How to Calculate Hip Roof Rafter Fascia Angles When Using Square Cut Common Rafters
Framing a roof requires precision, especially when it comes to matching up your fascia boards on a hip roof. If you are working with square cut roof rafters, determining the exact angle for your fascia board can seem complicated. However, with a simple piece of plywood, a straight edge, and a standard framing square, you can calculate this angle quickly and accurately. In this guide, we will walk through a foolproof method using a 6 in 12 roof pitch as our example.
SETTING UP YOUR REFERENCE LINES To begin, you need a flat surface to act as your drawing board, like a scrap piece of plywood. Snap a perfectly straight reference line across the board. Next, grab your framing square. Position it so the top edge of the framing square sits completely flush and even with your reference line.
MARKING YOUR ROOF RATIOS For this specific example, we are using a 6 in 12 roof ratio. Here is how to mark your board:
On your framing square, mark the number 6. If your roof has a different pitch, such as a 4, 5, 7, or 8, simply move your mark to the corresponding number.
Next, mark out the common rafter ratio, which is 12 for our 6 in 12 pitch.
Finally, mark out the hip or valley ratio, which is universally 17 for this type of layout.
DRAWING THE ANGLE LINES Once your points are laid out, draw two diagonal reference lines starting from your initial pitch mark. Draw one line passing through the 12-inch mark to represent your common rafter, and another line passing through the 17-inch mark to represent your hip rafter.
After drawing these two lines, create a 90-degree angle from the 17-inch mark that connects back to the common rafter line. You can achieve this by aligning the elbow of your framing square with the hip rafter mark, drawing your line, and then using a straight edge to extend it until it intersects the common rafter line. Draw a final connecting line between these intersection points. This new geometry represents the exact fascia board lines for both your common and hip rafters.
TRANSFERRING THE ANGLE TO YOUR RAFTER To transfer this calculated angle to your actual lumber, follow these steps:
Place your framing square so that one leg runs parallel to the final fascia reference line you just drew.
Slide the square until it aligns with a clean number, like the 10 or 12 mark, making sure you note exactly where it sits on the square.
Take the framing square to your hip rafter.
Align the square on the board using the exact same measurements you just found on your plywood template.
Mark the angle across the wood.
CONCLUSION By creating a quick template on a piece of plywood, you eliminate the guesswork from cutting hip roof fascia boards. It is highly recommended to write down your framing square measurements for quick reference so you can replicate the cut easily. Add this mathematical technique to your carpentry toolbox, and your next roof framing project will go smoother than ever.
