Skylights and Roof Leaks: Preventing the Most Common Failure Points
Why skylights get a bad reputation
Most skylight leaks are not actually the skylight itself. They are the flashing around it. A skylight sits in the middle of a roof slope, and water has to move around it on four sides. Every side is an opportunity for a flashing detail to go wrong.
When a skylight leaks, homeowners blame the skylight. In our experience, nine times out of ten the unit is fine and the flashing was rushed.
Curb-mounted vs deck-mounted
Older homes often have skylights built on a curb - a small wooden frame that sits on top of the deck and the skylight clamps onto. Newer skylights (Velux, for example) mount directly to the deck with a factory flashing kit specifically sized for the unit.
Factory flashing kits are much more forgiving than site-built curb flashing. If you are replacing an old leaking skylight, upgrading to a deck-mounted unit with a factory flashing kit usually eliminates the leak problem for the life of the roof.
The four critical flashing points
Every skylight needs: a head flashing at the top that diverts water around the unit, step flashing on the sides laced into each shingle course, a sill flashing at the bottom that carries water down and onto the next shingle course, and an underlayment layer that wraps everything.
Each piece has to be installed in order and lapped correctly. Skip a step, and water finds the gap.
When to replace the whole unit vs re-flash
If the skylight itself is more than fifteen years old and the flashing is failing, replace them together. Modern skylights are dramatically better sealed, better insulated, and come with factory flashing kits that are effectively fool-proof. Pulling out an old unit and re-flashing around the same unit only to replace it in five years does not make sense.
What to ask your roofer
Ask whether they use factory flashing kits, whether they recommend replacing the unit as part of a reroof, and what their warranty is specifically on the skylight flashing detail.