Water leakage is a common problem with skylights. Do not select a skylight design that depends primarily on sealants to prevent leakage. The most reliable method of avoiding leaks is to use one-piece molded skylights that overlap the curb. Unfortunately, molded skylights are limited in size.
If a skylight must have joints, design it so that the joints are steeply sloped to shed water. Ideally, no joint or sealant should face uphill, but this condition cannot be met everywhere in built-up skylights. Instead, the frame extrusion should be designed so that it conveys any water that leaks through seals to the outside of the skylight.