Thermografie PV Flachdach: Inspektion gewerblicher Dachanlagen

Commercial flat-roof systems are the most common PV system class in Germany – from the 50 kWp logistics warehouse to the 2 MWp industrial roof. Despite their prevalence flat-roof systems present specific inspection challenges: mounting systems, roof structures, rooflights and limited accessibility for ground personnel. This article explains what to consider for thermographic inspection of flat-roof PV systems.

Flat-roof PV: the commercial standard

Over 60% of commercial PV installations in Germany are on flat roofs. Logistics centres, production halls, supermarkets and office buildings offer large continuous roof areas that are ideal for PV systems. System sizes between 50 kWp and 2,000 kWp are typical, with drone thermography becoming particularly economical from approximately 200 kWp.

Flat-roof systems have special characteristics compared with pitched-roof and ground-mounted systems: the modules are generally on a mounting system (tilt angle 10–15°) and do not lie directly on the roof surface. This has both advantages and disadvantages for inspection.

Mounting systems and their characteristics

On flat roofs modules are mounted on mounting systems fastened either with ballast weights (common) or roof penetrations. The mounting system determines the tilt angle and therefore also the thermographic accessibility. For east-west mounting systems the modules are tilted perpendicular to the roof – here the drone flight angle for the IR camera must be calculated differently than for south-facing systems.

Ballasted systems without roof penetration are non-critical for thermography. With penetrating fixings moisture can enter at the fixing points and lead to corrosion in the mounting structure – a thermographically detectable signal when moisture affects electrical connections.

Roof structures and obstacles

Commercial flat roofs are rarely free of structures. HVAC units, cooling aggregates, ventilation shafts, rooflights and walkways interrupt the PV area and create shading sources. These obstacles must be taken into account during flight planning – both for complete coverage of all modules and for the safety of the drone flight.

Particular attention must be paid to thermally active roof structures: a cooling aggregate emitting heat can influence neighbouring modules and generate false hotspot signals in the thermogram. Experienced thermographers can distinguish such environmental effects from genuine module defects.

Drohnenbefliegung auf Flachdächern

Safety aspects deserve particular attention when drone-surveying flat-roof systems in urban areas. Flat roofs are generally in the vicinity of other buildings, roads and possibly near controlled airspace. Before every deployment we check the applicable airspace restrictions and apply for necessary permits where required.

An advantage of flat roofs: the low height of the modules above the roof surface (typically 2–4 m) enables a close drone flight distance for high-resolution IR captures. This increases the thermogram detail resolution compared with ground-mounted systems surveyed from greater height.

BOS-Komponenten auf Flachdächern

On flat roofs BOS components are frequently housed in roof structures, plant rooms or weatherproof enclosures directly on the roof. Inverters in plant rooms are accessible with the IR hand camera through the door. For outdoor inverters and DC junction boxes on the roof we thermographically check for overheating that can be promoted by radiant heat from the roof substrate.

Flat-roof buildings are often equipped with a lightning protection system that must be integrated into the PV system. Surge protection devices damaged by lightning events show up in the thermogram as conspicuous heat sources – a frequent finding in flat-roof systems in southern German thunderstorm valleys.

Cleaning and maintenance on flat roofs

Flat-roof systems are more prone to soiling than tilted systems because the shallower tilt angle (10–15°) reduces rainwater run-off. Bird droppings, leaves and fine dust from nearby industrial sites accumulate on the modules. Thermography detects soiling patterns as local hotspots, providing an objective basis for the cleaning decision.

Cleaning work on flat roofs requires fall protection and appropriate access concepts. The thermographic inspection, by contrast, is possible without personnel setting foot on the roof – the drone carries out the inspection without contact and without operational interruption.

Typical damage in flat-roof systems

In commercial flat-roof systems compared with ground-mounted systems we more frequently find: hotspots from bird dropping soiling on individual modules, connector defects from UV degradation of cable sheaths (particularly in older systems without high-temperature cables), bypass diode faults in modules permanently stressed by partial shading from roof structures and inverter overheating from inadequate ventilation in enclosed plant rooms.

Advantages of thermography vs. visual inspection

Visual inspection of a commercial flat-roof system requires personnel to access the roof, fall protection and considerable time. For a 500 kWp system with 1,200 modules a thorough manual visual inspection takes 1–2 days. Drone thermography inspects the same system completely in 2–4 hours and delivers a standards-compliant document with geo-referenced findings – while the system remains in operation.

Frequently asked questions