Lifting shingles at the bottom of a skylight: when it means a leak is coming

A skylight flashing kit works as a set. Head flashing covers the top, L-shaped step flashings are woven up each side with the shingle courses, and an apron along the bottom sends runoff from around the unit onto the shingles below, according to 3D Established Roofing. The apron lies over the course below it and is left exposed so it can drain, The Roofing Brief notes in its replacement guide, so metal showing at the bottom edge is by design. A lifted lap is different: wind-driven rain can push in under a lap that sheds ordinary rain without trouble.

An overlay roof adds two complications. VELUX's warranty against leaks covers only the first installation, made over a single, correctly installed layer of roofing, The Roofing Brief reports, so a unit flashed into a second layer can fall outside that coverage. And once a roof carries two layers of any covering, section R908.3.1.1 of the International Residential Code bars another recover. That leaves a full strip down to the deck as the only option for the next reroof. Reroofing rules also require damaged or deteriorated flashings to be replaced and new flashing built to the manufacturer's instructions (sections R908.5 and R908.6).

Call a roofer now if stains show on the light shaft or ceiling, or if a wet trail appears after rain or while snow sits on the roof. Condensation forms an even film around the frame instead, and a real leak often surfaces a few feet from the skylight because water runs along the deck and rafters first, 3D Established Roofing says.

Caulk won't fix it. A proper repair pulls back the shingles, removes the failed pieces and puts back the apron, step pieces and head flashing in sequence. HomeGuide prices flashing repair at $150 to $500 and VELUX puts flashing and sealing at $300 to $800, according to The Roofing Brief, while a full replacement runs $800 to $2,400 and skylights typically last 15 to 40 years.