How to Calculate Rafter Cuts

Sometimes calculating a rafter cut requires a sharp pencil, a framing square, and a head for math. But there is also a few empirical methods that leave less room for error. Although calculating rafter cuts is greatly understood by knowing the geometry of roof framing, following a few literal methods can save you time and frustration.

Things You'll Need

• Chalk
• String
• Sliding Bevel
• Level
• Saw
• Rafters

Instructions

1. Put a chalk line on your plywood subfloor representing a rafter pair in relation to its plates and ridge. Layout your rafters as a full size diagram. This two dimensional setup will become a rafter pattern you can follow.

2. Cut the common rafters using your pattern layout. After the ridge beam is up, nail the rafters in place. If the full size layout is precise, the rafter cuts should be also.

3. Get up on the roof and measure the relationships between the rafter to be cut, and the existing plate and ridge; using a string and a sliding bevel. This second method involves dealing directly with the rafter components. What you see is what you get.

4. Nail a piece of scrap wood to the ridge beam. Nail another piece to the top plate where your rafter would sit. Attach a piece of nylon string to each of the wood pieces. Stretch the string so it is tight. Using the string to see the imaginary center line, visualize the rafter in place. Using the sliding bevel and a level, find the angle cuts.

5. Measure the actual distance when calculating rafter cuts instead of calculating imaginary ones. This method works best with longer and heavier rafters. Finding the length of the rafter and the angles of the cuts using actual distances and not guesswork, makes your job much easier.

Tips & Warnings

• When calculating the angle cuts using a piece of nylon string, be sure not to distort the string, or the rafter cuts will be off.

• Laying out rafters and calculating rafter cuts is done all the time using a framing square. But when calculating unusual angles or compound angles, it is best to deal directly with the problem.