When installing a skylight on a flat roof, where does the skylight frame sit?

When installing a skylight on a flat roof, the skylight frame is designed to sit over the base flashing. This placement is crucial for ensuring that water runs away from the skylight rather than seeping into the roof structure, which could lead to leaks or damage over time.

The base flashing, which is typically made from metal or a flexible waterproof material, creates a protective barrier that helps to guide water away from the skylight. By situating the skylight frame over this flashing, the design facilitates proper drainage and helps to maintain the integrity of the roofing system. It also allows for easier installation and better connection between the skylight and the roofing materials, enhancing the overall durability of the installation.

The other options do not provide an effective method for ensuring watertightness or proper drainage. For instance, if the skylight frame were placed on top of the membrane, it could disrupt the waterproof layer and lead to potential leaks. Additionally, placing the frame inside the curb would not allow for sufficient protection from water intrusion, while situating it underneath the base flashing could lead to severe issues with drainage and water retention, ultimately compromising the roofing system's effectiveness.