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.
