Things You'll Need
Proper amount of 2-by-4s
A gable roof consists of two upward sloping sides that slope at the same angle and from the end look like a symmetrical triangle. Gable roofs without ceiling joists provide a much higher and triangular-shaped ceiling inside the home. The shape and size of a roof depends on the span and the floor plan of the building; and the roof covering material determines the size--cross section area--of the roofing timbers used.
Mark a base line to represent the wall plate level on a level area of ground. On the base line mark the two ends of the span (points A and B) and the midpoint (point C). The span dimension is critical and should be double checked with the "as built" dimension for conformity.
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Set out a perpendicular line and mark the roof pitch (point D) from the marked midpoint (C) on the base line.
Take one timber length for the rafter and place it from the point D to point A and extend beyond the base line. Mark on timber a line corresponding to the perpendicular line from C to D. Cut the timber along the line. At point D, the point should be on the outer face of the rafter, while at point (A) it should be on the inner face.
Repeat Step 3 for the rafter on the opposite side but from point B to D.
Mark and cut out a slot where the ridge board will fit in allowing a 2-inch protrusion above the rafters.
Use the prepared rafter as a template to cut all the other rafters.
Move your operations up to the wall plate level. Mark the midpoint on both sides of the walls parallel to the rafter arrangement. Temporarily fix a vertical timber piece secured to the wall by bracing. Mark the roof pitch.
Set the ridge board on the temporary vertical piece in Step 7. Check that it is level with a level. Mark the position of the rafter on both the wall plates
Start nailing the prepared rafters on the marked positions. The ridge board will fit into the previously cut notch. At the point where the rafter rests on the wall plate, cut a bird mouth notch where the wall plate will fit in and secure the rafter. Check plumbness before permanently nailing the rafters, starting from the wall plate and onto the ridge board.
Nail all the rafters into place.