How to Lay Out Rafters

Check the exterior wall sections with a carpenter's level where the end of the rafters rest, to make sure they are square and level. Measure the width of the structure to find out the span -- the distance from one wall section to the other wall. Determine the run for the common rafter. This number is half the span. For example, if the span measures 26 feet, the run of the common rafter equals 13 feet. Verify the slope of the roof, such as 6-12. This means the roof has a rise of six inches for every foot. Use a construction calculator to avoid having to make the computations.

Use 2-by-6-inch lumber for the rafters. Make sure the board is as straight as possible. Lay out the common rafter -- this component runs from the ridge board to the outside wall. The ridge board refers to a long board that runs down the center of the structure's peak from end to end. Use a framing square to mark the lines for cutting the common rafter. This tool contains the measurements and framing tables, which provides guidance for rafter layouts.

Use a framing square to mark the lines for cutting the common rafter. The framing square has a narrow blade called the "tongue" and a wider blade referred to as the "body." This tool contains the framing table, which provides guidance for rafter lengths based on the type of rafter. For example, determine the length of the common rafter with a a 6-12 slope, look for the number 6 on the table. Below the number 6 is the number "13.42," which represents the length of the common rafters per foot run pitch. The table also contains the same information to compute the length for other types of rafters. Multiply 13.42 times 13 (the run as determined in Step 1"), which equals 174.46 inches (14 feet 5 inches) -- the length of the common rafter.

Make cuts for the rafter starting from the top edge of the board. Position the framing square based on the slope of the roof. For example, for a roof with a 6-12 slope, place the tongue on the board's edge at 6 inches and the body on board at 12 inches and mark the board for the plumb cut -- where the rafter meets the ridge board. Attach the measuring tape to the high point of the plumb cut, measure the length of the rafter -- 14 feet 5 inches, and mark the board.

Place the framing square on the mark, with the blades pointed down; make sure the narrow blade is on 6 and the wide blade on 12 along the bottom edge of the board. Mark the angle along the outside of the narrow blade. Place the framing square with the blades pointing up, adjust the framing square and measure 4 inches from the bottom edge of the board to the last mark. Make a mark for the "birdsmouth" The birdsmouth rests on the 2-by-4 inch top plate of the wall section.

Use a circular saw to make a vertical cut to where the two lines intersect, finish making the cut at an angle to create the "birdsmouth." Add the overhang -- usually 12 inches, and cut the rafter tail.