Why does my skylight only leak during heavy rain?
Short answer. A skylight that stays dry in light rain but leaks in downpours usually has flashing that works at normal flow but gets overtopped when a lot of water comes down the roof at once: a short or poorly lapped head flashing, debris damming the uphill side, or no self-adhered membrane under the flashing as a backup. Wind-driven rain can also push water sideways under step flashing with too little lap.
Why heavy rain is different
In a downpour, the slope above a skylight sends a sheet of water into its uphill side. If the skylight is wide or sits below a long run of roof, that volume can rise higher than the upturn of the head flashing and spill over, or back up under the shingles. Light rain never reaches that height, which is why the leak seems random.
Common causes
- Head flashing too short or too flat: it does not reach far enough up under the shingles above.
- No diverter on a wide unit: larger skylights need the water split around them, much like a wide chimney needs a cricket.
- Debris dam: leaves, pine needles or pollen packed against the head slow the water and make it pile up.
- Missing membrane wrap: without self-adhered membrane wrapped up the curb, any water that gets past the metal goes straight in.
- Wind-driven rain: Georgia thunderstorms push rain sideways and even upslope under step flashing with short laps.
- Low slope: skylights on low-pitch roofs drain slowly and need flashing made for that pitch.
What to do
- Note when it leaks: only in downpours, or only with wind from one direction.
- From the ground, look for debris piled above the skylight. Having it cleared may help in the short term.
- Have the skylight reflashed with membrane under the metal and a proper head flashing or diverter, and replace the unit at the same time if it is old.
If the skylight is near the end of its life, read replacing skylights with a new roof, and for the flashing pieces involved, see roof flashing explained.