Each has their own reasoning for a slightly different amount of overhang.
How much should a roof overhang a gutter.
Shingle manufacturers say to use a 1 4 to 3 8 inch shingle overhang over the eave where your roof ends.
Some slight overhang is recommended in conjunction with a drip edge flashing to prevent water from getting under the roofing and onto underlying wood.
In my opinion the shingles should never be exactly even with the edge of any drip edge flashing.
Unless the roof edge is quite uneven some shingles overhang significantly more than others it should have the same stickout all along.
About 1 inch just enough to assure the water goes into the gutter and not behind it.
The bottom tile or slate of the roof covering should project at least half way into the gutter the ideal distance is between half way and two thirds of the way over.
Anywhere from a 1 2 to a 1 overhang is acceptable as you can tell from the varying comments amongst the previous posters.
You can add 3 to 5 inches to the top.
The reason for this is that whenever rain runs down any type of roof when it gets to the lower edge it clings to the tile or slate by capilliary action this means that the water actually runs around the bottom edge and slightlly back up the rear of the tile before gravity allows it to drop into the gutter.
The brackets typically hold the gutter about 1 2 1 inch away from the fascia board if u type brackets otherwise if screw through type the gutter might be almost up to the fascia board.
A typical metal roof overhang may be 2 to 4 inches or even less.
We ve seen other references that suggest extending the shingles a bit more than the manufacturers suggest to compensate for wavy eaves but they do not indicate extending more than 1 inch over the eave.
This will keep rain away from your shed.
Metal is usually installed on rafters or strip sheathing rather than the solid decking used under other materials.
Here phil explains how much overhang a roof should have at the gutter.