VELUX No Leak skylights feature three layers of weather protection: The pre-installed deck seal is nailed directly to the roof deck. Adhesive underlayment covers the deck seal, the roof deck and up the sides of the skylight. Engineered step flashing is integrated with the roofing material and works with a saddle flashing piece to direct rainwater away from the skylight.
https://www.youtube.com/watch?v=8xcxQ5Xxiyo