Window cleaning is the only exterior maintenance on a building that comes round several times a year. A roof has its moss removed every few years, a rendered facade is cleaned every five to ten years, but a glass head office is often washed two to four times a year, sometimes monthly for shop windows and lobbies. This frequency changes the whole economics of access: every MEWP set-up, every rope-access descent, every commissioning of a suspended platform is a cost that recurs. Drone window cleaning changes that arithmetic, but only up to a point.
That is why glazing is the first market targeted by several makers of cleaning drones, from the Lucid Bots Sherpa to the heavy systems developed for towers. The argument is appealing. But glazing is not a rendered facade: you do not apply a biocide to it for the rain to rinse off later. What you want is a streak-free result, immediate and visible close up. That demands a precise technique, pure water, and it is this technique that determines what a drone can really do.
Window cleaning: why tap water leaves marks
A drop of tap water drying on a pane leaves behind everything it held in solution: calcium, magnesium, silica, sodium. In much of France and central Europe, mains water contains several hundred milligrams of dissolved salts per litre. Every droplet mark becomes a small mineral deposit, clearly visible against the light. That is why the traditional window cleaner uses a washer and a squeegee: he removes the water before it dries.
A drone cannot handle a squeegee. It sprays or rinses, then moves on. For the glazing to dry without marks, the water must therefore be stripped of its dissolved salts. This is the principle of pure water, produced by reverse osmosis and polished on ion-exchange resin, down to a dissolved salts content close to zero, often measured below 10 ppm. This water has a useful property: being very low in minerals, it actively dissolves dirt and dries without leaving any residue. The same principle applies to photovoltaic panels, as explained in our article on drone solar panel cleaning.
The technique was not born with drones. It has been used for years with water-fed telescopic poles, such as those from Unger, which make it possible to clean glazing up to around twenty metres from the ground. The drone extends it to heights that the pole can no longer reach.
It is still important to understand what pure water does not do. It does not replace mechanical action: a pole is fitted with a brush that scrubs the glass, and it is this scrubbing that loosens greasy dirt and old deposits. A drone that merely sprays pure water rinses off dust and pollen well, but greasy films of urban pollution and bird droppings much less well. Serious systems therefore often combine a pre-wash, a contact time and a final pure-water rinse.
The sensitive points of a glazed facade
Joints and structural glazing
Many modern curtain walls use structural glazing (VEC, vitrage extérieur collé, the French term for bonded external glazing): the glass is held to the frame by a bead of structural silicone sealant, with no visible glazing bead. This joint genuinely carries the weight and wind loads of the glass. It is designed to withstand the weather, but not necessarily a high-pressure jet aimed at its edge for years. Perimeter sealing joints and expansion joints also age: they harden, shrink and lose their adhesion.
The physics is the same as for the windows of a house. A water jet generates at the point of impact a pressure out of all proportion to that of the driving rain against which the facade was tested. Our article on drone insurance and damage explains why a window that has never leaked in a storm can let water in under a jet, without having suffered the slightest damage. On a curtain wall, the rule is simple: low pressure, sufficient distance and never a jet aimed into the joints.
Self-cleaning glass
Some glazing carries a photocatalytic titanium dioxide coating on the outer face, such as Pilkington Activ™. Under UV light, this coating slowly breaks down organic dirt and makes the surface hydrophilic: rainwater spreads out as a film and carries residues away. These coatings are thin and can be damaged by abrasive products, some detergents or silicone sealants deposited on the glass. Self-cleaning glass is not cleaned like ordinary glass, and the manufacturer publishes its own maintenance recommendations, to be consulted before any work.
Coated glass and laminated glass
Low-emissivity coatings and solar control coatings are generally placed inside the double glazing, where they are protected. But some solar control or decorative glass has a coating on the outer face, more sensitive to alkaline products and abrasion. Before any cleaning, you need to know which face carries what.
Frames and cover caps
The anodised or powder-coated aluminium cover caps on curtain walls are sensitive to strong alkaline products, and runoff from a poorly rinsed chemical pre-wash can leave marks. Pure water alone does not pose this problem.
Access methods for glazing at height
To compare honestly, you need to know the existing methods.
The pure-water-fed pole. Fast, inexpensive, no work at height. It is effective up to around twenty metres, beyond which the weight and flex of the pole make it difficult to handle. It is the benchmark method for low-rise buildings.
The mobile elevating work platform (MEWP). It allows traditional squeegee washing, with a perfect result. But it occupies the pavement or the road, must be constantly repositioned, and cannot reach facades that are set back or above obstacles. The INRS (in French), France’s national occupational health and safety research institute, sets out the conditions of use for these machines.
