For larger roofs though i use a roofing ready reckoner.
Roof cuts and rafter lengths book.
Subtract what you are using as a ridge and you have your length perfect every time.
You put in the width and the pitch and this book will give you the full length of the rafter.
Tail or eave length.
The most useful book ever if you want to cut a roof.
There are a few good ready reckoners.
With calculations diagrams and tables.
Roof sheathing cuts and angles.
Exact lengths of all rafters including all regular and irregular hips valleys commons and jacks.
Rafter book page 1 created date.
The respective lengths for those two rafters and add them together.
It will help you with plumb and side cuts for hip and rafter down and side purlin cuts and more.
Invaluable to anyone who wants to cut a roof by hand and not use trusses.
Roof cuts and rafter lengths hancock s 10th edition is a very popular best selling book.
Calculations for roof frames roof cuts and rafter lengths.
The tail or eave overhang must be added to the rafter length.
Each page has a different table depending on the pitch angle of your roof with all the figures you need.
Details cuts and bevels for all rafters.
Gives rafter runs to locate ridges and framing points.
This is the rafter length from the top cut to the seat plumb mark.