Where does the overflow go?
Box gutters, rainheads and the overflows that should protect them.
In short: An overflow exists for the day the downpipe blocks. Piped into the rainhead, it blocks with it.
The job of an overflow
When a box gutter or its outlet blocks in heavy rain, water must escape outside the building rather than back into the roof or wall.
What we found
Box gutter overflows discharging straight into the rainhead below, on a fixing inspection in July 2026 and a handover in August 2026.
Why that fails
If the rainhead or downpipe is what blocked, the overflow feeds into the same blockage and the water has nowhere to go but inside.
Rainhead slots
We also recorded rainheads relying on overflow slots that are not a deemed to satisfy method, which needs either a compliant overflow or a performance solution from the builder.
What to ask for
A compliant overflow that discharges to the open air, or written evidence of the approved alternative.
Written by our local inspection team from work carried out in Manor Lakes. Licensed building practitioners, Building & Plumbing Commission registration CDB-U 53425.
