What it does
Positioned above the chimney where the roof slope would otherwise direct water straight into the back of the stack, the back gutter channels that water safely around and down the roof instead.
Why it's high-risk
Because it sits at a low point collecting water from a wide area of roof above the chimney, a failure here can let in a lot of water quickly, often before it's visually obvious.
Sizing it correctly
A back gutter needs to be sized for the actual roof area draining into it — an undersized gutter on a wide chimney is a common cause of chimney leaks we're called out to fix.