Building a pitch roof is accomplished by installing rafters that form a truss system. The roof pitch angle (known as the rise) of the roof is determined either by a building code or a specific concern such as roof loads that endure heavy snowfall or heavy rains that require the water to ship off the roof more quickly than in dry climates.
In addition to the rise, the length, (known as the run) must be considered in order to determine the size and bearing load of the rafter material. The run is the distance halfway between the walls of a room, since this is the location of the roof ridgeline above.
Once the run and rise are established, the correct sized lumber is ordered and the roof is ready to be assembled.
The overhang is the portion of the roof that extends beyond the building line and affords shade or cover to a portion of the building. The point where the rafter contacts the top of the building walls is known as the plate line and the distance from the plate line to the ridge board is the length of the rafter run, plus the overhang.
- Begin by stacking several rafters on edge, atop a set of sawhorses, align the ends and secure these together by nailing small blocks to the horses. Measure the length of the overhang, for example 24” from one end and draw a line at this point perpendicular across the tops of the rafters. This will be the plate line.
- The rafters must have a seat cut at the plate line in order to provide for nailing attachment. Setting the depth of a circular saw blade to 1” exposed does this.
- Using a framing square, draw a line on a piece of wood that corresponds with the pitch of the roof. For example 4 & 12. Using an adjustable square, transfer this angle to the angle of the saw blade. For example, 4 & 12 is approximately a 17-1/2 degree angle.
- With the depth and angle set, cut across the line drawn on the rafters. Next, lay each rafter on its’ side and set the saw blade to zero degrees and full exposure.
- Lay the framing square in place, aligned with the cut and mark the opposite angle by drawing a line from the deep point of the cut to the bottom edge of the rafter. Mark each rafter in this way and make the saw cuts along the line, stopping the curcular saw blade when it intersects the first cut. Finish these cuts with a hand saw and you have the seat cuts. Repeat this for all the rafters.
- Next, the rafters must be cut to length with an angle cut that matches up to the ridge board. To do this, lay one rafter on its’ side and draw a tape measure from the seat cut to the opposite end of the rafter.
- Next, using a rafter book find the table for the corresponding pitch of your roof and that will tell you the correct run of the rafter. Mark that length and make the plumb cut (angle of the pitch).
- Using this rafter as a pattern cut the remaining rafters. Layout the ridge board and mark it according to the rafter layout which is usually 24” apart.
- Next, the skeleton must be installed. This is usually four rafters, two rafters attached at opposite sides of the room that meet at the ridge board. Nail the rafters at the seat cut then nail them to the ridge board. Secure the ridge board with an angle brace at each end and install the remaining rafters.
- Finally, cut plate blocks that will fit between the rafters and nail them in place at the seat cuts, along the top of the wall and flush with the outside face of the wall. Be certain to nail into the blocking from each side of the rafter, and also toe nail into the top wall plate.
The roof is now ready for sheathing.