When the roof is the brake: The load-bearing capacity problem in commercial PV
In Germany, more than a third of all commercial roofs have insufficient load-bearing reserves for a conventional photovoltaic system. Older industrial halls, storage complexes, administrative buildings, or older factories date back to a time when solar energy was not a planning factor. The result: millions of square meters of roof space lie dormant—not because there is no sunlight available, but because the roof construction cannot support conventional solar panels.
Lightweight modules solve this problem. With an area weight of typically 4–6 kg/m² or around 5–8 kg per module, they exert less than a third of the load of conventional PV systems on the roof—thereby opening up a market potential that has largely remained untapped until now. This technical article explains the technology, the fields of application, the installation logic, and the economic classification.
Why load-bearing capacity is crucial in PV planning
When Photovoltaics planned, the structural analysis of the roof is checked beforehand. This is not just about the weight of the modules themselves – the total load is made up of several components:
| Glass-glass high-performance modules | Conventional glass foil module | Commercial light module | |
| Areal density (module) | ~12–18+ kg/m² (heavier construction) | ~10–15 kg/m² (typically 11–14 kg/m²) | ~3–7 kg/m² (frequently 4–6 kg/m²) |
| Mounting system / Substructure | Aluminum rails, hooks, clamps: 2–6 kg/m² | Mounting system similar to 2–6 kg/m² | Adhesive/primer/base layer 0.5–3 kg/m²; lightweight profiles 1–3 kg/m² |
| Ballasting (flat roof, mounted on racks) | Ballast 5–20 kg/m² depending on the system | 5–15+ kg/m² depending on wind verification | usually no ballasting required |
| Total load incl. mounting system | frequently 20–45+ kg/m² for ballasted systems | often ≈ 17–36+ kg/m²; on pitched roof without ballast ≈ 12–21 kg/m² | ≈ 4–12 kg/m² (depending on the project) |
| Snow load (Zone 1–3, DIN EN 1991-1-3) | +0.75 to +2.5 kN/m² | equivalent (building physics) | equivalent (building physics) |
Critical for older buildings: For elevated flat roof systems with the suitable substructures and ballasting (without roof penetration) – as is customary on commercial flat roofs – the total load can increase to 30–45 kg/m². A roof structure with a permissible surface load of 20 kg/m² is thus immediately overloaded. Lightweight modules without mounting frames and ballasting structurally solve this problem.
Important: Even with lightweight modules, a structural analysis by an approved structural engineer remains mandatory. An evaluation of snow loads, wind loads, and the structural condition of the roof construction is strictly necessary—regardless of the module type.
What are lightweight modules? Technology and structure
Lightweight modules differ fundamentally from standard modules in their combination of materials. While conventional glass-foil or glass-glass modules are based on a solid glass pane as a front cover and an aluminum frame, the lightweight generation relies on alternative materials:
Design and materials
- Front cover: Polymer composite material, special plastic film, or thin, tempered special glass (significantly thinner than in standard modules)
- Cell technology: Monocrystalline silicon cells (PERC, TOPCon) – identical to standard modules
- Frame: Completely eliminated or achieved through minimal edge sealing
- Backsheet: Polymer-based, UV- and weather-resistant
- Thickness: Partly only 2–6 mm compared to 32–40 mm for standard modules
Flexible vs. lightweight rigid modules – an important difference
The terms are frequently used interchangeably in the market, but they refer to different characteristics:
| Lightweight rigid modules | Flexible Module | |
| flexibility | Rigid, flat | Bendable (limited) |
| Weight | 4–8 kg/m² | 2–5 kg/m² |
| Efficiency | 19–22 % | 15–20 % |
| Commercial application | Ideal (trapezoidal sheet metal, membrane roof) | Special cases, curved roofs |
| Lifespan / Warranty | 25–30 years is usual | Partially shorter, manufacturer-dependent |
| Certification | IEC 61215 / IEC 61730 | IEC 61215 / IEC 61730 |
Technical key figures of a typical commercial light module:
- Weight per unit area: 4–6 kg/m² | Single module: 10–14 kg
- Efficiency: 19–22 % (monocrystalline)
- Power: 360–450 Wp per module
- Size: approx. 1.7 × 1.1 m (unframed)
- Product Warranty: 12–15 years | Performance Warranty: 25–30 years (≥ 80 %)
- Certification: IEC 61215, IEC 61730 | System voltage up to 1,500 V
- Attachment: Adhesive bonding (no roof penetration)
Application areas for lightweight modules – Which roofs benefit?
