Box Gutters and Stormwater

Non Standard Sizes

NCC 2016 Volume 2

Area of NCC Requirements:

• DP1.1 – Roof Drainage Systems
• DP1.2 – Overflow Devices
• DP1.3 – Watertightness
• DP1.4 – Roof Drainage Installation

The Challenge

Box gutters are the unsung heroes of stormwater management. Yet, their design is tightly governed by AS/NZS 3500.3. In practice, site conditions often mean the “Deemed-to-Satisfy” sump sizes and overflow orientations can’t be achieved.

At a residential development in Victoria an inspection revealed that compliant sump widths couldn’t be installed, and the overflow orientation didn’t line up with standard DTS provisions. Without a performance solution, the roof drainage system risked non-compliance.

Why Roof Drainage Design is So Complex

Roof drainage isn’t just about moving water away—it’s about doing so safely under intense rainfall. An undersized sump or incorrectly oriented overflow could mean uncontrolled discharge, damage to property, or worse, structural compromise.

The NCC provisions are written to protect against these failures. But when dimensions can’t be met, careful engineering assessment is required.

The Solution 

The system was analysed using:

• Catchment flow rates for each roof zone
• Comparative analysis of gutter and sump capacities
• Verification against AS/NZS 3500.3 intent

Expert judgement was applied to confirm that despite dimensional departures, the system would perform under 1:100 ARI events.

Why This Matters

Bottom line:

With extreme weather events becoming more frequent, stormwater design resilience is critical. Performance-based assessments allow innovative or constrained projects to meet NCC intent without compromising safety.

Performance Solutions Australia ensures that when the standard rules don’t fit, buildings are still safe, functional, and compliant.

Have a similar problem? 

Reach out to our team for the solution.