1. General Principles for Roof-Mounted PV Systems
A roof-mounted solar panel system is not an independent structure but a technical system placed on an existing building structure, imposing long-term loads on it. Therefore, when designing support structures, the question is not "how to fix it," but rather how the loads are transferred to the roof structure, and whether the roof can safely and sustainably bear these loads.
Logic of Load Transfer
The loads generated by a solar panel system do not begin at the panels; they end there. The path of the load must always be imagined in reverse.
In physical reality, the process is as follows:
- wind and snow act on the panel surface,
- the panel transfers this load to the rails via the mounting clamps,
- the rails transfer the load to the roof hooks or fasteners,
- the fasteners transfer the load into the rafters, purlins, or slab,
- finally, the load reaches the ground through the building's load-bearing walls and foundations.
If any element in this chain is weak, the entire system becomes vulnerable. Therefore, the support structure should not be understood in isolation but always as part of the complete load path.
What Constitutes a Load-Bearing Structure?
From the perspective of a solar panel system, only those elements that are part of the building's original structural system are considered load-bearing structures.
These typically include:
- rafters,
- purlins,
- reinforced concrete slabs,
- steel beams in industrial buildings.
A solar panel system is never affixed to the roofing material itself but passes through it and connects to the load-bearing structure. The function of the roofing material is solely to protect against weather, not to bear loads.
The Role of Wind and Snow Loads
Many people overestimate the weight of a solar panel system, while the greatest stresses are not caused by its self-weight but by environmental loads.
Snow load depends on the roof's pitch, the panel's placement, and whether snow can slide off or accumulate. Panels often act as snow guards, which can cause significant localized loads. This is particularly true for low-pitch roofs or flat roofs.
Wind load is even more complex. Wind exerts a pressure effect on the windward side, a suction (uplift) effect at the edges and corners of the roof, and creates turbulent effects beneath the panels. Therefore, support structures must be designed not only for downward loads but also for uplift. With poorly chosen fixings, the wind will not just "press down" but will try to tear off the system.
Materials for Support Structures
The choice of material for the support structure is not an aesthetic question but one of corrosion resistance, strength, and lifespan.
Aluminum is the most common material. It is lightweight, corrosion-resistant, easy to work with, and stable in the long term. Its mechanical strength is adequate if the system components are used with appropriate cross-sections and spacing.
Steel provides greater strength and is therefore often used for ground-mounted structures or larger spans. Its disadvantage is its susceptibility to corrosion, so it cannot be used without surface treatment (galvanization).
Stainless steel typically appears in fasteners (screws, hooks) where corrosion protection and high mechanical stress are simultaneously important. Entire support structures are rarely made from it due to its high cost.
2. Tiled Roofs
Tiled roofs are the most common roof type in Hungary, so the vast majority of solar panel systems are installed on such surfaces. However, this does not mean it is a simple installation environment. Due to the layer structure, timber roof structure, and various tile types, this is where the most mistakes can be made.
The most important basic principle: the tile is not a load-bearing element. The loads from the solar panel system are transferred not through the covering, but through the rafters underneath.
Characteristics of Clay and Concrete Tiles
Fired clay tiles are relatively light but brittle. They are weather-resistant but sensitive to point loads and impacts. Concrete tiles appear more robust, but due to their rigidity, imprecise cutting or tensioning can cause hairline cracks that may later propagate due to freeze-thaw cycles.
In both cases, the tile is a covering, not a support structure. The fixing must always connect to the timber load-bearing structure.
Interlocking and Flat Tiles
Interlocking tiles hook into each other, which provides good waterproofing but makes the placement of hooks more precise. If a hook lifts a tile, it can cause water to back up.
For flat tiles, cutting is often required for the hook. Here, precision is key: too large a cut poses a waterproofing risk, while too small a cut stresses the tile.
The True Load-Bearing Structure
Beneath the tiles, there are roof battens and counter-battens, but these are not load-bearing elements. The solar panel hook must always be fixed to the rafter.
