FRP vs Aluminium: A Precise Comparison for Rooftop Solar Mounting and Offshore Platform Secondary Structures

Aluminium is the default lightweight metal for rooftop solar mounting, offshore platform walkways, and access structures where weight matters. It is lighter than steel, corrosion-resistant compared with uncoated carbon steel, and widely available in extruded structural sections. FRP is 35 to 50% lighter than aluminium, does not suffer pitting corrosion in saltwater, does not conduct electricity, does not cause galvanic corrosion at dissimilar metal interfaces, and expands at roughly a third of aluminium's thermal expansion rate. The two materials are not equally suited to the same applications. Here is the precise comparison.

Published by Reinforce Technology  |  20 July 2026

Aluminium has been the lightweight structural metal of choice for rooftop solar mounting systems and offshore platform secondary structures for the same reason it became the default material in those applications: it is substantially lighter than steel, and in a structural application where dead load matters, that weight advantage translates directly into installation cost, foundation requirement, and handling logistics. Aluminium's natural oxide layer provides meaningful corrosion protection in many outdoor environments without the maintenance that galvanised steel requires. And its availability as extruded structural sections makes it a familiar and accessible material for engineers and fabricators working in renewable energy and offshore sectors.

FRP pultruded structural profiles are not as familiar to engineers trained primarily on metallic structural materials. But in the specific environments where aluminium is most commonly specified for lightweight secondary infrastructure, FRP's material properties address each of aluminium's limitations in ways that matter across the 25 to 30-year operational life of a rooftop solar installation or an offshore platform's secondary structure. FRP is 35 to 50% lighter than aluminium by section. It does not pit or suffer intergranular corrosion in chloride-rich saltwater environments. It does not conduct electricity and creates no galvanic corrosion risk at connections to dissimilar materials. And its thermal expansion coefficient is approximately one third of aluminium's, substantially reducing the thermal stress that aluminium's high expansion rate generates at fixed connection points across the temperature range of a UK outdoor or offshore installation (GTOFRP, 2025).

This blog compares FRP and aluminium directly across the properties that matter for rooftop solar and offshore platform secondary infrastructure: density and weight, corrosion behaviour, electrical conductivity and galvanic effects, thermal expansion, structural performance, and lifecycle cost. It is a comparison between two lightweight materials, not between FRP and the conventional heavy metal alternative. Understanding where FRP and aluminium differ is the basis for making the correct specification choice between them in these specific applications.

Aluminium is the conventional lightweight structural metal for rooftop solar and offshore platform applications. FRP is 35 to 50% lighter than aluminium, non-conductive, immune to galvanic corrosion, and expands at one third of aluminium's thermal expansion rate. The comparison between the two determines which is the correct specification for each specific application.

Density and Weight: FRP vs Aluminium

Aluminium has a density of approximately 2,700 kg/m³. Pultruded GFRP has a density of approximately 1,750 to 2,100 kg/m³. For equivalent structural cross-sections, FRP structural profiles are 35 to 50% lighter than aluminium equivalents. In terms that are directly relevant to rooftop solar and offshore platform applications, this weight difference matters in three distinct ways.

On rooftop solar installations, dead load is the primary structural constraint. Every kilogram of mounting structure weight adds to the permanent load on the roof structure beneath. Roof structures in commercial and industrial buildings are designed to carry specified loads, and the available structural reserve for additional rooftop solar equipment is determined by the original building design and any subsequent structural assessment. A mounting system that is 35 to 50% lighter than an equivalent aluminium system either frees up structural reserve for additional solar panel capacity, reduces the structural assessment requirements for marginal rooftop installations, or enables rooftop solar deployment on structures where aluminium mounting systems would exceed available reserve. This is not a marginal advantage in the context of the UK's rooftop solar expansion: the Planning and Infrastructure Act reforms are accelerating deployment and the structural reserve argument is one of the most common constraints that rooftop solar projects encounter in the assessment stage.

On offshore platforms, weight reduction in secondary structures, walkways, cable management, and equipment supports, reduces the topside weight that determines the platform's stability, load distribution, and the crane capacity required for installation and maintenance. In jacket and semi-submersible platform configurations, reducing topside weight directly improves the platform's structural reserve and operating envelope. FRP secondary structural sections achieve this weight reduction without the structural performance compromise that lower-density polymer alternatives to aluminium would introduce: FRP profiles carry structural loads, span between supports, and form access systems that meet the same functional requirements as aluminium equivalents at substantially lower weight.

