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.
Valley gutters are installed on roofs with a pitch of less than 12° and are fully supported, but they should not be positively fixed, except at the head, because that will inhibit expansion and can produce noise. For this reason, the valley sole or upstand should not be returned or fixed under the roof cladding but should be terminated with a weather hook.
The dimensions of a valley gutter complying with 5.6E Metal Tile Valleyare suitable for most domestic roofs, with a minimum depth of 50 mm at the centre of the valley. Where the catchment area is less than 50 m², the valley dimensions should be increased by recessing the valley boards into the rafters or rescessing them between the rafters, supported on dwangs. See 5.6D Recessed Valley.
When the roof pitch is between 8° and 12°, the capacity of the gutter should be similarly increased, and the minimum depth should be increased to 75 mm. Alternatively, expand the width to increase the valley capacity by up to 50%.
Do not compromise the design capacity of the eaves gutter by cutting down the back of the gutter at the valley. If the valley is below the top of the eaves gutter, the gutter should be lowered at the discharge point, or the valley should be 'sprung' to this level.
If downpipes discharge into the catchment area served by a valley gutter the total catchment should not exceed the capacity of the valley.
Where the roof pitch is more than 35°, it is best practice to provide a central baffle, which can also act as an expansion joint. See 5.6D Recessed Valley.
When the roof pitch is 8° – 12° the valley should be made in one piece or the joints sealed.
The valley pitch will always be less than the roof pitch by a calculated value, eg where the roof pitch is 12° the valley pitch is 8.5°. See 8 Penetrations.
To find the valley pitch when the roof pitch is known, and the valley is at 45° to the rafter:
- Find the tangent of the roof pitch. See 17.2 Roof Pitch Tangent.
- Divide the tangent by 1.414(√ 2)
- Find the pitch.
|Roof Pitch 20°||=tangent||=0.364|
|New Valley Pitch||=Valley Angle|
The total clearance between the sheeting on either side should be 80 mm, enough to allow a tennis ball to pass freely. The valley should be free to expand, but should be positively fixed at the head to avoid creep caused by expansion or by snow on steep roof slopes.
Valleys should not have a "wing" or return under the cladding which would be penetrated by any fixing, inhibiting free movement and causing noise. The sides of the valley should have an upstand, a weather hook, or should be folded as shown in 5.6B Hook Valley to 5.6E Metal Tile Valley. The hook should extend full height to the underside of the roof cladding.
There are alternative means of securing the valley gutter to the substrate. A simple clip system allows for movement and security. A compatible washered nail or screw is a suitable alternative, but a bent nail can cause damage to the metal cladding and is not durable. See 5.6B Hook Valley.
Alternative valley designs with the valley boards are on top of the rafter (5.6B Hook Valley), or cut into the rafter (5.6D Recessed Valley) are acceptable. Where they are cut into the jack rafter, the rafter depth should be increased, or the valley board cut between the rafters.