Lightweight modules are not a universal product, but for a clearly defined building category, they are the only sensible PV option. The following overview shows the most important use cases:
Suitable roof types
- Corrugated metal roofs and sandwich roofs: Very common in industry, logistics and agriculture. Low load-bearing capacity reserves due to lightweight steel structures. Adhesive bonding directly onto the sheet metal possible.
- Single-ply membrane roofs (PVC, EPDM, FPO, TPO, Alwitra): Flat roofs on commercial and administrative buildings. No roof penetration required – ideal for ongoing roof sealing warranties.
- Bitumen roofs (with and without slate surfacing): Common in older low-rise buildings. Bonding is possible provided the substrate is clean and structurally sound.
- Flat roofs of older commercial buildings: Bungalow style, 1960s–80s administrative buildings, parking garages. Frequently only 10–20 kg/m² of permissible additional load.
- Lightweight halls and greenhouses: Lightweight steel structures with very low roof load capacity.
- Existing building with PV extension: When expanding an existing system without load-bearing capacity reserves for additional standard modules.
Compatible surfaces for adhesive mounting
Lightweight modules can be bonded to almost any smooth or slightly curved surface: trapezoidal sheet metal, bitumen (with or without slating), PVC membrane, EPDM, FPO, TPO, Alwitra membrane, concrete, Eternit, glass, and aluminum facades.
Not suitable: Green roofs, thatched roofs, heavily profiled or uneven surfaces without a leveling layer, as well as areas of the roof that are permanently exposed to water.
Assembly method for lightweight modules – bonding instead of drilling
Adhesive bonding is the technological centerpiece of the lightweight module concept. Roof penetrations, drill holes, aluminum profiles, and ballast are completely eliminated. The process at a glance:
Process of adhesive assembly
- Step 1 – Surface preparation: Cleaning of the roof surface with a suitable cleaning agent (manufacturer-specific)
- Step 2 – Adhesive beads: Three to four parallel adhesive beads of a structural adhesive are applied per module (minimum layer thickness 3 mm).
- Step 3 – Module support: Light module is positioned and pressed down gently. Assembly time per module: approx. 3 minutes
- Step 4 – Curing time: Depending on the adhesive; generally 24–48 hours until fully load-bearing
Key advantages of adhesive bonding technology
- No roof penetration → roof waterproofing remains intact, warranties remain valid
- No ballasting → no additional roof load from concrete blocks
- No mounting structure → significantly lower material requirements
- Lower windage area due to low-profile module system → more favorable for wind load calculations
- Dismantlable: Modules can be removed with low residue using cutting wire (similar to windshield technology) and reinstalled
Maintenance access
Since adhered modules must not be walked on, the layout is usually planned in double rows with defined maintenance aisles in between. This enables maintenance, cleaning, and module replacement without damaging neighboring modules.
Performance and efficiency
Efficiency of lightweight modules in practical comparison
A common misconception: Lightweight modules are generally considered to be significantly less powerful. The real gap is smaller than often assumed:
| Standard glass-foil module | Lightweight module (crystalline) | |
| Efficiency | 21–23 % | 19–22 % |
| Loss of power in the heat | –0.35 to –0.40 %/°C | –0.30 to –0.38 %/°C (depending on the manufacturer) |
| diffuse light | Good | Comparable, in part better |
| Annual yield/module (avg.) | 430–520 kWh | 360–500 kWh |
| Performance guarantee | 25–30 years ≥ 80–85 % | 25–30 years ≥ 80 % |
| Product warranty | 12–25 years | 12–15 years |
Crucial for the economic evaluation: The point of comparison is not lightweight module vs. standard module, but lightweight module vs. no photovoltaics. On roofs without sufficient load-bearing capacity, the lightweight module is not the second-best option – it is the only option.
Amortisation in the commercial context
- High self-consumption (>50 %): Payback period typically 5–8 years
- Low self-consumption / high feed-in: 8–12 years
- Levelized cost of electricity: approx. 5–7 ct/kWh (with favorable financing)
- Relevant funding: EEG feed-in tariff, possibly BAFA/KfW funding, municipal funding programs
Lightweight modules in the Contracting as a CAPEX-free alternative
For companies that do not want to or cannot capitalize investment costs, Contracting an attractive alternative to direct purchase. In this arrangement, an energy service provider – such as CUBE CONCEPTS – finances, builds, and operates the lightweight module system on the customer's roof. The user purchases the generated electricity at a contractually fixed price below the grid electricity price, without having to make any investment of their own.
