Stains on the ceiling near skylights
Most of us didn’t know that many skylights have an extruded aluminum frame with drain holes leading to the outside of the frame. On the inside of the frame is a small gutter (channel) system that leads to the small weep holes to the exterior. These weep holes allows the moisture in the gutter to drain onto the roof. They’re often located at the skylight corners and in some skylights may also be spaced every 24 inches. A few manufactures have a two gutter system.
These condensation gutters will catch most of the moisture that builds up on the dome or lens of the skylight, so that it won’t drip or run onto the ceiling drywall or finish.
Are the weep holes or condensate gutter clogged ?
If the weep holes get clogged, then the moisture overflows the small gutters, resulting in stains. The clogging may be from dust building up over the years. If remodeling has taken place, drywall dust and other small particles may have float about, which contribute to the clogging. A damaged or bent gutter (channel) can likewise result in moisture condensation over flow.
On the roof
When possible unclogging these small weep holes will be the answer as to why there are stains around the skylight. Remember to also check the curb of the skylight on the roof, because the weep holes may have become clogged from excessive mastic applied on the exterior curb and flashing of the skylight.