Step 1
Measure the distance between the outer surface of parallel sides of the octagonal gazebo's frame. This measurement is the gazebo roof's span. Divide the span by two and subtract half the width of the rafter bracket's center support. The result is the gazebo roof's run. Decide the distance from the top of the gazebo's wall frame to the top of the bracket. This dimension is the roof's rise. Rise is proportionate to slope; a higher rise equals a steeper roof.
Step 2
Add the square of the rise to the square of the run. The result is the square of the rafter's length. Find the square root of this number to determine the length of a rafter that runs from the bracket's outer edge to the outer edge of gazebo wall. Add the desired length of overhang to this dimension to determine the overall length of a single rafter.
Step 3
Divide the rise by 12 to determine the rise per 12 inch unit of run. For example, a roof with a rise of 36 inches and run of 144 inches has a unit rise of 3 inches per every 12 inches of run. Builders often refer to a roof's slope by its unit rise per run, such as "3 in 12."
Step 4
Attach a square stop to the square's tongue at the dimension that corresponds to the unit rise. The tongue is the thin, short side of the square. Attach a square stop to the square's blade at the 12 inches mark. The blade is the wide, long side of the square. Lay the tongue and blade over a piece of rafter lumber with the tongue near one end of the board.
Step 5
Butt the square stops against the board's edge. Draw a pencil along the outside edge of the tongue to draw an angled line across the board's face. This line represents the top plumb cut, which sits flush against the outside of the gazebo rafter bracket's center. The side of this line closest to the end of the board is the top of the plumb cut.
Step 6
Stretch the tape measure along the board from the top of the plumb cut to the dimension that represents the length of the rafter plus overhang. Mark the dimension on the board with a pencil. Flip the square and position the tongue facing away from the top plumb cut. Butt the square stops against the board's edge; you butted the stops against the opposite edge to lay out the top plumb cut.
Step 7
Align the tongue's stop with the overhang mark. Draw a pencil along the outside edge of the tongue to create a cut line for the bottom plumb cut. Cut along both cut lines with a circular saw. Use the cut rafter as a template for remaining rafters.