HeyBub wrote:
"Pitch" is the vertical distance over the horizontal distance. For
example, "5/12" means for every twelve foot horizontal distance, the
roof's elevation drops five feet. A "12/12" (or 5/5) means the roof
has a 45-degree slope.
That said, shingles are possible in the range of 4/12 to 12/12
(sometimes, depending on the manufacturer).

Shingles are used on way over 12/12 slopes, at least on my house.