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.
A valley is a gutter at the internal intersection of two sloping panes of roof cladding.
Valleys should not be positively fixed, except at the head, because that would inhibit expansion and can produce noise.
Alternative means of securing the valley gutter to the substrate include:
When the roof pitch is less than 12°, the valley should be made in one piece or the joints must be sealed. To ensure snug fitting, the valley angle should be matched to the pitch of the valley support. Having the valley too open will result in a diminished capacity, and too sharp an angle will make installation difficult.
| Roof Pitch | Internal Angle |
|---|---|
| 3° | 176° |
| 5° | 173° |
| 10° | 166° |
| 15° | 159° |
| 20° | 152° |
| 25° | 145° |
| 30° | 139° |
| 35° | 132° |
| 40° | 126° |
| 45° | 120° |
| 50° | 114° |
| 60° | 104° |
| Roof Pitch | 3° | 5° | 8° | 10° | 12.5° | 15 | 20° | 25° | 30° |
|---|---|---|---|---|---|---|---|---|---|
| A 3-fold | 12 | 18 | 29 | 41 | 70 | 106 | 146 | ||
| B standard | 25 | 34 | 47 | 63 | 99 | 140 | 184 | ||
| C Deep | 60 | 86 | 152 | 180 | 215 | 251 | 321 | 389 | 452 |
| D Tile | 17 | 22 | 33 | 45 | 57 |
For other pitches, rainfall intensity, and valley shapes refer to the 5.5.7 Valley Capacity Calculator tool.
For information about internal corners, refer to 5.4.3 Internal Corners.
The maximum recommended catchment area for a bifurcated valley is 10 m².
A change of roof pitch in a valley run will usually result in the change of angle in plan view. The change is acceptable, but the freeboard of the lower valley must be at least 20 mm to allow for turbulence.
Where opposing roofs of different pitches discharge into a valley‚ an asymmetrical valley is required. As these have reduced cross-section area compared to a symmetrical valley at the same (lower) pitch, it is often necessary to increase the valley dimensions. Increasing the depth has the biggest effect on capacity. Greater depth can be gained by using 10 mm ply valley boards, standing purlins on edge, or fitting valley boards flush with the rafter. The consequences that a deeper valley will have on the capacity of the gutter it discharges into must also be considered.
A valley baffle is recommended where the difference in roof pitches exceeds 10°. Valley baffles are also helpful in wooded locations to minimise lodging of debris under the roof overhang.
Before using this calculator, please read 5.3 Roof Drainage Design.
To calculate valley capacity, insert the required values in the designated fields. All valleys require freeboard.
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.
