The call we get after somebody else’s roof clean is almost always the same: the roof looks fine, and now the skylight leaks.
Skylights and roof windows are the weakest point in an otherwise sound roof, and they are the component most likely to be damaged by a wash. They are also routinely left out of quotes altogether, which is how owners end up discovering the problem during the next storm rather than on the day.
Why they fail when the rest of the roof does not
A tiled or metal roof sheds water by overlap and gravity. A skylight interrupts that, and the watertightness depends on flashing, sealant and gaskets rather than on the roof material itself. Those components have a service life measured in a decade or two, not the life of the roof.
By the time a skylight is twenty years old, the sealant has usually hardened and cracked, the gasket has perished, and the flashing may have lifted slightly at one corner. None of that leaks under normal rain, because rain arrives at an angle and at low pressure.
A pressure washer does not. Water directed upward under a flashing at several thousand psi goes places rain never reaches, and once it is inside the roof cavity it stays there.
The three failure modes
- Driven water. Spray forced under flashing and past perished seals, into the ceiling cavity. Often not visible for days.
- Cracked or crazed domes. Older acrylic domes go brittle with UV exposure. Thermal shock from cold water on a hot dome, or direct high pressure, can crack them.
- Displaced sealant. Old sealant that was holding on gets cut away by the jet, leaving a path open for the next rain.
How they should be treated
Skylights are hand-cleaned at low pressure, not blasted from a distance with the rest of the roof. Around the perimeter, the flashing is worked with the flow rather than driven under it. Where a dome is visibly crazed or a seal has clearly failed, the correct answer is to clean around it conservatively and tell the owner it needs a roof plumber, not to press on and hope.
This is part of the same judgement applied across roof cleaning generally: the method is chosen per component, not per roof.
Why the inside stays hazy
A frequent complaint after a legitimate skylight clean is that it is still dull. Usually the outer surface is now clean and the haze is on the inside of the dome, or between the panes of a double-glazed unit.
Internal condensation staining and failed double-glazing seals cannot be cleaned from the roof. That is a replacement, and no amount of exterior work changes it. Tubular skylights with a reflective shaft have their own version of this, where the diffuser at ceiling level is the dirty part.
The related version of this on glass balustrades and windows is covered under window cleaning, where etched or hazed glass is often mistaken for dirty glass.
What a quote should say
If the house has skylights, they should appear on the quote by number and by treatment, not be silently folded into a roof price. Ask specifically:
- How many skylights are there, and are they included?
- What method is used on the dome and on the flashing?
- What happens if a seal is found to be failed?
- Who is responsible if a leak appears afterwards?
The last question is the one that matters. A contractor carrying proper public liability and willing to put the answer in writing is in a different category to one who avoids the topic.
The best time to look at them
Before the roof is cleaned, not after. A skylight already at the end of its sealant life is better identified during the quote, so the roof plumber goes first and the clean follows. Doing it in that order costs less than doing it the other way and repairing a ceiling.
Written by the team at The Prestige Standard, exterior cleaning contractors working across Sydney. Fixed written quotes within 24 hours with every surface itemised, $20 million public liability. See our service areas or call 0494 810 609.
