Why does rain shoot straight over my gutter?

This is distinct from overflowing. An overflowing gutter is full and spilling; an overshooting one may be nearly empty while water sails past it. The distinction matters because the fixes have nothing in common, and people spend a lot of time clearing a gutter that was never blocked.

Safety

Step by step

1. Establish that the gutter is not simply full.Watch it in rain. Water arcing over a gutter that is not brimming is overshoot. Water welling up and spilling is an overflow, which is a different guide.
2. Note whether it happens along the run or at one point.At one point, look above it. A valley between two roof slopes concentrates a large catchment into a narrow discharge, and that is the classic overshoot location.
3. Consider the pitch and the slope length.A steep roof throws water further out. A long slope delivers more of it, faster. Both together make overshoot far more likely, particularly in the intense short downpours that are becoming more common.
4. Look at where the gutter sits relative to the tile edge.A gutter set too far back catches water that dribbles and misses water that is thrown. There is a relationship between the roof edge and the gutter position, and a replacement gutter fitted without regard to it commonly overshoots.
5. Check for a missing or displaced valley deflector.Where a valley discharges into a gutter, a deflector or a stop end is often fitted to stop the jet crossing the gutter entirely. They get dislodged.
6. Do not just fit a bigger gutter.Capacity is not the problem if the water never lands in it. A wider gutter sometimes helps by presenting a bigger target, but it treats the symptom.
7. Deal with the ground consequences meanwhile.Water repeatedly landing in one place next to the house saturates the ground against the wall. That is worth managing even before the roof edge is corrected.

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