For skylight cleaning in 2026, use filtered water under 50 ppm TDS, or a neutral detergent at pH 6-8 diluted to 0.1-0.3%. Avoid alkaline builders, ammonia and abrasives. On robot cleaning, pure water for the first pass and a light neutral mix for the second gives the cleanest finish without residue.
You would not think solution choice mattered this much. It does. Most “robot leaves streaks” complaints that reach a supplier are a water or detergent problem, not a machine problem, and the answer is usually on the label of whatever was in the bucket.
Is pure water enough, or do you need detergent?
For routine maintenance on a robot-maintained skylight, yes. Pure water at low TDS carries the dust away and dries without leaving minerals behind. That is the entire logic of the window-cleaning water-fed pole industry, and it applies to roof glass too.
Detergent earns its place when the soiling is not dust. Bird droppings, traffic film, tree sap, and the grey organic film that builds on north-facing glass all need a surfactant to lift. A neutral detergent at 0.1-0.3% does it. Stronger is not better, and on a robot it is actively worse because the recovery tank cannot hold a heavy lather.
What pH is safe for sealed glazing units?
pH 6-8. That is the band glazing sealants, EPDM gaskets and the silicone edge seal of an insulating glass unit are designed to tolerate. Outside it, you are slowly eating the seal, and a failed IGU on a roof costs far more than the cleaning contract.
- pH below 4: attacks the frame finish and the sealant; never on coated glass
- pH 4-6: acceptable for brief contact on uncoated glass, rinse immediately
- pH 6-8: neutral band, the default for any repeat cleaning routine
- pH above 10: strips low-E coatings and dulls anodised aluminium over time
Two specific offenders. Ammonia-based glass cleaner, which is fine on a mirror and wrong for coated solar-control glass: it can attack the coating edge. And any product labelled “heavy duty” with an alkaline builder, which on a warm roof in summer reacts faster than the label implies.
Why does hard water ruin a skylight finish?
Because the water evaporates and leaves what was in it. Above roughly 200 mg/L calcium carbonate, a pure-water rinse leaves visible spotting. On a horizontal skylight that spotting sits in place for months, and on a dark tinted pane it reads as a white haze from inside the building.
A deionising cartridge brings TDS down to under 10 ppm and stops it. Cost is modest: a refillable DI vessel runs 100-300 USD and processes several thousand litres. Compare that to a professional acid-based mineral removal treatment on a 200 m² roof, which is a four-figure invoice and a job you should not attempt on your own.
How much solution does a robot actually use?
Less than people expect. A roof-class machine typically sprays 0.2-0.5 L per 10 m², so a 300 m² roof uses 6-15 L per cycle. That means a 20 L tank covers most residential and small commercial roofs without a refill.
| Roof glass area | Solution per cycle | Tank size needed |
|---|---|---|
| 60 m² atrium skylight | 1.5-3 L | 5 L is fine |
| 200 m² | 4-10 L | 10-15 L |
| 500 m² | 10-25 L | External feed recommended |
Over-wetting is a real failure mode. Too much solution and the recovery tank fills mid-run, the pad floats, and on a pitched roof the runoff becomes a slip hazard on whatever is below. Two lighter passes beat one heavy one every time.
Who should not use a wet robot system at all?
Buildings with glazing over electrical installations, server rooms or retail stock. Runoff management there is a containment project before it is a cleaning project. Likewise, a fragile heritage skylight with cracked putty and perished seals: adding water accelerates the failure, and the correct solution is a dry microfibre method with a lamp survey first.
And if your roof glass is old single-glazed wired glass, check before you spray anything. Water through a hairline crack in wired glazing gets into the wire mesh and rust stains appear within weeks, permanently.
What else should be on the cart?
A separate set of pads for the first (dirty) and second (finish) pass. A squeegee for the frame edges where a robot chassis cannot reach. Microfibre cloths for the ridge caps. And a TDS pen, which costs 15 USD and settles most arguments about whether the water is the problem.
For machine selection around all this, our skylight robot review archive covers tank capacity, spray control and recovery behaviour on each model, and the ECOVACS WINBOT W2S PLUS review is useful if you are considering a dual-tank portable unit for smaller glass roofs.
Does the season change the solution you need?
Yes, more than most maintenance schedules account for. Spring brings pollen, which is a protein-based film and responds to a neutral detergent far better than to pure water. Summer in a hot climate bakes mineral deposits onto warm glass within minutes, so a pre-rinse and a shaded working window matter as much as the mix. Autumn leaves tree sap and gutter overflow, both of which need a second pass with a fresh pad.
Winter is the awkward one. Below about 4 degrees Celsius, water on a roof can turn to a thin film of ice behind the robot’s track, and grip disappears without warning. Some operators switch to an alcohol-based winter mix, which works, and which also attacks some sealants if it sits. If you are cleaning a glass roof in winter, clean in the middle of the day, and keep the solution warmer than the glass.
Key Takeaways
- Filtered water under 50 ppm TDS is the baseline; hard water above 200 mg/L calcium causes permanent-looking spotting.
- Neutral detergent at pH 6-8, diluted to 0.1-0.3%, handles organic soiling without harming seals.
- Avoid ammonia, alkaline builders and abrasives on coated solar-control glass.
- Robot spray rates of 0.2-0.5 L per 10 m² mean most small roofs need no refill mid-cycle.