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.
