Any sloped roof, whether it is a very shallow slope with a barely noticeable incline or a church steeple, is considered a pitched roof. Steep pitches are more common in northern climates that receive a high annual snowfall. A steep slope sheds snow and ice more easily and prevents a large build-up from over-loading the roof structure.
The pitch of a roof is expressed as a ratio; the rise (vertical projection) to the run (horizontal projection). A roof that slopes upward vertically 6 inches for every 12 inches measured horizontally is said to have a 6 pitch, a vertical rise of 8 inches per foot is an 8 pitch and so on. A roof with a pitch of less than 5 has a shallow slope, and a roof pitch of more than 8 appears to have a steep slope. In wood-frame construction, the pitch of the roof is determined by the way the framing components are laid out, cut, and assembled in place.
The primary structural members of a roof frame are the ridge board and the rafters. Rafters can vary in size from 2x6 to 2x12; a ridge board is typically 2x12. The top end of each rafter is nailed to the ridge; the bottom end is connected at the top plate of the stud wall. The rafter “tails” usually extend beyond the exterior wall a short distance to form the overhang or the “eave.” Here are some steps to help you build a pitched roof.
Step 1 - Layout the Pattern Rafter
After you decide to build a pitched roof, you can layout and cut a rafter pattern. Lay a framing square flat on the board near one end, so that the horizontal blade aligns with the bottom edge of the board at 12 inches and the vertical blade is aligned at the roof pitch. Trace the lines for the plumb cut and the level cut for the eave from the vertical and horizontal edges of the framing square.
Knowing the width of the overhang, you can slide the square along the bottom edge of the board and lay out the notch or “bird’s mouth” in the same way. The level cut for the bird’s mouth must be at least 3 ½ inches — a distance equal to the width of the top plate on which it rests.
Continue to slide the square along the length of the board at the same angle so that the run, or the total span of the rafter, is stepped off in horizontal, 1-foot increments. Use the vertical blade to layout the plumb cut for the ridge board, and cut out the rafter pattern.
Step 2 - Trace Out and Cut All Remaining Rafters
Using the pattern as a template, trace out and cut all the rafters. You can determine the total number of rafters needed from the length of the ridge board.
Step 3 - Install Layout Ridge
From the total rise of the rafters, you can calculate the elevation of the top of the ridge (overall building height). Install the ridge board on two end posts with temporary bracing and set it straight and level. A layout is a series of pencil lines 16 inches apart showing the location of each rafter. Mark the rafter layout on the ridge board and on the top plates.
Step 4 - Install Roof Rafters
Lean the rafters onto the ridge resting on the plates alongside each joist. Make sure that the long point of the top end of each rafter is flush with the top edge of the ridge. Install the rafters by first nailing through the ridge into the rafter. Then secure the rafters to the plate by nailing through the rafter into the joist and toe-nailing the rafter to the plate.
Step 5 - Add Bracing, Roofing, and Trim
Install collar ties, purlin and bracing as needed for the rafters and ridge to prevent sagging over time. Roofing materials like shingles, roofing paper, sheathing, and trim are then installed onto the completed frame.