.:o