The gutter is blocked
Debris collects at the outlet before it collects along the rest of the run, since that is where the water is trying to go. A gutter can look fairly clear across its length while the outlet beneath it is packed solid.
This is the cause to suspect when the overflow builds up over a season, when it shows up worse near one outlet, and when branches hang over that part of the roof.
The gutter is undersized for the roof
A gutter carries whatever the roof above it sheds. If the run or its outlets were sized for a smaller roof than what actually drains into them, the gutter overflows in heavy rain no matter how clean it is, and it happens at the same point every time.
This looks identical to a blockage from the ground. The difference shows up after cleaning: a blocked gutter stops overflowing once it is cleared, an undersized one does not.
- Can follow a roof addition or a re-roof that added shed area
The gutter has lost its fall
A gutter is hung with a slight slope toward its outlet. When a hanger works loose and a section drops, that slope flattens or reverses, and water sits in the low spot instead of draining. Once it sits there, it spills over at that point.
Standing water in the gutter after the rain has stopped is the sign to look for. A gutter with the right fall drains itself.
- Sometimes visible as a sag when you look up from the ground
- Fixed by rehanging the run to the correct fall
The downspout cannot keep up
A downspout blocked near the bottom, or simply too small for what is reaching it, backs water up the whole system above it. The gutter overflows, but the gutter is not the problem.
A blockage low in the downspout is easy to miss because everything above it looks fine.
- Overflow starts near the downspout and works back along the run
- Check the discharge end, not just the top of the run