Field guide

Roofer Skylight Leak Pages: The Flashing Kit Is the Roof

Skylights almost never leak through the glass

A skylight almost never leaks through its glazing. It leaks where the metal panels or shingles terminate against the curb or frame, and where the flashing pieces around that termination overlap. That is the same principle that makes flashing and valleys the most leak-prone details anywhere on a roof, skylight or not, and it is the first thing a roofer's page should say, because the homeowner is staring at the glass and blaming the glass. The glass is fine. The flashing is the roof, and the flashing is what failed. Every leaking skylight that gets traced back has one of three causes: the factory flashing kit for that exact model was not used, the ice-and-water membrane around the opening was skipped, or sealant was used as the primary defense instead of properly lapped metal.

The diagnostic step is worth printing, because it is what a good roofer does first and what a bad one skips. The visible water stain inside is rarely the actual source, because water travels along the framing or the light shaft before it drips. The first job is to separate a true flashing failure from condensation, which appears as moisture on the glass or frame during cold weather. If the leak only shows up during heavy rain, it is a flashing or seal problem. A controlled water test, hosing the roof slowly from the bottom up until the leak repeats inside, finds the height where the water gets in, and that height is the repair.

Curb or deck is a flashing decision

The two mounting approaches behave differently, and the choice is a flashing decision before it is a looks decision. A curb-mounted skylight sits on a raised, watertight frame built up from the roof deck, which gives installers a clean vertical surface to terminate the panels and flashing against, well above the plane where water runs. That is why it is the standard, most reliably watertight approach, and the right choice on low-slope roofs, about 3/12 pitch or less, and on flat roofs, where a deck-mounted unit cannot be installed at all. A deck-mounted skylight sits nearly flush with the roof surface for a sleeker look, but without the curb's height advantage the panel-to-frame flashing has to be precise, and it performs best on steeper-pitched roofs where water sheds quickly rather than lingering near the opening.

The numbers that decide it are small and specific. Most deck-mounted units specify a minimum slope, commonly around 15 degrees. Below that, a curb-mounted unit on a built-up curb is required so water sheds instead of ponding at the head. A deck-mounted unit needs its matched flashing kit. A curb-mounted unit needs an apron at the bottom, step flashing up the sides, a head flashing at the top, and membrane under all of it. None of that is optional, and none of it should be reused from an old unit. The page that prints the 15-degree line and the curb rule is the page that keeps a homeowner from ordering the wrong unit, which is the cheapest leak prevention there is.

The three failures that repeat

They repeat because they are all shortcuts, and the page should name them as shortcuts. The first is a site-improvised flashing detail instead of the manufacturer's kit for that unit on that covering, fitted as instructed. The second is a skipped ice-and-water shield at the opening, which is the membrane that catches the water the flashing misses. The third is roof cement or sealant used instead of a flashing, which cracks and fails over time while the metal underneath would have lasted. A VELUX flashing system installed by a Three-Star Specialist carries a 10-year no-leak warranty, and that warranty is the proof of the principle: the kit, fitted as instructed, is what keeps the opening watertight, and the warranty is only available because the kit is fitted as instructed. The roofer who sells the kit is the roofer who can offer the warranty.

Frequently asked questions

Why does my skylight leak?

Almost always a flashing failure, not the glass. The usual causes are the wrong flashing kit for the model, a skipped ice-and-water membrane, or sealant used instead of properly lapped metal.

What is the difference between curb-mounted and deck-mounted skylights?

A curb-mounted skylight sits on a raised frame, which gives a clean vertical surface to flash against and is the standard on low-slope and flat roofs. A deck-mounted skylight sits nearly flush, looks sleeker, and needs more precise flashing and a steeper pitch.

Can a deck-mounted skylight go on a low-slope roof?

Usually not. Most deck-mounted units specify a minimum slope around 15 degrees. Below that, a curb-mounted unit on a built-up curb is required so water sheds instead of ponding.

How do you find where a skylight leaks?

First rule out condensation, which shows as moisture on the glass in cold weather. Then run a controlled water test, hosing the roof slowly from the bottom up until the leak repeats inside.