Skylight cleaning robots leave spots when dissolved minerals dry on the glass. A two-stage filter plus a clean-water rinse cuts visible spotting dramatically, but tap water above roughly 250 ppm hardness still leaves residue.
Owners blame the brush. The brush is rarely the problem. The problem is that the machine just pushed dirty water around and let it evaporate in the sun.
Why does filtered water matter more on a roof than on a wall?
Roof glass dries fast. It catches sun all day, sits above building heat, and holds a film of water after the machine leaves. Vertical facade glass sheds water quickly; horizontal skylights hold it. Every milligram of calcium in that film stays behind when the water goes.
That means a machine that performs fine on a wall can produce visible spotting the first week it runs on a flat skylight. The water spec has to match the surface.
What filtration do you actually need?
At minimum, sediment filtration down to 5 microns to keep grit out of the brush and off the glass. Better setups add a carbon stage for organics and a deionising resin stage for total dissolved solids. Field experience suggests you want final rinse water below 100 ppm TDS, and ideally under 20 ppm for a spot-free finish.
| Water stage | Typical TDS | Result on skylight glass |
|---|---|---|
| Raw tap, hard region | 250-400 ppm | Visible spotting within minutes |
| Sediment + carbon | 150-250 ppm | Fewer spots, still a haze |
| Add DI resin | Under 20 ppm | Clean finish when squeegeed |
Does the machine need a separate rinse stage?
Yes, if you care about the finish. A single-brush machine that scrubs and lifts in one pass leaves a dirty film. Two-stage machines that scrub with recycled water and then rinse with clean water leave far less residue, and the difference is obvious in direct sun.
This is also why tank discipline matters. If the rinse tank is topped up from a hose that has been sitting in the sun, you have reintroduced bacteria and minerals. Fill rinse tanks from the filtered line only.
What are the real risks of getting this wrong?
Spots are cosmetic. Scratches are not. Grit carried in unfiltered water, or a brush that has picked up sand from a roof walkway, will abrade coated glass. Low-E and self-cleaning coatings are thin, and a scratch is permanent.
The second risk is biological. Standing water in a tank left on a roof through summer breeds biofilm; the machine then smears it across 2,000 m2. Drain and dry tanks after every cycle. Every time.
Who can skip filtration
Sites with genuinely soft water, under 80 ppm, cleaning low-visibility industrial skylights where nobody inspects the finish. In that case a sediment filter is enough and DI resin is wasted money. If your glass is on a mall atrium above a food court, skip nothing.
For machines in the Lingkong K3 and Lingyun Y3 class, check the tank and filter layout before you buy, because retrofitting a rinse stage is more expensive than specifying it. Our contact page can help match a machine to your water situation. Related reading: pads, brushes and filter maintenance.
How do you check results without getting on the roof?
Use the machine’s own pass log and a camera on a pole. Photograph the same three fixed points after every cycle and compare. If spotting appears on the south-facing run first, your rinse water quality has drifted. This beats climbing onto a 15 degree skylight to inspect by eye, and it builds a record that is useful when a tenant complains.
Does water temperature matter?
More than people expect. Hot water in a tank on a hot roof can leave more residue as it evaporates fast, and cold water on cool glass cleans less effectively on grease. Ambient-filtered water is the safe default. If you are chasing a spot-free finish on a visible atrium, keep the water out of direct sun rather than heating or chilling it.
What should be in the maintenance schedule?
Daily: inspect pad skirts and empty the tanks. Weekly: check the brush for embedded grit and rinse the filters. Monthly: verify the vacuum alarm with a deliberate pressure drop near the floor, and check the tether and its anchor. Quarterly: review the route file against the roof, and replace any pad set that has hardened or glazed. A schedule this simple, actually followed, prevents almost every failure we see in the field.
What does a typical cleaning cycle look like?
A full cycle is more than the time the machine spends cleaning. It is the walk to the roof, the water fill, the boundary setup, the run itself, and the pack-down. On a well-mapped roof the setup is 20 minutes and the run dominates. On a difficult roof with restricted access it can be 45 minutes before the machine lays down its first pass. Budget the whole cycle, not the pass, when you plan a shift.
How does glass type change the brush and pad choice?
Coated glass, laminated units and textured or fritted panels all behave differently under a rotating brush. Fritted glass wears pads faster and needs lower brush pressure. Coated low-E glass scratches easily, so a clean pad and a fine brush are not optional. Textured or prismatic glazing traps dirt in the pattern and often needs more passes, which quietly doubles your clean time on those sections.
Key Takeaways
- Spots come from dissolved minerals drying on horizontal glass, not from the brush.
- Target rinse water under 20 ppm TDS for a spot-free finish on visible skylights.
- A separate clean-water rinse stage matters more on roofs than on walls.
- Unfiltered grit scratches low-E coatings permanently.
- Drain and dry tanks after every cycle to prevent biofilm.