Work out the design flow for the lower roof gutter and downpipes when an upper roof discharges onto it through a spreader. Per AS/NZS 3500.3 Clause 3.4.5.
Enter both roof catchments and select a location. The lower roof drainage is sized for the total flow.
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The design rainfall intensity sets how much water each roof collects. Each plan area is increased for roof slope, then converted to a flow rate.
Water from the spreader runs down the lower roof into its gutter. The lower gutter and downpipes carry the upper and lower catchments together.
Metal sheet side laps are sealed, and tiled roofs are sarked, for 1800 mm either side of the point of discharge.
Our hydraulic engineers design roof drainage, stormwater, and overflow systems for commercial and residential projects.
This spreader calculator helps hydraulic engineers, roofers, and builders size the lower roof drainage where an upper roof downpipe discharges onto a lower roof through a spreader. Select your project location for the design rainfall intensity, enter the plan area and slope of both roofs, and the calculator gives the flow at the spreader and the total design flow for the lower roof gutter and downpipes. Spreaders are commonly used at step-downs and where an upper roof sits above lower roof sheeting.
Calculations follow AS/NZS 3500.3 Clause 3.4.5, Higher catchment area. The standard allows a spreader where the lower roof is sealed or sarked for 1800 mm either side of the point of discharge, and it requires the lower catchment gutter and downpipes to carry the total flow from both catchments. The design rainfall intensity is the 5-minute event at 5% AEP (20-year) for external eaves gutters, or 1% AEP (100-year) for box and valley gutters. Plan areas are increased for roof slope using the catchment area multiplier in Table 3.4.3.2.
This tool provides a quick check for straightforward spreader arrangements. It does not check the water-carrying capacity of the lower roof sheet under the concentrated discharge, which depends on the sheet profile and slope and should be confirmed against the roofing manufacturer's data. 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.