The Difference Between Right and Wrong
In Architecture, like most things, right and wrong matters but it's not always clear-cut. This is particularly true when it comes to gable-end gutter details. There is a right way, but almost everyone does it wrong. You may ask, "if everyone does it wrong, doesn't that make it right?". No. Let me explain why, but first let's look at two details.
The "right" image (conveniently situated on the left), shows the gutter mitering into the rake molding, and the lower half of the fascia returning over the corner board. This creates an elegantly proportioned classical composition, where the column supports the pediment. By mitering the rake into the gutter, a continuous line is created that defines the roof.
The "wrong" image shows the gutter returning horizontally, and the rake terminating on top of the mitered gutter. This visually disconnects the rake from the gutter. It also puts a gutter in a place where no gutter is needed on the return. This detail was never done on older homes in their original detailing.
How Did We Get Here?
In the 18th, 19th, and first half of the 20th centuries, gutters were typically made of wood, as was the rake molding they mitered into. The "Boston Pattern" gutter had a rounded profile that matched the rake, which enabled the same rake molding to miter into the gutter at a variety of pitches (conventional wisdom indicating that you must develop a difference rake profile for each roof pitch is incorrect). The problem began in the 1950's with the introduction of roll-formed aluminum gutters. In order to make this thin material stiff enough, the profile was changed to the "K" shape common today. Also, around this time, the wood gutter profile was simplified to make it less expensive to produce, resulting in the flat bottom wood gutter common at lumber yards today. Neither of these profiles could miter into a rake molding, so the detail was largely abandoned (some knowledgeable old-timers would still buy the correct profile at custom millwork shops).
A New Detail Emerges
Since builders couldn't miter aluminum or flat bottom wood gutters into the rake, they had to figure out a way to terminate both the rake and the gutter. By returning the gutter onto the gable end, they maintained the appearance of a mitered corner, and created a projection that the rake could die into.
Why Is It Bad?
As mentioned above, the elegance of the original detail is that the horizontal gutter seamlessly turns the corner and goes up the rake, providing a consistent edge around the roof plane. A rake molding simulates the top of a column, and should be the furthest out projection. When the gutter returns horizontally, it projects out past the plane of the rake molding, which wrecks the proportions. Also, as a practical matter, it adds more labor as it creates an additional miter corner on the return which is unnecessary.