If the fixing is only to the battens, it can loosen, shift, or even lead to leaks in the long term. The battens' role is solely to hold the roofing, not to bear additional loads.
Roof Hook Fastening
On tiled roofs, the most common solution is hook fastening. The hook is screwed into the rafter, passes through the plane of the roofing, and supports the rail system.
The design of the hook must ensure that:
- it does not lift the tile,
- it does not stress the roofing,
- the rail runs at an appropriate height.
The spacing of the hooks cannot be determined by feel. Wind and snow loads, as well as the length of the rail, determine the required density. The hook must be aligned with the center line of the rafter. Inaccurate fastening reduces load capacity and increases the risk of timber cracking.
Tile Cutting and Waterproofing
The purpose of cutting is not just for the hook to "somehow fit," but for the roofing to continue to properly drain water. The replaced tile must lie stably, not wobble, and the overlaps must remain intact. A tiled roof is not a waterproof system, but a water-draining system. If the hook lifts a tile, or the underlayment is damaged, leaks may not appear immediately but years later.
The measure of good installation is not that it doesn't leak today, but that it won't leak twenty years from now.
3. Metal Roofs
The common characteristic of metal roofs is that the covering is a thin metal sheet, which is not a load-bearing element but works on the timber or steel structure beneath. Therefore, for solar panel mounting, one must not rely on the "strength" of the metal sheet, but rather on how the load is transferred to the supporting structure.
The two most common types are standing seam metal roofs and trapezoidal sheet metal roofs, and these require entirely different fastening logics.
Standing Seam Metal Roofs
A standing seam roof consists of longitudinal, upright seams. These provide the panel's rigidity and waterproofing, and the solar panel mounting is also done to them.
The biggest advantage is that there is no need to drill through the roofing. The fastening is done with special seam clamps that grip the seam. If the clamp is of the correct type and tightened to the torque specified by the manufacturer, the system's waterproofing remains secure.
The most common mistake here is overtightening. The metal sheet expands with heat, and if the fastening does not allow for this, the seam can deform or eventually crack. Another typical problem is using an incompatible clamp that does not precisely fit the profile. On standing seam roofs, the load is distributed along the seam, which is advantageous, but higher wind loads must always be considered at the edges and corners. In these zones, the fastening layout should not be underestimated.
Trapezoidal Sheet Metal
Trapezoidal sheet metal is a ribbed profile metal sheet that provides some rigidity, but the supporting structure still bears the load. Fastening here typically involves drilled screws.
The screw passes through the sheet and grips the underlying timber or steel structure, bearing the load there. This is a stable solution, but every drilled hole is a potential water ingress point, so sealing is crucial.
The EPDM washer ensures waterproofing and vibration damping. If tightened too loosely, leakage can occur. If too tight, the washer loses its elasticity and can also lead to leaks over time. With trapezoidal sheets, special attention must also be paid to corrosion. Drilling damages the surface protection, so the screw material and the quality of the seal must be compatible with the roof material. Otherwise, galvanic corrosion can start, which will weaken the fastening in the long term.
4. Flat Roofs
Solar panel systems installed on flat roofs operate according to a completely different logic than pitched roof solutions. There is no natural slope here, water drainage is a sensitive issue, and wind load is much more pronounced. The central question of design is not merely fastening, but how the system remains stable without damaging the roof's layer structure.
A flat roof typically consists of layers: load-bearing slab, slope formation, thermal insulation, waterproofing, and sometimes gravel or paving slabs. Of these, only the slab is considered a load-bearing element structurally. The loads of the solar panel system must ultimately reach this, either directly or indirectly.
Ballasted Systems
In a ballasted solution, the support structure is not mechanically fixed to the roof. Stability is provided by its own weight and the placed ballast. Panels are placed on stands with a predetermined pitch, under which concrete slabs or weight blocks are placed.
The system does not simply remain "pressed down" on the roof, but also counteracts the uplift force and overturning moment caused by the wind. Therefore, design always occurs according to wind zones: more ballast is required at the edges and corners of the roof than in the internal areas.
