Calculate the maximum catchment area for an upper roof discharging onto lower roof sheets via a spreader. Per AS/NZS 3500.3:2021.
Enter roof geometry and select a location to calculate maximum catchment area.
The design rainfall intensity determines the volume of water the upper roof collects. Flow rate is calculated from catchment area, rainfall, and roof slope.
Each roof profile has a maximum water-carrying capacity per metre width. The spreader must distribute flow within this limit.
The calculator compares the design flow against the lower roof's capacity to determine if the spreader arrangement is adequate.
Our hydraulic engineers design roof drainage, stormwater, and overflow systems for commercial and residential projects.
This spreader design calculator helps hydraulic engineers, roofers, and builders determine the maximum upper roof catchment area that can safely discharge onto a lower roof via a spreader. Select your project location for the design rainfall intensity, choose the lower roof sheet profile, and the calculator will determine whether the upper roof runoff can be spread evenly across the lower roof without exceeding the sheet's water-carrying capacity. Spreaders are commonly needed at step-downs, clerestory walls, and where upper roof areas terminate above lower roof sheeting.
Calculations follow AS/NZS 3500.3:2021, Clause 3.5, which addresses roof water discharge onto lower roof areas. The design rainfall intensity uses the 5% AEP (20-year) event at a 5-minute storm duration from Bureau of Meteorology IFD data. Each roof profile has a rated water-carrying capacity per metre width, and the calculator checks that the design flow from the upper roof does not exceed this capacity when distributed across the spreader width.
This tool provides a quick check for straightforward spreader applications. Where the upper roof catchment exceeds the lower roof sheet capacity, a gutter and downpipe system must be used instead of a spreader. For complex roof junctions, multiple levels, or projects with specific overflow requirements, consult a qualified hydraulic engineer to ensure the design complies with AS/NZS 3500.3 and local council requirements.