how long should rafters be
To find the length of your rafters, multiply the run in inches by the rafter length factor and divide by 12. For example: The run is 96 x 12.65 (length factor for a 4/12 roof) = 1214.4 divided by 12 =101.2 inches.Not quite done yet!dd that to the 102.2 inches (101.2 plus 14.75 = 115.95 or 116 inches). Your rafters will need to be at least 116 inches long. Porch Home / Build a Porch / Roofing Materials Comparison. If your rise and run (or slope) is 4/12, multiply the square footage above (504) by 1.06.