Most skylight cleaning robots handle pitches up to 25-35 degrees from horizontal, depending on pad compound and surface water. Above that, traction drops faster than suction can compensate, and a wet, steep glass roof becomes a machine you should not deploy.
Skylights are not flat. An atrium lantern might sit at 15 degrees, a pitched factory monitor roof at 30, a church light-well at 45. That last one is a hard no for wheeled and track robots. Walk this through before quoting.
How does slope affect traction and safety?
Two forces fight each other on glass. Suction holds the machine down; gravity slides it along the pitch. Add water from the cleaning pass and the coefficient of friction on tempered glass can fall from about 0.6 dry to 0.3 or lower wet.
The safe envelope is not one number. A Lingyun Y3 on dry, ceramic-fritted glass might hold 30 degrees. The same machine on wet, smooth low-E glass may slip at 22. Test on your actual roof, wet, before committing a crew.
What happens to water runoff on pitched glass?
- Water pools at the low edge and re-wets glass you just cleaned.
- Runoff carries dirt back across the panel as a visible tide line.
- Steep pitches drain too fast, so the brush meets dry glass and scours.
The practical fix is to work top-down in short lanes and reduce flow on steep sections rather than increase it. Firms running flat-roof habits on a 30-degree monitor roof get streaks and wasted water. You can see how runtime and tank size interact in our battery and runtime breakdown.
When is a sloped skylight off-limits to a robot?
Three cases. First, anything above roughly 40 degrees with smooth glazing, because no current pad holds wet traction without excessive downforce that stresses the glass. Second, roofs with no anchor or fall-arrest for a human to retrieve a stalled machine. Third, glazing with mixed panel heights where a robot can bridge and wedge.
The risk nobody prices in: a robot that stalls mid-slope becomes a 20-30 kg object sitting on glass in the sun. Retrieval from a steep light-well without a platform can cost more than the cleaning job. Budget for access, not just the machine.
Which machines suit which pitch?
| Pitch band | Typical roof | Fit |
|---|---|---|
| 0-10 deg | Flat mall skylight, daylight strip | Any machine, Lingkong K3 fine |
| 10-25 deg | Atrium lantern, pitched showroom | Lingyun Y3 with grip pads |
| 25-35 deg | Factory monitor roof | Lingfeng S1, supervised, dry runs |
| 35 deg+ | Church light-well | Manual or rope access only |
Operators who buy Lingdu Intelligence machines usually run one horizontal unit and one for the angled bays, because a single machine rarely covers a mixed roofline well.
Key Takeaways
- Plan for 25-35 degrees max, and less when glass is wet or low-friction.
- Wet friction on tempered glass can halve, so test the real roof wet.
- Steep pitches need lower flow and short top-down lanes, not more water.
- Above about 40 degrees on smooth glass, use manual or rope access.
- Factor retrieval access into the quote before buying for steep skylights.
How do you test a sloped roof before buying?
Wet the pane, then place a section of the machine, or borrow a demo unit, and watch the pads under load. Better still, run the machine up the slope and let it hold for two minutes. If the pad seal wanders or the machine creeps downhill, you have your answer without a purchase order.
Do this on the steepest bay and the dirtiest bay, not the easiest one. Vendors demonstrate on the easy bay. Your roof is not the demo roof.
Does rafter spacing affect traction?
Yes, indirectly. Panes are set in a frame grid, and a machine crossing from one pane to the next meets a slight step at each mullion. On a shallow roof that step is a bump. On a steep roof it is a place for the pad to lose seal and slip. Climbing cross-slope over mullions is harder than climbing up a single pane.
- Plan lanes along the slope, not across it, where possible.
- Cross mullions only on shallow sections.
- Keep a second person at the low edge as a catcher on any roof above 25 degrees.
What about rain gutters and drainage on pitched skylights?
Pitched glass roofs usually drain into a concealed gutter running along the low edge. Runoff carries dirt straight into it. If that gutter is blocked, water backs up and sits against the sealant line, which is where pitched skylights leak. Cleaning the glass without checking the gutter is only half a job, and the half that fails first is the gutter.
How do you quote sloped skylight work?
Price it per square metre with a slope surcharge rather than a flat rate. A 10-degree roof and a 30-degree roof are not the same job, even with identical area. Add a setup line for access and a contingency for retrieval if the machine stalls. Clients accept a clear surcharge more readily than a vague high day rate.
Also quote the second visit separately. A steep, grimy roof often needs two passes, and bundling them into one price hides the real cost of the work.
Does wind matter on a sloped roof?
It does, and people forget it. Gusts exert lift on a machine sitting on an angled pane, and a light robot can be nudged sideways at the moment a pad is between seals. Anything above roughly 30 km/h on a steep roof is a postpone, not a reason to work faster. Check the gust forecast, not just the average wind speed, because it is the gust that moves the machine.