The biggest advantage is that the waterproofing remains intact. The biggest limitation, however, is the load-bearing capacity of the roof. For older buildings, the additional load of ballast requires serious structural analysis.
Penetrating Fixings
In penetrating solutions, the support structure is mechanically fixed to the slab, through the waterproofing. In such cases, loads are transferred directly to the load-bearing structure, so less ballast is sufficient.
This is structurally more favorable but more sensitive in terms of waterproofing. The design of penetration points cannot be an improvised solution: multi-layer insulation and manufacturer's system solutions must be used. A poorly designed penetration does not leak immediately but causes hidden damage over years.
Wind Load and Aerodynamics
On flat roofs, wind creates vortices and, by getting under the panels, generates uplift. Edge and corner zones are particularly vulnerable.
- the pitch of the rows and their distance from each other are also wind engineering questions,
- edge zones require special reinforcement,
- design cannot be generalized; it must always be based on site-specific calculations.
Incorrect design does not necessarily cause spectacular failure but slow displacement that damages insulation and cabling.
Drainage and Accessibility
On flat roofs, water drainage is always critical. The solar panel system must not block drains or create areas of stagnant water. The design must take into account the direction of slope, the location of drains, and maintenance routes.
A well-designed flat roof system is not only structurally stable but also maintainable and safe in the long term.
5. Ground-Mounted Structures
The greatest advantage of ground-mounted solar systems is freedom: they are not restricted by the roof's orientation, pitch, or load-bearing capacity. However, all structural issues must be solved from scratch. A ground-mounted support structure is not a simple stand but an outdoor engineering structure exposed to wind, snow, and thermal expansion for decades.
Fixed Ground Mounts
A fixed mount is the simplest and most reliable solution. Panels are typically placed at a constant pitch, usually facing south. There are no moving parts, so the system is stable long-term and requires low maintenance.
The key design issue is the pitch. A steeper angle improves winter production but increases wind load and structural demand. A shallower angle means less mechanical stress but lower yields in winter. The choice is always an energy and structural compromise.
For industrial and higher-capacity systems, this is the most common solution because it is easy to calculate and size.
Adjustable Tilt Systems
Adjustable systems allow for seasonal modification of the tilt angle. In theory, this can improve annual production, especially if a steeper angle is used in winter.
In practice, however, these systems are more complex. More connections, more structural elements, and regular intervention are required. Adjustability does not mean automatic sun tracking but manual adjustment. Therefore, it is more justified for special applications, such as off-grid systems where winter production is critical.
Foundation Solutions
The stability of a ground-mounted support structure depends on its foundation. Two typical solutions are ground screws and concrete foundations.
In ground screw foundations, steel screws are twisted into the ground, and the support structure is attached to them. Advantages include quick installation and the elimination of concrete work. However, it is highly soil-dependent: in cohesive, stony, or layered soil, the load-bearing capacity can be uncertain. In such cases, soil mechanical investigation is necessary.
With concrete foundations, the support columns are set in concrete. This is more predictable and provides greater security for high wind loads or large fields but means slower installation and more difficult removal.
The choice always depends on soil conditions and the size of the system.
Row Spacing and Shading
For ground-mounted systems, one of the most important geometric questions is the distance between rows. If the rows are too close, the shadow of the front row will fall on the back rows in winter. When designing, the low winter sun angle must be considered, not the summer one.
Greater row spacing yields better specific output but reduces land utilization. A denser arrangement allows more panels in the same area but results in greater shading losses. Here, an energy and economic compromise is always made.
Maintenance Aspects
Ground-mounted systems are more accessible than roof-mounted ones, which is an advantage for cleaning and inspection. However, one must account for vegetation growth, dust, splashing mud, and the possibility of mechanical damage.
Good design here also involves foresight: appropriate row spacing, walkable paths, and stable structural design ensure that the system is not only buildable but also sustainable.
