Keep routine skylight robot cleaning between pH 6 and pH 8. Anything outside that range risks low-E coating damage. Hard-water minerals need pH 3 to 4 short-contact acid, and only after you confirm the glass is uncoated or acid-tolerant.
Why does pH matter more than brand?
Marketing on a detergent bottle tells you nothing about what the product does to a coating. The number that matters is pH, and the second is contact time. Low-E coatings are thin metal-oxide layers measured in nanometres. A strong alkaline cleaner at pH 11 lifts that layer over repeated passes and the glass loses its heat rejection. A strong acid etches the surface if it sits too long. Routine cleaning detergent should sit near neutral, pH 6 to 8, and should never be left to dry on the glass. Rinse it off and the risk drops sharply.
| Task | Target pH | Contact time | Coated glass OK? |
|---|---|---|---|
| Routine wash | 6-8 | Normal | Yes |
| Hard-water / cement | 3-4 | Under 5 min | Only if confirmed |
| Bird droppings | 6-8 | Short | Yes |
| Heavy alkaline grime | 8-9 | Short | Check supplier |
What pH for hard water and cement haze?
Hard-water spotting and cement residue are alkaline deposits and often need acid to dissolve, typically pH 3 to 4, applied with short contact time and an immediate rinse. This is a specialist pass, not your routine fluid. Before using it, confirm with the glass supplier whether the pane is coated, and get the maximum contact time in writing. On coated glass, an acid pass done wrong leaves a permanent dull patch that looks like a smudge but never wipes off. If you cannot confirm the coating, use a mechanical pad and neutral fluid instead.
How do you test a new detergent safely?
Do not test on the roof. Take two spare panes of the same product, or clean an inconspicuous corner, and run the detergent at working dilution. Leave it for double the intended contact time, rinse, and inspect under raking light from both sides. Check for hazing, colour shift and coating loss. Repeat ten cycles to simulate a season. If the glass looks unchanged, the product is safe for your building. This twenty-minute test has saved owners from tens of thousands in reglazing, and it costs almost nothing.
What damages coatings even at neutral pH?
Two things. First, abrasive pads and gritty brushes scratch the coating mechanically regardless of chemistry, which is why pass-one dry grit removal matters. Second, letting even neutral detergent dry on hot glass leaves a residue that bonds to the coating. Clean early in the day on west-facing panes, rinse generously, and never run a robot that recycles water without filtration, because dissolved minerals concentrate and re-deposit. Check the water source too. Municipal water at 300 ppm hardness acts like a mild alkaline cleaner all by itself.
What water quality should feed the robot?
Hardness is the quiet saboteur. Water at 300 ppm leaves mineral film even with a neutral detergent, because the minerals stay behind when the water evaporates. A simple inline filter or a softener fixes most of it, and it costs far less than the repeated cleaning needed when you skip it. On coastal or limestone-area buildings, test the water before the first cycle. Operators who blame the detergent for spotting often find the real cause in the supply line. Fix the water and the detergent works as advertised.
Are eco-labelled cleaners worth it?
Sometimes, but read the pH, not the leaf logo. A biodegradable neutral cleaner at pH 7 is a good default for routine work on coated glass. An eco label does not guarantee a safe pH range, and some green cleaners run alkaline enough to threaten a low-E coating over many cycles. Check the technical data sheet for pH and coating compatibility, and run the same ten-cycle glass test you would run on any product. Sustainability and glass safety are separate questions, and only one of them is printed on the front of the bottle.
Related reading: cleaning frequency and schedule guide, and the robot vs manual crew safety and cost study.
Key Takeaways
- Routine robot cleaning detergent should stay between pH 6 and pH 8 to protect low-E coatings.
- Acid passes at pH 3-4 need written supplier approval, short contact time and an immediate rinse.
- Test any new detergent on spare coated panes through ten cycles before touching the roof.
- Abrasive pads scratch coatings mechanically, and recycled unfiltered water re-deposits minerals.
How often should you replace the water filter?
Follow the run hours, not the calendar. A filter on a dusty industrial roof clogs in a fraction of the time it lasts on a clean urban atrium. Check it monthly and replace when flow drops or the element discolours, whichever comes first. A clogged filter is not just an efficiency loss: it lets minerals through to the glass, which is exactly the spotting problem you installed it to prevent. Log the replacement hours and you will find a predictable interval for your site.
Mildly alkaline process water and high-hardness municipal supplies both shorten filter life. Test the supply hardness once and set the replacement interval from the result, rather than guessing.
How do you store cleaning chemicals on site?
Store them sealed, labelled and out of sun, and never decant into unmarked bottles. A mislabelled container is how an acid ends up in a routine wash and a coating gets ruined. Keep the technical data sheets in one folder at the roof access point so an operator can check pH before mixing. Rotate stock so old product is used first, and dispose of anything past its date. Chemical handling looks like admin until the day it damages a pane, then it becomes the whole story.