Water above about 150 ppm total dissolved solids will leave mineral spots as it dries, no matter how good the brush is. In 2026, run skylight cleaning robots on filtered water below 50 ppm, or accept a final spot-free rinse as standard.
Every streak complaint we have chased came back to water, not technique. The machine was fine. The supply was not. Water quality is the cheapest upgrade in glass roof cleaning and the most ignored.
What does TDS actually mean on a glass roof?
TDS is total dissolved solids, measured in parts per million with a cheap handheld meter. Tap water in a hard-water city can run 250-400 ppm, mostly calcium and magnesium. As the film of water dries, those minerals stay put and form a faint white spot. On dark or coated glass the spots show badly.
| Water source | Typical TDS | Result on glass |
|---|---|---|
| Hard municipal tap | 250-400 ppm | visible spots |
| Softened tap | 80-150 ppm | faint marks |
| Carbon filtered | 40-90 ppm | mostly clean |
| RO or deionised | under 20 ppm | spot-free dry |
Do you always need reverse osmosis?
No, and pretending otherwise wastes money. If your roof is cleaned monthly and the glass is plain and light in colour, softened or carbon-filtered water at 80 ppm is often fine. Save RO for the cases that need it: coated or dark glass, infrequent cleans where dirt bakes on, and any roof where the client has a written spotless standard. An RO unit for a single machine is a real cost, so match the treatment to the glass.
How do you rinse without leaving marks?
The rule is simple: whatever dries on the glass is what stays. So either dry the glass mechanically with a squeegee follow-up, or rinse with water cleaner than the film already on the pane. A robot that scrubs with 300 ppm water and squeegees dry can leave fewer spots than one that rinses with 300 ppm water and lets it air-dry.
- Scrub with the main water supply, whatever its TDS.
- Do a final rinse with the cleanest water you have.
- Squeegee or dry-pad the rinse so nothing air-dries on the pane.
- Test TDS at the machine outlet, not at the tap; filters drift over time.
What about reclaimed or harvested rainwater?
Rainwater is soft, often 10-30 ppm, which makes it good for rinsing. The catch is biological load: pollen, bird matter and biofilm. Run harvested water through a 5-micron filter and a carbon stage before it reaches the machine, or you will spread organics across the glass and feed a green tint problem. On roofs with a greywater system, check the chemistry first; some greywater has surfactants that leave a film.
Where does water quality bite hardest?
Two places. First, desert and arid sites, where tap water is often 400 ppm and above; if that is your supply, budget for RO from day one. Second, sandy or dusty roofs, where the first pass makes a mud that dries into brown marks, so start with a lower-pressure pre-wet to lift grit before the scrubbing pass. Our notes on filtration and hard water cover the setup.
And do not forget the tank. A tank left with standing water grows biofilm and every clean then adds a faint organic film. Empty and rinse the tank weekly.
Who can ignore all this?
Anyone with soft municipal water under 100 ppm and a monthly clean cycle on plain glass. You still get better results from drying the glass, but you will not lose a contract over spots. Everyone else should own a TDS meter; they cost almost nothing and settle arguments instantly.
How do you test water on site?
A handheld TDS meter is the first tool in the bag. Test at the tap, then at the machine outlet, because a filter that is overdue will read close to the tap on the outlet side. Log the reading with the date so you can see the filter drifting before it fails. Anything above 150 ppm on the outlet is a spot risk on most glass.
For coated or dark glass, add a hard-water test on a small pane: clean it, let it air-dry, and look at a low angle the next day. A faint milkiness means your current supply is not clean enough for that facade.
How often should filters be changed?
Carbon and sediment stages are consumables, not permanent fixtures. In a hard-water area, change the sediment stage every few months and the carbon stage on the manufacturer’s interval, or sooner if TDS climbs. RO membranes last far longer but need a pre-filter and a periodic check, and a membrane run without a working pre-filter clogs fast and gets expensive.
What is the cheapest spot fix?
Dry the glass mechanically. Even with 200 ppm tap water, a squeegee or dry-pad finish leaves far fewer marks than an air-dry, because the minerals leave with the water instead of settling on the pane. It costs nothing but operator care, and on many roofs it is the difference between a clean glass and a spotted one. Filtering helps; drying proves it.
Key Takeaways
- Water above 150 ppm leaves mineral spots on drying glass.
- RO or deionised water under 20 ppm gives a spot-free dry.
- Match treatment to glass: RO for coated, dark or spotless-standard roofs.
- Rinse with your cleanest water and squeegee dry, never air-dry tap water.
- Empty and rinse the tank weekly to stop biofilm forming.