The suspended platform or BMU. Many towers are fitted with a permanent facade maintenance installation, a trolley on the roof that lowers a cradle along the glazing. These suspended access platforms fall under standard EN 1808, and their use is regulated (in French) by inspection and training rules. It is the most productive access on a large equipped building, but the installation is expensive and requires its own maintenance.
Rope access technicians. Rope access work makes it possible to reach almost any facade, but the French Labour Code reserves it for situations where collective protective equipment is not possible and subjects it to precise conditions, notably double anchorage and the presence of a second person (in French). It is a flexible method, but slow, and safety depends entirely on the quality of the anchorages and the training.
The drone. It eliminates work at height and prolonged occupation of the ground. It is fast on large flat surfaces. But it does not scrub, or only a little, and it cannot squeegee.
Drone window cleaning: what you can reasonably expect
Effective rinsing of light soiling
On regularly maintained glazing, where the soiling is mostly dust, pollen and rain marks, a pure-water rinse by drone gives a satisfactory result, comparable to that of a pole without vigorous brushing. This is the most solid use case: frequent maintenance between two more thorough traditional washes.
A poorer result on ingrained soiling
On an urban building beside an expressway, greasy films of combustion particles do not come off with water alone. On glazing neglected for years, limescale deposits and streaks from joints or cover caps require mechanical action, or even specific products. The drone is then only one step, not a complete solution.
Flow rate and endurance that depend on the ground supply
A drone that carries its own water is limited to a few litres per flight, which is not enough to rinse a large glazed area. Professional systems are therefore supplied from the ground through a hose connected to a pure-water production unit, often also by an electrical cable that removes the battery constraint. The maximum height then depends on the length of hose, the pressure available and the mass the aircraft can carry. High-rise towers remain the domain of BMUs and rope access technicians.
A safety perimeter incompatible with some sites
A cleaning drone falls under the Specific category of the European regulations and requires a controlled ground area beneath the facade being treated. On a city-centre tower bordered by a very busy pavement and a traffic lane, this area may be impossible to set up during the day. Work then moves to early morning or the weekend, which has a cost.
Glazing and privacy: an issue specific to office and residential facades
A drone cleaning glazing holds position one or two metres from the windows, with an on-board camera. Behind those windows are offices, meeting rooms, sometimes homes. The images captured are personal data within the meaning of the GDPR as soon as they make it possible to identify people. The company must inform the occupants in advance, limit capture to what is necessary for piloting and documenting the job, and set a retention period. Our article on drones, privacy and neighbours details these obligations. For offices, informing the occupant’s security and compliance departments prevents a flight from being mistaken for an attempt at espionage.
Cost comparison for an office building
The following figures are expressed in the reference unit common to our articles, one drone cleaning pass on a reference house. Scenario: a six-storey office building, around 2,500 m² of external glazing, with no BMU. These are indicative ranges for a complete wash.
| Method | Cost per wash (× unit) | Time on site | Quality of result |
|---|---|---|---|
| Pure-water pole (up to ~20 m) | 1.5–3× | 2–4 days | Good, with brushing |
| MEWP + squeegee | 5–9× | 5–10 days | Excellent |
| Rope access technicians | 4–8× | 5–8 days | Excellent |
| Pure-water drone | 2–4× | 1–2 days | Good on light soiling |
The conclusion can be read in the last column. The drone is competitive on price and unbeatable on duration, but its result depends on the condition of the glazing. Many property managers adopt a mixed strategy: one complete traditional wash per year, and drone rinses in between, where the building’s image matters.
How often should a building’s glazing be cleaned?
The frequency depends on the surroundings and the use. An office building in a dense urban area, beside a heavily trafficked road, builds up greasy films of particles within a few months. A building on the outskirts, surrounded by green spaces, gets dirty mainly with pollen and dust, which are easier to rinse off. Shop windows and entrance lobbies, seen close up by visitors, justify much more frequent passes than the upper floors.
Frequency has a direct bearing on the method. The closer together the passes, the lighter the soiling at each visit, and the more a simple pure-water rinse is enough. That is what makes the drone economically attractive on glazing: it is not the best tool for rescuing neglected glazing, but it is well suited to keeping already clean glazing clean.
Drone window cleaning: when to choose it
The drone suits medium-height buildings whose glazing is maintained regularly, facades out of reach of the pole and not fitted with a BMU, sites where long-term occupation by a MEWP is a problem, and frequent maintenance programmes where speed of intervention matters more than the perfection of each pass.
It is poorly suited to heavily soiled glazing, self-cleaning glass or glass coated on the outer face without the manufacturer’s approval, curtain walls with ageing joints, towers fitted with a BMU in good condition, and sites where no safety perimeter can be established during working hours. In every case, ask for the dissolved salts content of the water used, the pressure at the nozzle, the working distance, and how the company will handle the images captured in front of the windows.
