1. Prep the Boards
I prep each rafter by squaring one end of the board. Rough-sawn and pressure-treated material usually isn't completely square, or it can be rough-cut on the ends. I do this either with a miter saw or a circular saw with a Speed Square as a saw guide. I then measure and cut each rafter to its finished length.
Finally, I cut a 3-foot scrap of a 2 x 8. This serves as the blank from which I'll cut a pattern.
2. The Pattern
I use a compass to mark the round portion. I spread my compass to 4 inches, then pin the point on the corner of the 2 x 8 scrap and scribe the circle. A 4-inch spread leaves 3-1/2 inches on top of the circle, and I think it makes an elegant proportion.
Using a jigsaw with a sharp blade, I cut out the semicircle along the line.
Next, I make the 1/2-inch clip cut, which I do on the miter saw. Once the pattern is cut to my liking, I trace its location on the saw deck with my pencil.
3. Making the Cuts
For the circle cut, I place the rafter on sawhorses and use the 3-foot pattern board to trace its edge onto each rafter's ends.
Using the jigsaw as before, I cut the circle.
For the clip cut, I line up each rafter with the round line previously marked on the saw. I pass the blade through at a 45-degree angle, clipping about 1/2 inch off the two corners. Note: Since the pieces are long, the key to making this work is out-feed support at the end of each board, to hold the work parallel to the saw deck height. You can use a roller stand, or, in my case, a JawHorse did nicely.
If you're short a miter saw, a simple 45-degree clip off the bottom of each end is elegant and can be done accurately with a circular saw and Speed Square. The design is open to interpretation--alter this one however you like, or employ its principles to create your own.