GRP Valley Troughs

In all cases and with all inclined roof valleys, valley troughs and valley lining materials, including membranes should be supported by valley boards or sark boards.

Valley boards may be inset or continuous over the rafter.  The width of the board should be sufficient for the valley being used and a minimum of 12mm thick plywood (or 19mm softwood) and supported on bearers of 50mm x 25mm or similar fixed to the rafter sides and set at a depth to finish the valley board flush with the top of the rafters.

Continuous overlaid valley boards may be used and should be a minimum of 6mm thick plywood butt jointed only over supporting rafters.  On rafter spacings above 400mm, it is recommended that support noggins of 75mm x 50mm minimum are securely fixed under the outer edge of the valley board between the rafters.

The fascia or barge board may be trimmed to allow the valley trough to pass through without flattening the profile. Alternatively, a lead or lead replacement soaker may be used if required.

Many years ago when these products became popular, the issues with lead were less understood and the wide range of flexible lead replacement products that we now have were not available. While still permissible, it is now considered to be slower and more costly to use lead, so it is rapidly falling out of favour.

Conventional ‘open cut’ tile or mortarless valley troughs are limited to 15°. The Danelaw dry fix valley trough can be used with a roof pitch differential of up to 20°.

Generally, valleys are not recommended below a 17.5° rafter pitch, and rarely used in rafter pitches above 60°. GRP valley troughs can accommodate this range, subject to roof area, valley length and pitch differential considerations.

The Danelaw design of dry fix valley trough can accommodate a greater pitch differential and can be used for larger roof areas and longer valley lengths. For further guidance on applications falling outside our standard published criteria, please contact out Technical Department.

We always recommend that the valley trough is installed with the central upstand pinched together. This minimises the visible opening which can easily be filled with a good quality external grade grey sealant such as ‘Leadmate’.

No – these additional components are not required with the Danelaw design.

Yes. The design, function and operation of valley trough remains the same. The height of the central upstand should be at least as high as the highest point of the lapping slates or tiles, but for aesthetic reasons, the profile should be chosen to best suit the external roof covering.

No, unless the recommended minimum overlap cannot be achieved (see previous answer).

The requirement for all unsealed laps in roofing is generally 150mm when measured vertically, and this rule is applied to all Danelaw strip flashing products such as slate and tile valley troughs, Bonding Gutters, Conti-soakers etc.

For a roof pitch below 22.5°, the overlap should generally be 350mm. This can be significantly reduced using a sealant in the lap, but because of the additional time, cost and unknown reliability of this method, it is rarely used.

Lead has traditionally always been used as a versatile flashing product to form a saddle detail. This is still an option, but with the increasing concerns over health, environment and theft, there are now many lead-free flashing options available – some better than others and some good products but with limitations.

Fast Flash from Hambleside Danelaw is one of the best options here. It is an easy to use, high quality product with fully adhesive properties market leading stretch ability.

It is now recognised that the use of a good quality lead replacement product is no less reliable, and both quicker and more cost effective.