Skylight leaking water — Barcelona, Catalonia

A leaking skylight can cause stained ceilings, rotted framing, mold growth, and damaged insulation if not addressed promptly. Not every drip is a true leak — condensation on the interior glass is common in winter when warm, humid indoor air meets the cold skylight surface, and it can look identical to a roof leak. A true leak usually appears during or shortly after rain and is caused by failed flashing around the skylight curb, dried or cracked sealant between the glass and frame, improperly installed step flashing where the skylight meets the shingles, or ice dams forcing water uphill under shingles. A roofer can diagnose the source with a water test. Re-sealing the skylight frame with appropriate roofing sealant costs $150–$400. Replacing deteriorated flashing around the curb — which requires removing surrounding shingles, installing new step and counter flashing, and re-shingling — runs $400–$1,200 depending on skylight size and roof pitch. If the skylight unit itself is cracked, fogged between panes, or over 20 years old, full replacement costs $800–$2,500 including the unit and installation. Deck-mounted skylights are easier to replace than curb-mounted ones. Adding a proper cricket (small diverter ridge) above a skylight on a low-slope section prevents future water pooling ($300–$600).