A.Will Holladay responds: Using a sample roof pitch of 6/12, the first step is to calculate the rafter run for the 2-foot difference in rafter heights. This is done by dividing the difference in inches (24) by the rise ratio of 6/12 (.5), to arrive at a 48-inch run (24 ÷ .5 = 48).
Next, add 48 inches to the 24-foot span to create a phantom 8-foot-high wall outside the 10-foot wall, as shown in illustration B. Now, figure the rafter lengths for the 8-foot wall using the 28-foot span.
to read the full article.