A skylight is a hole in your roof. The glass is not what keeps water out.

First: is it actually leaking?

This is worth settling before you spend anything, because a large share of "my skylight is leaking" turns out to be condensation, and the two look identical from a sofa.

Condensation appears on cold mornings, sits in the lower corners of the frame, is worst in winter, and gets worse when the house is humid — a bathroom or kitchen skylight is the classic. It is warm indoor air meeting cold glass. Nothing on your roof is broken.

A leak appears during or shortly after rain, regardless of temperature, and usually stains outward from one corner or runs down one side of the shaft. It gets worse with wind-driven rain.

The tell is timing. If it happens on a dry, cold morning, it is condensation, and the fix is ventilation and humidity — not a roofer.

Why skylights leak when they do

The flashing kit was reused

By far the most common cause. Every skylight ships with a flashing kit matched to its model and to the roof covering. When a roof is torn off and replaced, that kit should be replaced too. Reusing bent, fatigued metal — or worse, sealing around the old kit — is quicker, cheaper, and the reason a brand-new roof leaks at the skylight within two winters.

The seal between the panes failed

Not a roof problem at all. A failed insulated glass unit fogs permanently between the panes and cannot be cleaned. The unit is done; the roof around it may be perfectly sound.

Deck-mounted on too shallow a pitch

Deck-mounted units sit low and look better. Curb-mounted units sit on a built-up curb and shed better. On a shallow slope, a deck-mounted skylight is asking more of its flashing than the flashing can give.

The rule that saves people the most money

Replace the skylight with the roof.

A twenty-year-old skylight left under a brand-new roof is a callback with a date on it. When it fails — and a skylight has a shorter service life than the shingles around it — the new shingles have to be cut back to get at the flashing, and you pay twice for the same square metre of roof.

The marginal cost of swapping a skylight during a tear-off is a fraction of doing it as a standalone job later, because the roof is already open and the crew is already there. If you are replacing your roof and there are skylights on it, this is the single easiest decision on the whole estimate.

What we look at

• Is the staining consistent with rain, or with cold mornings?
• Flashing kit: original and model-matched, or reused and sealed?
• Curb-mounted or deck-mounted, and is that right for this pitch?
• Fogging between the panes — a failed seal, not a leak
• Condition of the shaft drywall, which tells you how long this has been happening
• Attic humidity and whether the bathroom fan actually vents outside

Straight answers