There are three basic kinds of roof rafter: common, hip and jack. Common rafters run from a centre peak to a wall on each side. Hip rafters run from a peak to the corners of walls. Jacks are short rafters of varying length that connect to hip rafters and help support the hip end of a roof. Hip roofs slope on all four sides of a house to a common eave. Hip rafters are more difficult to lay out than common rafters because they slope at an angle from the centre peak.
Use a basic framing square to lay out hip rafters. The square has a table on its wide (blade) side to calculate the length of rafter needed for common, hip or jack rafters. Get the pitch or slope of the roof and the rafter run, the distance between the peak and corner, from a house plan. Make sure the run is measured from the outside of the corner. Install common rafters, which form the centre and peak of a hip roof, before beginning hip rafter layout.
Figure and cut one common "king" rafter from the centre peak to the centre of the end wall, using the common rafter table on the square. Determine the basic distance between the centre peak and the corner and the pitch, then find the "length of hip rafter per foot of run" line on the square table. Assuming a 6/12 roof, which slopes 6 inches up for every foot of run, and a hip run of 14 feet, for instance, the pitch mark (6) would show 18 or 18 inches of rafter for every foot of run. Multiply 18 times 14 and you get 252 inches or 21 feet.
Determine the overhang needed to match the overhang of the common rafters in the centre of the roof, and add that to the rafter length. Mark the 252-inch mark, where the hip rafter will meet the wall corner, and draw a 3 1/2-inch by 1-inch notch to fit the rafter over the top board at the wall corner. Use a tape measure to mark that spacing correctly.
Calculate the angle for the top cut of the hip rafter by setting the rafter with the 2-inch side up. Place the square with the 12-inch mark on one leg at the end point of the rafter and the length from the rafter table on the other leg on the edge of the rafter. That forms the angle for the cut to fit on the ridge board at the peak. The length will be on the bottom line of the table -- for a 6-pitch rafter it is 11 5/16. Take off 3/4-inch at the top to allow space for a ridge board.
Lay out jack rafters, which go between the hip rafters and the common rafters on either side and the "king common" rafter in the centre of the hip roof, using the table on the square. The two centre lines on the table show the length variations of jacks for 16 and 24-inch spacing of rafters. The next line gives the side cut figures to calculate jack rafter tops.
Things you need
- Framing square
- Tape measure