Operating principle: CUBE CONCEPTS handles all aspects of the project, including planning, financing, installation, and operation. The client pays a monthly rate per kWh—typically 10–20 % below the current grid electricity purchase price via PPA. The contract term is typically 10–20 years. In certain cases, if necessary, we will carry out roof retrofitting as part of the Contracting program in order to install the system.
| Instant purchase | Contracting | |
| Investment | CAPEX investment by operator | No CAPEX – zero investment |
| Operation & Maintenance | By operator | By CUBE CONCEPTS |
| Electricity price | Generation cost approx. 5–7 ct/kWh | Fixed electricity price, typically 10–20 % for grid-purchased electricity |
| Operating expenses (OPEX) | By operator | By CUBE CONCEPTS |
| Risk | At the operator's | With the contractor |
| Typical runtime | – | 20 years of predictable electricity prices |
| Suitable for | Companies ready to invest | CAPEX-sensible companies |
Limits and what to consider during planning
Lightweight modules are a high-performance specialist solution – but not a universal alternative to standard modules. The following points must be considered during project planning:
- Structural analysis remains mandatory: Even with a module weight of 4–6 kg/m², snow and wind loads must be calculated in accordance with DIN EN 1991 and evaluated by a structural engineer.
- Substrate quality is crucial: The adhesive only adheres to clean, structurally sound substrates. Dilapidated roof waterproofing must be repaired beforehand.
- Product warranties partly shorter: While standard modules often offer a 25-year product warranty, the range for lightweight modules is 12–15 years, depending on the manufacturer. Performance warranties are comparable.
- Not for all surfaces: Green roofs, thatched roofs, and permanently damp areas are ruled out.
- Manufacturer quality differentiated: The longevity of the polymer composites depends heavily on the material used. Certifications according to IEC 61215 and IEC 61730 are a minimum standard; for projects, it is essential to pay attention to product data sheets and reference projects.
- Uneconomical despite sufficient load-bearing capacity: Where standard modules can be installed without problems, they are generally the more economical choice – higher efficiency, longer product warranty, lower unit costs.
Frequently asked questions about lightweight modules (FAQ)
How much do lightweight modules weigh per m²?
Modern crystalline lightweight modules for commercial buildings weigh 4–6 kg/m² including adhesive. Ultra-lightweight systems (e.g., glass-free module types) sometimes achieve under 3 kg/m². For comparison: conventional glass-foil modules weigh 9–13 kg/m², and ballasted pitched flat roof systems sometimes 30–50 kg/m².
Do I still need a structural engineer despite lightweight modules?
Yes, always. The inspection by a certified structural engineer is required by law and is necessary regardless of the module type. Snow loads, wind loads, and the condition of the roof structure must be evaluated. Lightweight modules significantly reduce the dead load, but do not replace the structural analysis.
What is the efficiency of lightweight modules?
Current crystalline lightweight modules achieve an efficiency of 19–22 %. In practice, the difference compared to premium standard modules (21–24 %) is minimal. For roofs without sufficient load-bearing capacity, this difference is not a deciding factor anyway.
How are lightweight modules mounted?
The standard method is adhesive mounting: structural adhesive is applied to the substrate in beads, the module is placed on top and pressed down. No screws, no roof penetrations. If necessary, modules can be removed again with little residue using a cutting wire.
Are lightweight modules as durable as standard modules?
Performance warranties are largely comparable (25–30 years, ≥ 80 % residual performance). Product warranties range from 12 to 15 years, depending on the manufacturer. The longevity of polymer composites depends heavily on material quality. Market-leading systems use UV-resistant, weather-resistant materials comparable to those used in aircraft glazing.
For which building types are lightweight modules most sensible?
Primarily for commercial and industrial buildings with trapezoidal sheet metal roofs, foil roofs, or bitumen roofs and limited load-bearing reserves. Also suitable for retrofitting existing PV systems on areas without available structural engineering data.
Conclusion: Lightweight modules unlock untapped potential
One-third of Germany’s commercial roofs are waiting to be retrofitted for energy efficiency—not because of a lack of sunlight, but because of insufficient load-bearing capacity. Lightweight modules are the technological solution to this structural problem. With weights per unit area of 4–6 kg/m², IEC-certified efficiencies of 19–22 %, and an adhesive installation method that requires no roof penetration, they open the door to PV projects that simply could not be realized with standard modules.
The decision in favor of lightweight modules is not a compromise—it is often the only engineering-justified solution. Decisive factors for project success are careful substrate assessment, specialized structural planning, and the selection of a high-quality, certified product.