In the context of COP Roof drainage, NZBC E1 requires that water discharging into a gutter must have less than a 2% probability of entering a building.  This means that those drainage paths inside the building envelope, including valleys and internal gutters, must be designed to accommodate a 50-year rainfall intensity over a short period.  Roof Drainage Design discusses how the COP addresses this in more detail.

The above caveat equally applies to the limitations on run length of roof cladding, drainage capacity around penetrations, and discharge of spreaders, which are dealt with in sections XXXX

Gutter catchments outside the envelope, which can discharge freely without entering the building, can be calculated using less rigorous safety factors

Revision Category: 
2 - Editing and rearrangement
Revision Detail: 

Clause promoted, replacing ES/AS1 at postion 6.2, and content of clause reviewed.