The important parts of any type of rafter layout is of course determining the "Theoretical Length" of the member in question. This can be done many ways:
-Step off method with the framing square,
-Use Pythagoreas method (run squared + rise squared=Length squared,
-Trig method,
-Ratio and Proportion method besed on the slope triangle given (eg. 6:12)
-Use the "system" on a good framing square. When you buy a square you should get a booklet with it explaining the basic triginomic tables imprinted on the face of the square. You might try to locate a booklet from a good tool supplier that explains the use of the tables.
The second step in rafter layout is actually locating the "Theoretical Lenths" of the rafter on the stock. Then "shortening" from these marks or "working points" the amount of stock that must be cut off to allow for half the stock that the rafter is joined to. This is because layout of rafters is always considered from the center of the rafter on the top or bottom edge.
This is a very condensed explanation, a good book with sketches will help. When all else fails snap out the plan view 1:1 on the floor, or a good scaled drawing.
Jeff Wharton, Apprenticeship Carpentry Instructor, Alberta, Canada
