When people talk about the risks of drones, they instinctively think of the dramatic scenario: an aircraft that loses power and crashes onto a car, a conservatory or a person. That case is rare and, when it does happen, liability is generally clear. The disputes that take up the most time on building cleaning jobs are quieter. A tile is found cracked the week after cleaning, but was it already cracked before? A stain appears on the ceiling below a window that has leaked in every storm for three years, but is the cleaning the cause this time? A glass balustrade has a haze that will not polish out, but is it the product, the water or ten years of weather? That is where drone insurance earns its keep, or fails to.
Insurance pays out on the basis of causation, and causation is established through evidence. Yet the evidence that settles these disputes is gathered before the aircraft takes off. A generous cover limit is no help to an owner who cannot show the condition of their building before the work. This article sets out the cover that exists, the physical mechanisms by which cleaning causes damage, and the three practices that make it possible to settle a claim: a pre-flight inspection, a complete record and a written condition report.
Drone insurance: the cover that exists and its gaps
In Europe, third-party liability insurance for drone operators falls under Regulation (EC) No 785/2004 and national law, and how it applies to small drones has varied between countries and over time. In France, the official application form for an operational authorisation has the operator declare that the corresponding insurance cover, if required, will be in place; this authorisation from the DGAC (France’s civil aviation authority) is required for most cleaning flights in built-up areas. For a client, the safest course is to ask for the insurance certificate and read it, rather than rely on a general rule. Most reputable companies also take out professional liability insurance covering defective workmanship: it covers damage caused by the way the work was carried out, as opposed to damage caused by the aircraft as a vehicle.
These are separate policies, with different wordings, and it is in the gaps between them that claims fail. Three of these gaps are worth understanding.
Damage to the property being worked on. Many general liability policies exclude or cap damage caused to the property the insured is working on at the time of the loss. They were designed so that a company’s insurer does not become the guarantor of its own work. Yet, for a cleaning job, the property being worked on is precisely the building most likely to be damaged. Look for an explicit clause including damage to the customer’s building during the work, and check for any sub-limit.
The aircraft or the product. Third-party liability insurance linked to drone operations generally applies to damage caused by the aircraft: impact, fall or debris. Damage caused by the sprayed product may be excluded from it. Overspray on a neighbour’s car, discoloured plants or etched glazing then fall under a general policy or workmanship cover, which may itself carry a pollution or chemicals exclusion. Sometimes these policies only cover ‘sudden and accidental’ releases and exclude gradual effects. Drift during a cleaning pass is arguably sudden, but that argument takes place after the damage, not before.
The work itself. Workmanship cover generally pays for damage caused by defective work, but not the cost of redoing it. If an unsuitable product leaves streaks on a facade, the policy may cover restoring the facade, but not a second cleaning pass.
One last point: some countries impose long-term guarantees on construction work, others do not. Find out what applies locally and check whether cleaning or surface treatments fall within it.
Drone roof and facade cleaning: four damage scenarios and their physics
The scenarios below are physical, not legal, which is why they apply in every country. Each one includes a mechanism, a typical pre-existing condition that blurs causation, and an order of magnitude of the financial exposure. This is expressed as a multiple of the reference unit, namely one drone cleaning pass on the reference house. The ranges are indicative. They depend heavily on access, materials and local labour costs, and are intended to give an order of magnitude, not to predict a specific claim.
Cracked tiles and slates
Mechanisms. The feed hose of a tethered cleaning drone is a heavy line. When full, it weighs in the order of 0.2 kg per metre. If it rubs on a ridge or the edge of a slate, it applies a concentrated load to a fragile, thin-edged element. A water jet on the edge of a tile exerts a force proportional to the mass flow rate multiplied by velocity: the higher the pressure, the greater the load, precisely where tiles are weakest.
Pre-existing conditions. Clay tiles subjected to years of freeze-thaw cycles have microcracks. Frost resistance is exactly what the EN 539-2 (in French) standard tests, because that is how clay tiles deteriorate. Thick cushions of moss hide cracks and lift tile edges. When the moss dies and is rinsed off, cracks that were already there become visible for the first time. Old fibre cement slates are a separate problem. On buildings that predate the EU-wide asbestos ban, in the mid-2000s, they may contain asbestos. High-pressure cleaning can release fibres: their composition must therefore be established before any cleaning, independently of any insurance question.
Exposure. Replacing a few tiles generally represents a fraction of the reference unit. The cost rises when the model is no longer made and a matching batch has to be found, or when the roof pitch requires specific access equipment.
Damaged seals and gaskets
Mechanisms. Window seals, glazing gaskets and perimeter sealants age under UV and temperature cycling. They harden, shrink and lose adhesion over a service life commonly estimated at 10–20 years depending on the material, a figure to be treated as indicative. A close-range jet can lift a hardened gasket, open a debonded sealant joint or drive water into the edge of a double-glazed unit. Chemistry can also play a part: oxidising cleaners, such as hypochlorite solutions (in French), degrade some elastomers faster than others. The product documentation should state its compatibility with seals, which it often omits.
Pre-existing conditions. Failure of the edge seal of a double-glazed unit shows up as condensation inside, often several weeks after moisture has got in. A unit that mists up a month after cleaning may have been damaged by it, or may have been failing for a year.
Exposure. This is where claims become disproportionate. A single large cracked or failed glazing unit generally costs several times the reference unit once made to measure, delivered and installed, especially above ground floor, where installation requires its own access equipment. As an indication, that is around 2–5×, and more for laminated, coated or oversized units. In other words, a single damaged window can cost more than the entire cleaning job, and that is the reason to check the insurance certificate.
