Skylights

Leak or condensation

Water appearing in cold weather with no rain is condensation.

Water appearing during or just after rain is a leak.

Condensation runs from the glass itself, uniformly, and shows along the bottom edge of the frame.

A leak usually shows at one corner or on one side, and stains the shaft drywall from the top.

Condensation is worse in a bathroom or a kitchen, which is where the moisture load is.

And a skylight that does both is the common case, which is why fixing the leak does not always end the complaint.

The condensation fix

Reduce the indoor humidity and improve the airflow at the skylight. Run the bath or kitchen fan longer, keep indoor humidity moderate in winter, insulate the shaft properly, and widen the shaft at the bottom so warm air can move across the glass instead of sitting against it. An old single or double glazed unit will condense regardless and is a replacement question.

Where skylights actually leak

The flashing kit, particularly where it was not installed as a kit — a site-fabricated flashing is the most common cause of a leaking skylight.

The head flashing at the top, where water coming down the roof has to be diverted around the unit.

The step flashing up the sides.

The curb, where a curb-mounted unit sits on a built-up frame that has rotted.

The seal between the glass and the frame on an older unit, which perishes.

And ice damming above the skylight, which is a Toronto particular — water backing up gets in at the head flashing.

Repair or replace

A flashing problem on a sound modern unit is a repair, and it is worth doing properly with a manufacturer flashing kit.

A perished seal or a fogged sealed unit is a replacement of the unit.

A rotted curb is a carpentry job before any roofing.

An old unit in a roof being replaced should generally be replaced at the same time, because the flashing and the roof go in together.

A skylight that has leaked for years frequently means the shaft framing and drywall need attention too.

And where somebody wants the light but the skylight has been trouble, a modern unit with a proper flashing kit in a properly insulated shaft is a different experience from a thirty-year-old one.