Four specs decide a roof cleaning robot skylight job: suction in pascals, cup footprint against mullion width, water tank volume, and tether length. Everything else on the sheet, from app control to LED lighting, changes the experience without changing whether the machine can do the roof.
Which suction figure should you trust?
The one measured on a slope, not on a vertical pane. Suction quoted in still air on flat glass tells you the pump’s best case. A skylight at 30 degrees asks a different question, because the machine has to resist sliding as well as falling.
Practical bands: below 2,500 Pa suits near-flat glass only. 2,500-3,200 Pa covers most commercial skylights. Above 3,200 Pa buys margin for dirty, steep or wind-exposed surfaces, and costs battery time.
Why does cup footprint beat almost every other spec?
Mullions interrupt glass roofs on a fixed grid. If the cup array cannot straddle the frame, the machine stops at every joint or, worse, tries to cross and loses seal. Measure your narrowest mullion before ordering, and compare that number with the cup spacing on the machine.
A machine with smaller cups crosses more frames. A machine with larger cups holds better on wide panes. There is no single right answer, only a match or a mismatch.
How much water does a roof job need?
Dusty urban roofs use more than clean suburban ones, and a 400 ml tank may be exhausted before a large bay is finished. Refilling means climbing down or opening a housing, so tank volume is really a labour spec in disguise.
Which Lingdu model suits which roof?
Lingkong K3 for vertical facade panes, Lingyun Y3 for framed glass roofs where frames interrupt the path, Lingfeng S1 for steeper roof panels. The Lingfeng S1 review covers slope handling; the reviews archive has all three side by side.
What should be crossed out on the sheet?
Any spec without a unit. “Strong suction” is not a number. “Long battery life” is not a number. If a supplier leads with adjectives instead of pascals, millimetres and minutes, the spec sheet is marketing and you should test on your own roof before signing.
What does battery capacity mean on a rooftop?
Less than it means indoors. Heat degrades cells, and a roof in direct sun runs well above ambient temperature. A pack rated for 90 minutes in a test cell may deliver 55-70 minutes at midday in summer, and the drop is not linear across the season.
Schedule heavy roof passes for early morning where possible. The glass is cooler, the cups hold better, and the same charge covers more square metres. That scheduling decision costs nothing and buys more runtime than a larger pack would.
How do you spec a roof you have never measured?
Measure it. Pitch in degrees, mullion width in millimetres, glass thickness, and the height of the lowest edge above anything that could be damaged. Four numbers, one site visit. Suppliers will happily spec a machine from a photo, and that is how mismatches get shipped.
If the site is not accessible, ask for a drawing with the pitch marked in degrees rather than as a ratio. Ratios get converted wrong, and a 1:4 roof is 14 degrees while a 1:3 roof is 18, which is enough to change the cup spec.
What is the most overrated spec?
App connectivity. On a roof, the operator is usually within sight of the machine, and a remote with physical buttons is faster and works with gloves. App features matter on a large facade where the machine travels out of view; on a skylight they mostly add a firmware dependency.
What does setup time mean for labour cost?
Setup is the hidden labour line. Anchoring a tether, placing the bottom stop, positioning the machine on the first pane and checking cups takes 10-20 minutes. On a 40 minute roof job that is a third of the labour, and it does not shrink with machine quality.
Buildings that keep the anchors installed permanently cut that to a few minutes. Retrofitting anchors after the fact costs more than specifying them when the roof is built, which is an argument for involving the cleaning contractor during construction rather than after handover.
How do you match a machine to a glazed roof with a ridge?
Treat the ridge as a boundary, not a path. Machines that try to cross a ridge usually stall or lose seal, and the workaround is two separate passes from each side with the machine repositioned. That is slower, and it is the honest answer for almost every ridged glass roof.
Ask the supplier directly whether their machine is intended to cross a ridge. A vague answer means it is not.
What does a realistic daily output look like?
Plan on 150-250 m2 per operator day on a clean, simple roof with two charges and one filter change. A roof with many frames or heavy soiling can halve that. Quoting from the manufacturer’s peak figure is how schedules slip in the first week.
Key Takeaways
- 25 degrees is the number that decides most jobs, ahead of any feature on the box.
- Plan around 30 m as a practical limit; beyond it the machine stops being the right tool.
- Maintenance routines beat spec sheet comparisons over a two-year horizon.
- Test on one pane with the safety device fitted before committing a full roof.
- Compare against a rope-access quote before buying, not after.