How to Frame a Pitched Roof

Decide on the pitch of the roof you're going to build. Pitch is expressed by the number of inches of vertical descent that occur over 12 horizontal inches. A 12/12 roof descends 12 vertical inches across 12 horizontal inches, meaning it's at a 45 degree angle. A less steep roof might have a 6/12 or a 7/12 pitch.

Lay out the pitch of the roof on the face of a 2-by-8-inch board using a framing square. For a 45 degree angle, lay the square diagonally across the plank so the numbers that line up with the plank's edge are identical. This places the tongue of the framing square at a 45 degree angle across the plank's face.

Cut the rafter with a circular saw and use it as a template for the other ones you need.

Cut a bird's mouth in the end of each rafter with the circular saw. A bird's mouth is the notch that fits around the top edge of the wall on which the roof will sit. It needs to be at the same angle as the cuts on the ends of the rafter so its top and side faces sit flush on the top and outside face of the wall.

Nail a pair of rafters together at their top ends using 3-inch nails. Tilt the pair up and position the bird's mouths at the end of each rafter over the tops of the walls on each side of the house. Brace the pair of rafters by nailing a 2 by 4 onto them, angling it down to the ceiling joists and nailing it there. Ensure the rafters are vertical by checking them with a 4-foot level.

Repeat this process, positioning the rafters so they are 16 inches apart on center. When all the rafters are built, strengthen them by installing braces between them.