A skylight cleaning robot uses about 15 to 25 litres of water per 100 m2 for a standard wet pass. With a two-stage wash and recycling on board, that can drop to 7 to 10 litres per 100 m2, which matters where water restrictions cap daily use.
Most buyers ask about suction force and speed. Almost nobody asks about water until the first summer under a restriction order, and by then the machine is already on the roof.
How much water does a wet pass really need?
The physics are simple. You need enough water to dissolve dust, carry it off the glass and then rinse the residue. Cutting flow too far leaves a thin film that dries into spots, which is worse than not cleaning. The trick is separating the wash from the rinse.
| Method | Litres per 100 m2 | Spotting risk |
|---|---|---|
| Single-pass wet clean | 15-25 | High on hard water |
| Two-stage wash + rinse | 10-14 | Low |
| Two-stage with recycle | 7-10 | Very low |
| Dry microfibre only | 0 | Dust re-deposits |
Does deionised water pay for itself?
On south-facing glass in a hard water area, yes, usually within the first year. Untreated mains water at 180 ppm hardness leaves mineral haze that tenants read as dirt. The acid wash needed to remove it costs more than a season of resin and filters. A Lingfeng S1 with a small deioniser unit solves this on site without a tanker.
What changes on a drought-restricted site?
Clean less often but more thoroughly. Stretching the interval and doing an aggressive high-solids wash uses more water per event but less per year. The alternative, frequent light rinses, wastes water removing almost nothing.
- Clean at first light to cut evaporation loss before the glass heats.
- Use a reclaim mat or on-board recycle where available.
- Never rinse a hot roof; the water flashes off and leaves the minerals behind.
- Log litres per clean so you can defend your allocation at renewal time.
Who should worry about this, and who should not?
Sites in southern Europe, the Middle East, Australia and parts of western China should design around water from day one. Sites in temperate, wet regions mostly should not; rain does part of the work and water is cheap. The risk of over-engineering is real, since recycle systems add weight and maintenance. Do not buy recycling if you clean twice a year in a rainy climate.
Can you clean glass properly with almost no water?
Partly. Dry microfibre removes loose dust well and uses nothing, but it leaves the fine film that makes glass look grey under bright light. The honest answer is that water does real work, and trying to remove it entirely produces a visibly worse result. What you can do is use it more cleverly: small volumes, applied where they lift dirt, then removed before they dry.
That is the logic behind two-stage washing. The first pass wets and loosens at low volume with detergent. The second pass rinses with clean deionised water and a squeegee. Because the detergent does the lifting, you need far less clean water to finish, and the squeegee removes it before minerals can dry on the pane.
What does a recycling system actually recover?
A well-designed on-board recycle loop can recover 40 to 60 percent of the rinse water, filtering out particles and reusing it for the wash stage. It never gets to zero, because some water leaves as run-off and some is lost to evaporation. The gain is real on a large roof where tank refills cost time, not only water. The trade is extra weight and another filter to maintain.
- Recycle is worth it on roofs over 8,000 m2 and on restricted-water sites.
- Skip it on small roofs; the maintenance burden exceeds the saving.
- Always keep a clean-water tank separate for the final rinse.
How do you prove water savings to a client?
Meter it. Record litres drawn per clean, glass area per clean, and refills required. After four cleans you have a defensible litres-per-100-m2 figure that you can put in a sustainability report or use to argue for a larger water allocation. Sites that do this are the ones that never get caught short when a restriction lands mid-season.
Does water quality matter as much as water volume?
More, on glass. Two litres of clean deionised water do more than ten litres of hard mains water, because the clean water removes residue instead of leaving it. Total dissolved solids above about 50 ppm start to show as spotting on a sunny pane, and above 150 ppm the roof looks worse after cleaning than before. Test your water source before you design the schedule; it changes the whole plan.
Rainwater and recycled rinse water vary widely. Rain collected off a dirty roof is not clean, whatever it looks like. Treat every non-mains source as unknown until you have measured it, then decide whether filtration is enough or whether you need full deionisation.
Can rainwater harvesting clean glass roofs?
On some sites, yes, with constraints. Collected rainwater is soft, which is good, but it carries whatever it touched on the way down. Rain from a clean metal roof, filtered and dosed correctly, can rinse glass well. Rain from a roof with dust, bird droppings or industrial fallout needs heavy filtration before it touches a pane. The saving on water is real, but only after the treatment cost.
The practical approach is to treat harvested water as a wash-stage supply and keep a small clean-water reserve for the final rinse. That splits the risk: any residue in the harvested water is removed by the clean rinse, and the roof still gets the benefit of a low-cost first pass.
Key Takeaways
- Expect 15-25 litres per 100 m2 wet, down to 7-10 with two-stage plus recycle.
- Deionised water usually pays back within a year on hard-water glass.
- On dry sites, clean less often but thoroughly rather than rinsing frequently.
- Clean at first light to reduce evaporation losses.
- Log your water use to justify allocation renewals. See specs via our water systems guide.