This section is from the book "The Carpenters' Guide", by Harvey Miller. Also available from Amazon: The carpenters' guide.
Use the 2' square to determine the number of inches rise and pitch.
1/2 Pitch is 1/2 of 24" or 12" rise to 12" base.
1/3 Pitch is 1/3 of 24" or 8" rise to 12" base.
1/4 Pitch is 1/4 of 24" or 6" rise to 12" base.
1/6 Pitch is 1/6 of 24" or 4" rise to 12" base.
1/8 Pitch is 1/8 of 24" or 3" rise to 12" base.
(See fig. 24.) Or take 1/2, 1/3 or 1/4 of the width of the building for the pitch of the roof.
Fig. 24
 
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