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.
The pitch is the angle between the horizontal and the roof line. It is also the relationship between the rise and the horizontal span of the roof. See 18.2 Roof Pitch Tangent for the tabulation of these values and a calculation tool.
| Profile | Min Height | Min Pitch | Check Capacity after run of... |
|---|---|---|---|
| Trapezoidal Asymmetrical | 20 | 4° | 20 m |
| 27 | 3° | 40 m | |
| 36 | 3° | 65 m | |
| Trapezoidal Symmetrical | 20 | 4° | 15 m |
| 30 | 4° | 25 m | |
| Secret Fix | 30 | 3° | 60 m |
| 25 | 3° | 30 m | |
| Standing Seam | 30 | 3° | 85 m |
| 25 | 3° | 35 m | |
| Corrugate | 16.5 | 8° | 15 m |
| 21 | 4° | 20 m | |
| 35 | 3° | 65 m |
Buildings designed with widely spaced purlins and widely spaced portal frames may require an increased design pitch to comply with the minimum recommended as-laid pitches.
Low pitched roofs require greater attention to flashing details. The ability of side laps or end laps to withstand water penetration also becomes more critical at low pitches, but the good design of flashings can ensure weathertightness in extreme conditions.
Runoff is the ability of the roof cladding to discharge maximum rainfall without water penetrating through side laps, end laps or flashings and depends on rainfall, the catchment area, the roof pitch, and the profile geometry.
The roof pitch determines the rate of flow; steep slopes shed water faster than shallow slopes. The rib height and spacing of trapezoidal profiles also affects its shedding ability.
For example: At a rainfall intensity of 200 mm/hour, a five-rib trapezoidal profile at 3°, with a rib height of 27 mm can have a run of 90 m.
The 7.1.4 Maximum Run Calculator can calculate capacities for any known profile.
The capacity of a roof profile to drain water is determined by its geometry and the roof pitch. The catchment area is the distance between rib centres times the length, and the effective cross-section area and the wetted perimeter is taken to the height of the overlap on corrugate, or capillary bead on trapezoidal and trough section profiles.
The 7.1.4 Maximum Run Calculator gives the maximum length that a roof can drain at a given pitch and rainfall intensity. The manufacturer’s data can be accessed from the drop-down box; for other profiles, the data can be entered manually into the worksheet.
Where the flow of water is concentrated by penetrations or spreaders, go to the 9.4.4 Maximum Area Above Penetration Calculator.
Before using this calculator, please read 5.3 Roof Drainage Design.
The Maximum Run Calculator calculates the maximum roof length to achieve effective roof drainage for any profile, pitch, and rainfall intensity. Insert the values in the designated fields.
For an explanation of each element, please click on the corresponding question mark.
For rainfall intensities, refer to NIWA’s HIRDS tool or the 5.3.2 Rainfall Intensity.
