Hip Roof Framing Calculation Methods

Calculation Basics

• The essential elements for calculating hip roof rafters are the pitch or slope of the roof, the distance in rise from each wall, and the rafter run or space each rafter must cover from a wall to a peak. Start with a basic house plan, showing the length and width of the outside walls, a line to mark the center or peak, lines across the width to show where the hip angles start and 45-degree diagonal lines from each end corner to the center of that point.

Rafter Types

• Hip roofs require common rafters for the center portion, a single common rafter on each end from the peak to the end wall, diagonal or hip rafters from each corner to the roof peak and "jack" rafters to connect those hip rafters to the outer walls. Each type of rafter requires a separate calculation, based on roof pitch and rafter run.

Framing Square

• A standard framing square has all the information needed to calculate common hip and jack rafter angles and lengths. A table on the wide blade of the square shows the length of common and hip rafters per foot of run, the differential between the horizontal length and the angled length from a peak to a wall. It also has those differentials for jack rafters and information on how to figure the end angles for those rafters.

Figuring Angles

• A framing square has two sides -- a 1 1/2-inch-wide tongue and a 2-inch-wide blade. These meet at a 90-degree angle. The inch marks on the two sides of the square are used to form the roof pitch, the inch mark on the tongue for the pitch of the roof and the 12-inch mark on the blade to show the slope per foot. A 5/12 roof, for instance, slopes 5 inches per foot of run, so the 5-inch mark and the 12-inch mark are aligned to create the angles.

Alternative Methods

• There are other ways to calculate hip roof rafters. A Swanson speed square can be used much like a framing square; these typically come with an instruction manual giving specific directions. Several websites offer online calculator programs that figure rafter angles and lengths based on basic data such as roof size, rafter lumber size and roof pitch. There also is an "adjustable hip square," a patented device developed by a carpenter to expedite the hip rafter calculations.