How To Calculate, Layout And Cut Jack Rafters For Hip Roof Framing Without Difficult Math Formulas
Simplify Jack Rafter Layout: One Measurement for Consistent Spacing
Jack rafters, those shorter members filling the space between hip/valley rafters and common rafters, can be a headache to lay out accurately. But here's a streamlined method using a single measurement to ensure consistent spacing and simplify your roof framing.
Understanding the Challenge
Jack rafters decrease in length as they approach the hip or valley. Calculating each individual rafter length can be time-consuming and prone to errors. This method eliminates those individual calculations.
The "Common Difference" Method:
Calculate the Longest Jack Rafter: This is typically the jack rafter closest to the common rafter. Determine its length using the Pythagorean theorem or a rafter table based on your roof slope and run.
Find the "Common Difference": This is the incremental length difference between each jack rafter. Here's how to find it:
Using a Rafter Square: Most framing squares have a scale specifically for "difference in jack rafter lengths." Find the scale corresponding to your roof slope (e.g., 7/12) and your on-center spacing (e.g., 16"). The number indicated is your common difference.
Using the Pythagorean Theorem:
Imagine a right triangle where the base is your on-center spacing (e.g., 16") and the height is the unit rise of your roof slope (e.g., 7" for a 7/12 slope).
Calculate the hypotenuse of this triangle. This is your common difference.
Mark the Longest Jack Rafter: Lay out your longest jack rafter and mark its length.
Subtract to Find Shorter Jacks: Starting from the mark for the longest jack, measure and mark the "common difference" towards the hip/valley. This is the length of your next jack rafter. Continue subtracting the common difference to mark each subsequent jack rafter.
Example
Let's say your longest jack rafter is 40" long, your roof slope is 6/12, and your on-center spacing is 16".
Using your framing square, you find the common difference for a 6/12 slope and 16" o.c. is 17 7/8".
Mark your longest jack rafter at 40".
Subtract the common difference (17 7/8") from 40" to get 22 1/8". This is the length of your next jack rafter.
Continue subtracting 17 7/8" to find the lengths of the remaining jack rafters.
