01
Plan the field around the dormer
Adjust nearby slate widths so valley and sidewall cuts remain substantial. Course lines on the dormer roof should relate cleanly to the main roof where the architecture makes alignment visible.
02
Separate each flashing function
Valley lining, step flashing, apron or sill flashing and headwall flashing have different jobs. Counterflashing or cladding should protect their upper edges while allowing water to return to the roof surface.
03
Check window integration
Window pans, trim, cladding and sealant joints can leak independently of the slate. Coordinate the entire opening and preserve weeps or drainage paths required by the window assembly.