Quick answer: A robot window cleaner cannot clean: deeply textured or patterned glass (the suction seal can't form), cracked or loose panes (never risk it), curved glass, leaded/stained glass, panes smaller than the robot's own footprint, and glass so caked in grit that a damp pad would grind it around. Smooth frosted (acid-etched) glass, tinted glass, coated low-E glass and frameless edges are all fine. Knowing the limits is how you avoid buying the wrong tool — or blaming the right one.
Any robot cleaner brand will tell you what their machine can do. Here's the list they bury: what suction robots — ours included — genuinely can't clean, and why.
1. Deeply textured and patterned glass
Suction needs an airtight seal, and a deep texture is a thousand air channels. Victorian patterned glass, "rain" and "flemish" obscured styles, and heavily hammered glass all break the vacuum — the robot alarms and won't grip, exactly as designed.
The nuance: smooth satin and acid-etched frosted glass usually works fine, because the etching is microscopic and the seal still forms. Rule of thumb: run a fingernail across it. If the texture is deep enough to feel as bumps and valleys, it's hand-clean territory.
2. Cracked, chipped or loose glass
Never put suction on damaged glass. A vacuum robot applies real force (the CleanBot R1 pulls 6,500Pa), and a crack concentrates that stress. The same goes for panes rattling in perished putty on old frames. Fix the glazing first — this rule protects you, not just the robot.
3. Curved glass
Suction pads are flat; curved orangery glass, bowed shopfronts and porthole windows can't seal. No exceptions among suction robots.
4. Panes smaller than the robot
Georgian-divided sashes, leaded lights, door vision panels: if the uninterrupted glass is smaller than the robot's footprint, it physically can't sit on the pane. The UK window-type compatibility guide covers which British styles this rules out (fewer than you'd think).
5. Leaded and stained glass
Even where panels are large, the raised lead cames make the surface a relief map, and antique glass is both soft and irreplaceable. Museum-grade hand cleaning only: soft brush, deionised water, patience.
6. Builder-grade filth (at first)
Honest one: a robot with a damp microfibre pad is a maintenance cleaner, not a demolition tool. Glass coated in gritty construction dust, paint specks or years of crust should get one manual restoration clean first — otherwise any pad (robotic or human) is just sanding the grit around. New-build owners, your specific guide is here. After that first reset clean, the robot maintains the result indefinitely.
What surprisingly IS fine ✅
- Tinted, low-E coated and self-cleaning glass — coatings live inside the unit or are hard pyrolytic layers; a microfibre pad doesn't touch them chemically.
- Frameless edges — edge detection (part of the R1's 8-layer safety system) stops the robot at open edges.
- Mirrors, shower screens, glass partitions, solar panels — smooth flat glass is smooth flat glass.
- Vertical, sloped and fully overhead glass — suction doesn't care about gravity's direction.
FAQ
What happens if I try it on textured glass anyway?
Nothing dramatic — the robot fails to build vacuum and alerts you rather than starting. The safety rope is your backstop during any experiment.
Can it clean glass with stick-on films or decals?
Intact professional window film: generally yes, gently. Bubbling, peeling film: no — the pad will lift the edges further.
Internal glass doors?
If the glass panel is bigger than the robot, yes — though most people find internal doors quicker by hand and save the robot for the glass they can't reach.
If your glass is on the ✅ side of this list, the rest of the buying decision is just arithmetic: the honest cost breakdown is here.