Skylights are windows designed and manufactured for installation in a roof assembly. They must be installed carefully because they are exposed to severe climatic conditions and their remote location makes them difficult to maintain. Skylights must be securely attached to the roof framing and sealed to the air barrier system in the roof, at the walls of the skylight opening and at the ceiling.
When the air barrier in a wood-frame building consists of polyethylene attached to the inside surface of the roof framing, the polyethylene must be extended up and sealed to the inside face of the skylight frame. When the air barrier is a membrane on the exterior of the roof sheathing, it should be sealed to the outside of the skylight frame and covered with a prefinished metal flashing to protect it from exposure to ultraviolet radiation, water, snow, ice and mechanical damage. Some skylight manufacturers supply step flashings specifically designed for their units and for a variety of roof slopes and finishes, which enables the installer to effectively integrate the skylight unit into the roof waterproofing system. Figure 135 shows a stepped base flashing for a skylight unit on a pitched roof.
Source : Canada Mortgage and Housing Corporation (CMHC)