The handling and installation dimension of the weight difference is equally practical. FRP sections that are 35 to 50% lighter than aluminium equivalents can typically be positioned by one or two operatives without mechanical lifting assistance. On offshore platforms where lifting operations require crane scheduling, permit-to-work authorisation, and the full logistics of offshore heavy lift, the ability to position and connect secondary structural sections manually without crane support is a meaningful reduction in installation cost and programme duration (MEP Solutions, 2025).

Corrosion: The Saltwater Environment Distinction

Aluminium's corrosion resistance is one of its primary advantages over carbon steel. The natural aluminium oxide layer that forms on the surface of aluminium sections provides meaningful protection against atmospheric corrosion and general outdoor weathering without the zinc coating that galvanised steel requires. This is why aluminium has become the standard mounting material for rooftop solar: it performs well in the urban atmospheric environment of most UK commercial and industrial rooftop installations without the coating maintenance that steel demands.

The limitation of aluminium's natural oxide protection becomes apparent in two specific environments that are directly relevant to offshore platform secondary structures and coastal rooftop solar installations. The first is chloride-rich saltwater environments. In the presence of high chloride ion concentrations, the aluminium oxide layer breaks down locally, initiating pitting corrosion that penetrates through the oxide film into the underlying aluminium. Pitting corrosion in aluminium offshore structures is well-documented and is the primary mechanism by which aluminium secondary structures on offshore platforms lose structural cross-section over time. Pitting depth is difficult to detect visually until it is advanced, and the structural consequence of undetected pitting in load-bearing aluminium sections can be sudden and significant.

The second is galvanic corrosion at interfaces with dissimilar metals. Aluminium is anodic relative to most structural metals it is likely to be connected to in a mixed-material installation. When aluminium comes into direct electrical contact with stainless steel fasteners, carbon steel structural members, or copper earth bonding conductors in the presence of an electrolyte, a galvanic cell forms that accelerates corrosion of the aluminium at the contact point. Managing galvanic corrosion in aluminium offshore structures requires insulating bushes, barrier materials, and detailed attention to the electrochemical compatibility of every material interface in the assembly. It adds complexity and maintenance requirements to an installation that aluminium's lightweight properties were selected to simplify.

FRP has no corrosion mechanism of any kind. It is non-metallic, contains no iron, zinc, or aluminium, and has no electrochemical corrosion pathway. It does not pit in saltwater, does not lose structural section over time in chloride environments, and does not form galvanic cells at interfaces with other materials because it carries no electric charge and creates no electrochemical potential difference at material boundaries. In offshore environments where saltwater immersion and splash, high atmospheric chloride loading, and the combination of dissimilar metal interfaces are routine, FRP eliminates the corrosion maintenance programme that aluminium manages but does not eliminate (IncomePultrusion, 2026).

Electrical Conductivity and Galvanic Effects

Aluminium is an electrical conductor with a conductivity of approximately 37.7 million siemens per metre, placing it among the better electrical conductors of structural metals. In rooftop solar mounting applications, this conductivity has two implications. First, aluminium mounting frames must be earthed and bonded as part of the electrical installation of the solar PV system, connecting the metallic mounting structure to the earthing system of the building and the PV array. This earthing programme is required by BS 7671 and adds materials, labour, and inspection cost to the installation. Second, in the high-voltage DC environment of a rooftop solar installation, a fault current in the cable management reaching the aluminium mounting structure creates a conducted path through the earthed metallic structure that may affect the performance of the PV array's isolation monitoring and earth fault detection systems.

FRP is an electrical insulator throughout its full cross-section. It has volume resistivity of 10¹² to 10¹⁶ Ω·m, classifying it as a non-conductor across all voltages encountered in rooftop solar and offshore electrical systems (IntechOpen, 2022). FRP mounting frames and cable management require no earthing or bonding. They create no conducted fault current path through the mounting structure. And they introduce no galvanic corrosion risk at connections to other materials because they carry no electrochemical potential. In offshore platform applications, where structural members near high-voltage electrical systems must not create shock or arc flash hazards, FRP's non-conductivity is a specific and documented safety advantage over aluminium secondary structures in those zones (IncomePultrusion, 2026).

Thermal Expansion: The Rooftop and Offshore Fatigue Case

Aluminium has a coefficient of thermal expansion of approximately 23.1 parts per million per degree Celsius, the highest of the common structural metals and approximately twice that of steel. In the context of a UK rooftop solar installation subject to diurnal and seasonal temperature cycling, aluminium mounting frames expand and contract at a rate that generates recurring thermal stress at the fixed connection points between the mounting frame and the roof structure, and between the mounting rails and the solar panel frames. Over 25 to 30 years of daily temperature cycles, this thermal fatigue accumulates at connection interfaces and is a specific and recognised mechanism for loosening of fastened connections in aluminium solar mounting systems.

