The fact that he wrote 6/12, instead of 1/2 is a hint: 6/12 in roofing
terms is not the same as 1/2 slope = 0.5 slope = 45 degree slant = 100%
grade. It is short for 6 in 12, like, 6 inches drop per foot. Steep,
but not crazy. 4/12 is your classic american suburban house. 12/12 is a
45 degree slant. Your "1/5 pitch" would I guess mean 2.4 in 12, which
is nothing much, since 2 in 12 is considered a flat roof in these
parts, requiring membranes or tar and such, rather than shingles.

- Kevin