If you're looking for information on "how to design a roof", you probably know that a "rafter tail" is the end of the rafter that overhangs the wall. What you may not know is that the design you choose can determine whether you have a leaky roof edge or not. If you're going to attach a fascia board, you basically have two choices...
How to Design a Roof -- Plumb Cut Rafter Tails
Metal Cutting
Plumb cut rafter tails have a vertical fascia board. That's handy for installing gutters because the back side of most gutters is also vertical. But without gutters, much of the water coming off the roof runs down the fascia, where it can rot the wood over time, especially at the joints.
But a far bigger problem is the angle of the fascia to the sloped roof. It varies with the slope of course, but standard drip edge comes with just one angle, which is 90 degrees. Of course the roofer could have custom drip edge fabricated, but more often than not, he "makes do" with the standard stuff, trying to bend it by hand on site to accommodate the angle.
The typical result is a drip edge that not only sticks out at an unsightly angle, but creates a little "dip" at the very edge of the roof. Water lays in the dip, coming in at the seams in the roofing. With the leaks coming through vinyl or aluminum soffits outside the house, they can go undetected for years, resulting in a significant amount of rotted wood.
How to Design a Roof -- Square Cut Rafter Tails
Square cut rafter tails are perpendicular to the rafter. That slopes them down and away from the drip edge. I'm not a big gutter fan, but if you want them, brackets are available to provide a plumb mounting surface. And when gutters aren't used, water can run off the drip edge without ever touching the fascia board.
But the biggest advantage with square cut rafter tails is they accommodate standard 90 degree drip edge perfectly. The face of the metal is flush up against the fascia board and the flange lies flat on the roof, just like it should. That makes it easier for the tradesman to do a good job, which is an important principle when learning how to design a roof or anything else.