Water ingress through an existing defect
Mechanism. This is the scenario owners find hardest to accept, and the one physics explains most clearly. Windows and facade elements are designed to withstand driving rain, and the EN 12208 (in French) standard classifies the watertightness of windows by subjecting them to water spray under a pressure difference. Test levels range from zero to several hundred pascals for the best-performing classes. A 10 m/s wind produces a dynamic pressure of about 60 Pa (½ × air density × velocity²).
A cleaning jet is in a completely different league. The stagnation pressure of a water jet is ½ × water density × velocity², and water is about 800 times denser than air. Even after spreading out over a metre of travel, a low-pressure jet can load a seal at several thousand pascals, and a high-pressure jet far more. A window that has never leaked in a storm can therefore let water in under a jet without having suffered any damage at all. The water finds a path that the design never had to resist. On roofs, the same mechanism drives water upwards and under tile laps, flashings, the mortar fillets of chimney stacks and failed sealants around roof penetrations.
Pre-existing conditions. Almost always present. The typical water ingress claim combines an already existing defect, such as a cracked flashing, an open joint or a slipped tile, with a jet that found it.
Exposure. Water inside a building is the most variable exposure: damaged plaster, floor coverings, insulation and electrical installations, plus the cost of drying out. It can range from about one reference unit to many multiples of it. This is also the case where causation is most disputed. The company’s insurer will argue that the defect already existed. The building’s insurer may exclude gradual deterioration and damage caused by a contractor. Without prior records, the dispute has no solid starting point.
Marked panels, glazing and finishes
Mechanisms. High-pH cleaners attack the silicate network of glass. Hypochlorite solutions are generally strongly alkaline, and prolonged or concentrated contact, particularly on hot glass where the solution evaporates and concentrates, can leave permanent etching. Anodised aluminium is also sensitive to alkaline attack. Aged powder coatings may show patches from unevenly removed chalking. Hard water dries leaving mineral marks. Runoff carrying copper or iron can stain light-coloured stone and render. The anti-reflective coatings of photovoltaic modules can be damaged by chemicals or abrasion.
Pre-existing conditions. Aged coatings, old etching from previous cleanings and manufacturing variations in finishes make any before/after comparison difficult without good photographs.
Exposure. Restoring glazing or refinishing panels generally represents anything from a fraction of the reference unit to a few multiples, and replacement costs more. Overspray on a car parked just outside the safety perimeter can on its own cost about one reference unit if a body panel has to be resprayed. This figure is indicative and depends on the vehicle and the damage.
Drone cleaning: the three practices that make it possible to settle a claim
1. A pre-flight inspection
The inspection must cover every surface that will be sprayed and every surface the spray could reach. That includes neighbouring properties, vehicles in the drift zone and glazing below the work area. A close-range inspection flight with a zoom camera documents the condition of the roof better than anyone can from the ground, and a walk around the building makes it possible to check seals, flashings and finishes at eye level. Occupants should be asked about known leaks, because interior stains are part of the condition report.
The inspection must lead to decisions, not just photos. Areas with known defects are excluded, treated at reduced pressure or protected beforehand, and these decisions are recorded in writing.
2. A complete record
Record the job, not just the result:
- Flight logs: position, height and timestamps, which modern flight controllers record as standard.
- On-board video of the spraying: where the jet hit, for how long and from what distance.
- Application parameters: product, dilution, pressure and flow rate at the nozzle, read from the pump unit where possible.
- Weather conditions at the time of the job: wind at working height, temperature and rain, which affect drift, contact time and seal behaviour.
- Before/after photos taken from the same viewpoints, with timestamps and location data.
A one-word-against-another dispute can then be settled by the footage of the window concerned at the minute concerned.
3. A written condition report
Both parties should sign the inspection findings before work starts: defects observed, areas excluded, precautions agreed. A short equivalent report drawn up afterwards records what has changed. This single document protects both parties more than any policy wording. It protects the owner by fixing the condition of the building before the work, and it protects the company against being accused of causing defects that were already there.
Limits and trade-offs of drone facade cleaning
An inspection does not see everything. A cracked underlay beneath sound tiles, a failed interior seal or a flashing hidden under render will not show up in any photo. The condition report should state its own limits, that is, what was not inspected, so that its silence is not later interpreted as a certificate of good condition.
Documentation cuts both ways. A complete record can show that damage was caused by the work. That is the whole point, and a company reluctant to record is sending you a signal.
Subcontracting blurs cover. If the company signing the contract is not the one flying the drone, the relevant policies may be the subcontractor’s. The certificate should name the entity actually present on site, or the main company’s cover should explicitly extend to subcontracted work.
Low pressure reduces the risk without eliminating it. Low-pressure cleaning greatly reduces the jet forces behind water ingress and seal damage. The chemical risks to glass, metals and coatings remain, as does drift.
The claims procedure matters. Report the damage promptly, photograph it and, apart from urgent measures to stop it getting worse, avoid any permanent repair before the insurer’s loss adjuster has visited. A repaired defect can no longer be examined.
Drone insurance in practice: what to check before signing
Before signing, ask for the insurance certificates and check five points on them:
- the named insured matches the company present on site,
- the policy’s period of validity covers the dates of the work,
- the declared activity includes aerial work by drone and spraying or cleaning,
- damage to the property being worked on is covered, not excluded,
- the limits are proportionate to the building, taking into account in particular an urban location and large glazing.
Then set a single damaged glazing unit, at several times the cost of the job, against the excess and sub-limits shown on the certificate. For most owners, this comparison quickly shows whether the cover is sufficient.
Drone cleaning is well suited when surfaces are sound, the inspection and condition report have been carried out and recording is systematic. It is poorly suited when a building has known unrepaired leaks, ageing seals on large glazing or fragile roof coverings. In that case, even a perfect record will only document the damage: better to repair first and clean afterwards.