FRP has a longitudinal coefficient of thermal expansion of approximately 6 to 8 parts per million per degree Celsius — roughly one third of aluminium's rate. Across an equivalent temperature range and the same number of daily thermal cycles, FRP mounting frames generate substantially less thermal movement and substantially less fatigue accumulation at connection interfaces. Over 25 to 30 years, this difference in thermal behaviour translates into reduced connection maintenance requirements and lower risk of thermally induced loosening at the fastened interfaces between the mounting system and the building structure (GTOFRP, 2025).

On offshore platforms in the North Sea, where the temperature range between winter low and summer high can exceed 40°C and the combination of thermal cycling with wave-induced dynamic loading creates a particularly demanding fatigue environment, the lower thermal expansion of FRP secondary structural sections provides a material contribution to the fatigue life of the secondary structure's connection interfaces that aluminium's higher expansion rate does not.

FRP's thermal expansion coefficient of 6 to 8 ppm/°C is approximately one third of aluminium's 23.1 ppm/°C — reducing thermal fatigue at connection interfaces across 25 to 30 years of daily temperature cycling on UK rooftop solar and offshore platform installations.

The Head-to-Head: FRP vs Aluminium

Reinforce Technology FRP Products for Rooftop Solar and Offshore Applications

Reinforce Technology supplies FRP pultruded structural profiles, cable trays, grating, and handrail systems for rooftop solar mounting infrastructure and offshore platform secondary structures across the UK. Available in polyester and vinyl ester resin systems, with fire-retardant formulations for enclosed offshore and building applications. Non-conductive, non-magnetic, and fully immune to saltwater and marine atmospheric corrosion across a 25-year design life.

Contact us to discuss your rooftop solar or offshore project and whether FRP is the correct specification for your specific application, environment, and operational horizon.

Final confirmation of structural suitability for any specific application, including deflection calculations and fire performance classification, remains the responsibility of the appointed project engineer. Reinforce Technology provides technical guidance and material recommendations based on information supplied to us, but specification sign-off should always sit with the qualified professional responsible for the design.

References

GTOFRP (2025) FRP vs Aluminum Structural Profiles: Technical Cost, Strength, and Weight Evaluation. Available at: https://www.gtofrp.com/FRP-vs-Aluminum-Structural-Profiles-Technical-Cost-Strength-and-Weight-Evaluation-id44793085.html [Accessed: 20 July 2026]. [FRP profiles 35 to 50% lighter than aluminium; lower thermal expansion; field-tested under saltwater and acid exposure; no maintenance cycles].

IncomePultrusion (2026) Fiberglass Structural Profiles: Materials, Design, Applications, and Lifecycle Cost. Available at: https://incomepultrusion.com/fiberglass-structural-profiles-technical-guide/ [Accessed: 20 July 2026]. [FRP immune to galvanic corrosion; no cathodic protection required offshore; non-conductivity advantage near high-voltage systems; saltwater chloride 35,000 ppm above steel corrosion threshold].

IntechOpen (2022) 'Fibre-Reinforced Polymer (FRP) in Civil Engineering', in IntechOpen Engineering Series. Available at: https://www.intechopen.com/chapters/84203 [Accessed: 20 July 2026]. [Volume resistivity 10¹² to 10¹⁶ Ω·m; non-conductive throughout full cross-section; no corrosion mechanism; 25-year-plus design life in marine environments].

MEP Solutions (2025) Fiberglass Reinforced Plastic Cable Tray — Offshore Applications. Available at: https://sfsp-ikk.com/solutions/mep-solutions/cable-management-systems/fiberglass-cable-tray.php [Accessed: 20 July 2026]. [FRP ease of handling reduces hoisting accidents and shipping costs; manual positioning without crane support on offshore platforms].

NACE International (2016) International Measures of Prevention, Application and Economics of Corrosion Technology (IMPACT). Houston, TX: NACE International. Available at: http://impact.nace.org/economic-impact.aspx [Accessed: 20 July 2026].

ScienceDirect (2025) 'Sustainable composites for metal replacement: Environmental assessment and material selection of fiber-reinforced polymer across industries', ScienceDirect, doi: 10.1016/S2667-3789(25)00051-3. Available at: https://www.sciencedirect.com/science/article/pii/S2667378925000513 [Accessed: 20 July 2026]. [Pultruded GFRP manufacturing emissions approximately 60 to 70% lower per tonne than primary steel, cradle-to-gate].

Younis, A., Ebead, U. and Judd, S. (2018) 'Life cycle cost analysis of structural concrete using seawater, recycled concrete aggregate, and GFRP reinforcement', Construction and Building Materials, 175, pp. 135-144. doi: 10.1016/j.conbuildmat.2018.04.183.