The NZ Metal Roof and Wall Cladding Code of Practice is a comprehensive design & installation guide, and a recognised related document for Acceptable Solution E2/AS1 of the NZ Building Code.
All downpipes that discharge onto a lower roof must have a spreader to dissipate energy and ensure wide distribution of the water. A spreader should be used over multiple troughs.
For corrugate, a spreader should not discharge into a lapped trough. When using the COP calculator, discharge may be into a lapped trough of a trapezoidal or a trough profile.
The area of discharge holes from a spreader should equal the cross-sectional area of the downpipe.
The 5.7.1 Maximum Area Above Spreader Calculator enables users to determine the maximum upper roof area that a lower roof can discharge for a given combination of rainfall intensity, roofing profile, and lower roof pitch.