They are rarely a problem with steep pitches and small roofs.
However things can really go wrong:
- in flat roofs
- draining large roofs – especially at the gutter end
- when valleys change direction i.e. dogleg valleys.
To me the major issues Contractors should get right are:
- choose the right valley size for area of roof, slope and rainfall
- NEVER flatten the return fold on both sides during installation
- top of valley is always turned up / bottom of valley is ‘dressed’ over fascia AND down past back of gutter
- valley is one continuous piece wherever possible or joints are 200mm minimum and riveted and sealed
- sarking should drain into valley. Insulation MUST never extend in to the Valley and NEVER press on the return fold.
- avoid silly mistakes like fixing through the Valley, unsupported cut sheet corners / Valley tiles
Finally, please plan your downpipe location so one is close to the base of the Valley. And tell your Client to clean the Valley every 6 months if there are lots of trees above it.
Happy Valleys!
