Plan first, cut rafters, then assemble the base
After drawing the plan and figuring out the various framing lengths and angles, I pick the best stock for one common rafter to use as a pattern for the rest. Then I start building the base. I like to build the entire turret on the deck near where it will be located, but away from the edge so that my crew can frame, sheathe, and roof the entire structure without staging or extension ladders.
The first step is to snap the turret-wall outline on the third-floor deck. I plan for double top plates so that I can overlap them at the corners, which helps to stabilize the structure as I build on it.
Once the walls are built, I block them up off the floor to make room for the rafter tails that, when in place, will extend past the floor deck to meet the fascia of the regular roof. Then I recheck and adjust the octagon until its measurements are exact.Build the base, and raise the eight common rafters
Find the common-rafter length as you would for a conventional roof. First, determine the adjusted run by dividing the footprint of the roof in half; then subtract half the thickness of the ridge.
Overlapping double plates give the base greater strength. When complete, the base is blocked up so that the rafters with tails can be installed. Ridge detail. Three pieces of 2x stock cut a couple of inches longer than the hip plumb cut are ripped and beveled to form the octagonal ridge block that measures 3-3⁄4-in. sq.