Measuring Angle Cuts For Roof Construction

An angle cut is a very common cut needed for constructing rafters for any roof. Knowing how to make this calculation will make your roof construction project easier and will ensure the safety and success of your roofing project once completed.

Rafter Angle Cuts

Angle cuts on roof rafters set up the entire framing for roof construction, so it is essential that angle cuts be calculated correctly and accurately for all of the rafters. Rafters with angle cuts that are incorrectly calculated are subject to the roof sagging or failing soon after the entire roof weight has been placed on them.

The primary weight of the roof falls upon the rafters, which support not only the center roof beam but also the panels that enclose the roof and the materials that complete the roof enclosure, such as shingles and tiles.

There are two methods of calculating the angle cuts for rafters.

Method 1 – Using Framing Square:

1. Decide the Slope/Pitch of the Roof - You should be able to determine the slope/pitch of the roof you will be constructing by looking at the building's scale drawings. Usually, the slope/pitch is detailed in the exterior elevation plan.
2. Set Unit Rise - Hold a framing square so that the blade is facing you, pointing to the right side, and the tongue is pointing away. Screw the square gauge on the tongue at the marker for the slope/pitch of the roof or the unit rise.
3. Set Unit Run - Place the other square gauge at the run unit measurement mark. Screw the gauge in place.
4. Set Cut Angle for Rafter - Place the framing square so the blade is horizontal on the rafter and the tongue is on the rafter and draw lines. The tongue is the cut angle for the rafter.

Method 2 – Using Rafter Angle Scale:

1. Draw Vertical Rafter Line - On an already constructed rafter, draw a straight vertical line using a level.
2. Calculate Angle - Using a rafter angle square, place the pivot point at the precise location where the vertical line you've just drawn crosses the bottom edge of the rafter. Align the ruled edge of the rafter angle square with the vertical line. The first edge is set at a right angle to the flanged base of the framing square and is marked with a short ruler. The second edge is the long side of a right triangle and is marked with a degree scale.
3. Calculate Angle Cut - Note where the bottom edge of the rafter crosses the degree scale on the rafter angle scale. If there is a slot cut parallel to the degree scale on your rafter angle scale, this is the vertical rise indicator. This is the degree measurement and where you will make your angle cut.