Size box gutters and rainwater heads per AS/NZS 3500.3:2021. Select a gutter variant, enter your project details, and get instant results.
Select a gutter variant and enter project details. Uses 5% AEP (20-year) rainfall intensity per AS/NZS 3500.3.
Manning's equation sizes the box gutter for the design flow. Gutter width and slope determine the required depth.
Walls adjacent to box gutters add catchment from wind-driven rain. AS/NZS 3500.3 requires adding 50% of the vertical face area.
Side and vertical overflow provisions handle flows exceeding the primary system capacity. Sized using weir and orifice formulae.
CCC Engineering delivers complete hydraulic design packages, including roof drainage, box gutter sizing, overflow provisions, and on-site detention systems.
This box gutter calculator helps hydraulic engineers, roofers, and building designers size box gutters for commercial and residential roofs. Enter the catchment area, gutter dimensions, slope, and local rainfall intensity to calculate the required flow depth, total gutter depth including freeboard, and overflow capacity. The calculator also accounts for vertical face contributions from adjacent walls and sizes rainwater heads for the downpipe connection.
Calculations use Manning's equation for open channel flow with a roughness coefficient of n = 0.014 for sheet metal gutters, as specified in AS/NZS 3500.3:2021. The design rainfall intensity is based on the 5% AEP (20-year) event at a 5-minute storm duration. A minimum 35 mm freeboard is added to the calculated flow depth. Overflow provisions are designed for the 1% AEP (100-year) event using either side overflow (weir formula) or vertical overflow (orifice formula) methods.
This tool provides sizing results suitable for straightforward box gutter applications. For complex roof geometries, long gutter runs with multiple downpipe outlets, or projects in areas with specific council stormwater requirements, consult a qualified hydraulic engineer. Final designs must account for thermal expansion, fall direction, and connection details that affect real-world gutter performance.