· by Michael Blair, Nashville's Window Cleaning
A skylight sits at the angle that catches everything: pollen in spring, dust and sap through summer, leaves in fall, and the algae that grows on the shaded side of anything damp in Middle Tennessee. Solar panels sit at a similar angle and lose a measurable share of their output under the same film. Both are on the roof, which is where most people stop. Cleaning them is a routine part of window cleaning for companies equipped for it, and the methods matter, because the wrong one scratches a skylight or voids a panel warranty.
Why roof glass gets so dirty
Vertical glass sheds rain, and rain rinses it. Glass at a roof pitch holds water in a film at the bottom edge and along the frame, where dust settles into it and dries, and the next rain deposits more. Pollen in a Nashville April coats everything at that angle in yellow within a week. On the north side, or under trees, the film stays damp long enough for algae and lichen to take hold, and they are what turn a skylight green at the edges.
Skylights have the extra problem of being a low point on the roof where debris collects against the curb. Leaves and needles pile up on the uphill side, hold water against the flashing, and that is how a skylight leaks. Cleaning the glass without clearing the curb misses the part that matters.
How they are cleaned
Most skylights and panels within reach of a pole are cleaned from the ground or from a ladder at the eave with a water-fed pole: a brush on a telescoping pole, fed with purified water, that scrubs the glass and rinses it with water so clean it dries without spots. No squeegee, no ladder on the roof, no one walking on the panels. Poles reach a story or two up a moderate pitch, which covers most residential skylights and arrays.
What a pole cannot reach, or a skylight with a heavy buildup of lichen, needs someone on the roof with the right fall protection, and that is a job for a company that does it, not a homeowner with a garden hose. Never use a pressure washer on either: it forces water past the skylight seals and can crack the glass or the panel and void the panel warranty.
- Water-fed pole with purified water from the ground: the standard method
- Soft brush and mild soap, never abrasives or ammonia on plastic-domed skylights
- No pressure washing, no standing on panels
- Clear the debris above the skylight curb every time
Skylight materials and what scratches them
Older skylights, and many on additions and sunrooms, have acrylic or polycarbonate domes rather than glass. They scratch easily and cloud with age, and any abrasive pad, any ammonia-based cleaner and any solvent damages them. They are cleaned with a soft brush or cloth, mild soap and a lot of water. Glass skylights take the same cleaning as any window, but the seals and the flashing around them are the parts to be careful with; scraping at the edges lifts the sealant.
A skylight that is fogged between the panes, or a dome that has yellowed and crazed, is not dirty and will not clean. That is a failed seal or an aged dome, and it is a replacement.
Solar panels: whether cleaning pays
Rain cleans panels enough in most of the year that scheduled cleaning is not always worth its cost, and the panel makers say so. The exceptions are the pollen season, a panel array under trees, and a dry stretch that leaves a film, all of which reduce output noticeably. If the monitoring app shows output falling behind what the weather explains, a cleaning is the first, cheapest thing to try. Bird droppings are worth removing when they appear, because a single shaded cell can cut the output of a whole string.
Panel cleaning uses the same water-fed pole and purified water, in the morning or evening when the panels are cool, since cold water on a hot panel can crack it. Follow the installer’s guidance on what the warranty allows, and use a company that knows the difference between washing a panel and washing a window.