Why is my skylight leaking at the corner?

Which corner tells you a lot

• Upper corners: water running down the roof hits the head of the skylight and has to be steered around it. If the head flashing is short, the membrane underneath does not wrap the corner, or leaves and needles pile up there, water gets behind the step flashing. Wide skylights without a proper head diverter are especially prone to this.
• Lower corners: the sill or apron flashing must lap over the shingles below, and the last step flashing piece must lap over the apron. A reversed lap or a missing piece sends water under the shingles.
• Any corner, inside the frame: a failed corner seal on the frame, a cracked acrylic dome, or a broken glass seal lets water into the unit itself.

Leak or condensation?

Condensation forms on the inside of the glass and runs to the lowest point, usually a lower corner, so it can look exactly like a corner leak. If the drip appears on cool or humid mornings with no rain, suspect condensation and check bathroom and kitchen ventilation nearby. If it appears only during or after rain, it is a leak.

The fix

Corner leaks are rarely solved with caulk, because the problem is usually a lap or a missing membrane under the flashing. A proper repair removes the shingles around the unit, wraps the curb corners in self-adhered membrane, and installs a correct flashing kit. If the unit itself has failed, replacing it makes more sense than reflashing an old skylight. See replacing a skylight with the roof and roof flashing explained.