How to Calculate Roof Framing Cuts

Using Common Rafter Tables

• The framing square is an essential tool for any roofing carpenter. The tables listed on the side of the square give the length of a common rafter per foot of run. For every 12 horizontal inches of horizontal run, the table indicates the hypotenuse length for a given unit of rise. For example, under the number three on the rafter table you will see the number 12.37. If the total run for your roof is 13.5, you multiply 13.5 times 12.37 to find the total inches along the slope. This measurement is important for determining where to place your framing cuts.

Plumb Cuts for Common Rafters

• Plumb cuts are made at the point of intersection between a rafter and the ridge line. To calculate the plumb cut, you must know the roof pitch. Roof pitches are usually expressed as a ratio, such as 3:12, 4:12 and 8:12. For example, 3:12 would indicate 3 inches of rise for each 12 inches of run. If you place both the 3 mark on the blade (the short side of the framing square) and the 12 mark on the tongue (the long side of the framing square) against the top edge of a given rafter, the outer side of the blade will correspond to the plumb cut.

Bird's Mouth Cuts for Common Rafters

• Bird's mouth cuts start with a plumb mark. Measure the length of the roof minus half the ridge board thickness and make a mark indicating this distance from the top of the ridge plumb cut. Mark a plumb line at this location in the same manner as the ridge plumb mark. To finish calculating the cut, you must mark a perpendicular line from the plumb mark that corresponds to the thickness of the wall. For a 2- by 6-inch exterior wall, a 5.5 inch line must be drawn perpendicular to the plumb line marking the exterior wall surface.

Hip, Valley and Other Rafters

• Calculating hip, valley, jack and other types of rafters is more complicated than calculating common rafters. These rafters involve compound miter cuts, and the rafter lengths vary depending on how close the rafters are to the ridge line or exterior walls. A construction calculator is a useful tool for calculating these cuts. If your roof includes these cuts, it's wise to build a model before you attempt to build the actual roof.