Why gutters overflow in heavy rain
Five causes of overflow, how to tell them apart, and the fix for each.
Sydney gets short, violent downpours. The National Construction Code uses a design rainfall of about 200 mm per hour to size eaves gutters here, far more than most cities. A gutter with any weakness shows it in a storm.
1. Blocked gutter or downpipe
The most common cause. If the overflow is worst above a downpipe, the downpipe is probably blocked. If it spills along the whole length, the gutter itself is full. Fix: clean out and flush the downpipes.
2. Not enough fall
Eaves gutters need a fall of at least 1:500 toward the outlet. Brackets loosen and fascia boards warp, and the gutter ends up flat or falling the wrong way. You will see water sitting in it days after rain. Fix: reset the brackets.
3. Too few downpipes
No more than 12 metres of gutter should drain to one downpipe, and less where a valley concentrates water. Extensions and re-roofs often add roof area without adding downpipes. Fix: add a downpipe.
4. Gutter too small
A 115 mm quad gutter under a large or steep roof simply cannot carry a Sydney storm. Fix: replace that run with a larger profile.
5. Water overshooting the gutter
On steep metal roofs and where gutter guard has been fitted badly, water can sheet straight over the front lip. Tiles that overhang too far do the same. Fix: adjust the gutter position or refit the guard.
Where the overflow goes matters
Overflow should spill outward, away from the house. If water runs back toward the eaves or into the roof space, the gutter has been fitted without proper overflow provision, and that should be corrected before it damages ceilings. If water is getting inside, treat it as urgent.
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