COP:roof-drainage; valley-gutters

5.6 Valley Gutters 

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.

Valley gutters must be fixed at the head to avoid creep, but must not be through fastened to the roof cladding. Valley gutters must discharge into a rainwater head, sump or eaves gutter, which must have an adjacent downpipe within 2 m of the valley discharge if the total catchment area serviced by the downpipe exceeds 50 m².
Valley gutters must be designed for the greatest rainfall likely in the area in a 50-year return period (ARI). The valley capacity must be able to discharge the total roof catchment area above the valley plus half the area of any dormer. See 8.5 Dormer Junctions.
A valley gutter must not be used where the roof pitch is less than 8°, but an internal box gutter, complying with the design criteria outlined in 5.3.2 Capacity Calculations, can be used.

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:

5.6A Example

Roof Pitch 20°=tangent=0.364
Valley Pitch=0.364 
 ÷1.414(√ 2)
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.


5.6.1 Valley Catchment 

The capacity of a valley gutter is determined differently to standard gutters, which are flat or very near to flat. A valley is a gutter at the internal intersection of two sloping planes of roof cladding, where the roof pitch is more than 8°.

Because the minimum slope of a valley is more than 5.5°, the discharge rate is greater than that of a flat gutter. Roof pitch increases the rate of flow, and catchment area increases proportionately.

Research by Martin & Tilley, from CSIRO Australia, has been used to show the increase in 5.6.1A Valley Catchment Capabilities.

With the pitch of the roof known, the maximum catchment is determined from the graph.

5.6.1A Valley Catchment Capabilities has been calculated by allowing for 10 mm freeboard and roof cladding interference.



5.6.2 Bifurcated Valleys 

5.7.2A Oblique Gable Roofs with Different Widths shows the detail which is needed when placing a gable roof at an oblique angle to two other gables, but at a different width. Because there are two gable roofs at 90°, the catchment area increases and water should be channelled away from this vulnerable area.

    5.6.2A Diverting Bifurcated Valley shows water being diverted away from this point and  5.6.2B Bifurcated Valley (Baffle Details) shows a bifurcated vally with baffle details.

    The NZMRM COP emphasises that silicone sealant should not be used as the primary defence against water ingress. The design principle is to use a diverter which channels the water away from the line of flow using vertical overlapping flashing design. The maximum recommended catchment area for a bifurcated valley is 10m².

    Although 5.5.4 Internal Gutters advises not to change direction within an internal gutter, bifurcated valleys offer three differences.

    • An internal gutter is normally laid flat, whereas 5.6 Valley Gutters requires a minimum roof pitch of 8°.
    • The deviation from a straight line is only 22.5°, which is not regarded as a sharp change in direction.
    • At the confluence, the catchment area is small and measured only in fractions of a square meter.