A balcony drain smaller than the standard allows

In short: A Cobbitty handover recorded the balcony waste drain pipe as undersized against AS/NZS 3500.3.

What was recorded

On a Cobbitty two storey home the balcony waste drain pipe was smaller than AS/NZS 3500.3 requires. It was classed as a major defect.

A balcony collects heavy rain fast. If the drain cannot carry it, water backs up against the door sill and finds its way inside.

An overflow is the safety valve when the main outlet blocks. Its position and the membrane around it decide whether it works.

On another Cobbitty job, balcony drainage had not been installed at all at fixing. Checking at that stage gives time to get it right.

Ask the builder to confirm the pipe size and outlet against the hydraulic design, and to show the overflow detail.

Written by our local inspection team from work carried out in Cobbitty. Licensed building practitioners, NSW Fair Trading licence 366177C.

More pci / handover guides for Cobbitty

25 March 2026 · 5 min read

Can I see the drain size myself?

Sometimes from below or at the outlet. The report records what is visible and what needs confirming.

Is the membrane checked at handover?

No, it is covered by then. That is a waterproofing stage item.

What happens if water comes inside?

Stop using the balcony in heavy rain, notify the builder in writing and keep photographs.

Is this a plumbing or building matter?

Both. The plumbing standard sets the size; the builder is responsible for the result.