Extra Head
The slates in the valley must be longer than the standard slates on the roof if step flashing is not being used in the valley. The headlap should extend not just three inches under the second overlying course, which is standard headlap; it should extend completely up to and under the third overlying course by an inch or so. The extra headlap is necessary to prevent water penetration since the valleys carry more water and have a lower slope than the rest of the roof.
ROOF SLOPE, VALLEY LENGTH, AND OTHER CONSIDERATIONS
Slated valleys cannot be installed on low slope roofs by roofing contractors Toronto. It is recommended that slated valleys be installed on roofs with slopes no lower than 30' (7:12), although much steeper slopes, such as 20:12, are recommended.
The longer the valley, the more water it must carry, especially at the bottom. Therefore it is not recommended that rounded valleys he designed on long runs, with 20' being a recommended maximum length.