Photovoltaic for flat roof | SENS

Flat-rooftop photovoltaics

Putting industrial building rooftops to good use


Photovoltaic systems are suitable for a wide range of roofs, though flat rooftops are most common in industrial settings. The solar modules are usually elevated, letting you adjust their alignment and the distance between rows to optimise the safety and economic efficiency of the photovoltaic system. Logistics halls and industrial buildings are particularly suitable for flat-roof photovoltaics, as they can generate their own electricity in the workplace cheaply and in an environmentally friendly way. To achieve the best result possible, however, the installation of a flat-rooftop PV system must be well planned and professionally executed. Read on to find out more details.


Counteract rising energy prices

Achieve the optimum yield

PV with and without roof penetration

The safety of your system

PV and green roofs

Advantages and disadvantages

For industrial buildings, flat rooftops are often the most cost-effective option for covering large areas. They’re also useful for integrating special electrical equipment such as skylights, making a photovoltaic system a natural addition to help with energy efficiency. This aspect is becoming increasingly important for companies, especially in times of rising energy prices. You can use flat-rooftop PV systems on warehouses or production buildings to generate the electricity you need in a cost-effective and environmentally friendly way. Keep in mind: The larger the rooftop surface, the more modules can be set up and thus the greater the amount of solar electricity produced.

Alignment and slope angle influence electricity yield

The solar installation’s alignment must be carefully analysed from a static point of view and with technical considerations of yield in mind. A southern orientation without shading achieves the highest specific yield, where the modules are positioned at an angle of 30 to 35 degrees (depending on the exact location). With a pitched roof, the slope angle is usually predetermined so cannot be modified. The situation is different for solar on flat rooftops, as the modules are installed on a mounting system, which means the distance between rows and alignment are flexible and can be adjusted to achieve the optimum yield. Another advantage of elevation is that the solar modules virtually clean themselves, since rain can drain off easily.

However, the southern alignment with a 30-35 degree slope comes with a considerable catch: On the one hand, a larger gap must be left between the module rows to prevent shading and thus yield losses. An east-west orientation at approx. 10 degrees, on the other hand, has the advantage of achieving a yield distributed more evenly over the course of a day, thus usually covering its own consumption better. The photovoltaic modules are alternately aligned at a slope angle of about 10 degrees to the west and to the east. With this alignment, the angle is shallow enough to prevent as much shading as possible yet steep enough to allow self-cleaning from rain and snow (as described above). Although the yield per module is lower than with a southern alignment at approx. 35 degrees on account of the low angle, the PV modules can be placed more densely, allowing you to put more modules on the same rooftop area. This, in turn, has a positive effect on the overall yield.


Graphic for flat roof pv system | SENS

South elevation and east-west elevation in comparison

Checking rooftop suitability for a PV system beforehand

Not every flat rooftop is suitable for the installation of a PV system. Companies should therefore have their building rooftops carefully inspected. Many factors come into play, but three are decisive:

  • Load-bearing capacity: With a flat rooftop installation, more weight rests on the rooftop surface than with a pitched rooftop installation. The substructure, solar modules and ballasting add a considerable load to the roof, depending on the project.
  • Roof cladding: Photovoltaic systems can be installed on almost all rooftop sealings. The mounting system you choose depends on whether the roof cladding is made of bitumen, trapezoidal sheets or foil.
  • Insulation: The insulation material must have a certain pressure load capacity, as prescribed for “used rooftop surfaces”.

SENS also offers such a suitability test as part of the design of rooftop systems for its customers.

Mounting photovoltaics on a flat roof

Basically, there are two ways to fix an elevation on a flat roof: Either the construction is firmly screwed to the roof or it is weighted down on the flat roof with ballasts. Both methods have advantages and disadvantages:

1. Flat roof PV with roof penetration

This mounting method ensures stable positioning and prevents the modules from coming loose in strong winds. For this purpose, holes are drilled into the roof in order to attach the modules directly. However, such roof penetration can compromise thermal insulation, protection against moisture and statics. Nevertheless, many companies opt for roof penetration because it offers a more stable and durable option for installing photovoltaics on a flat roof - especially in regions with strong wind or snow loads. A PV installation with roof penetration should definitely be carried out professionally. It also makes sense to use an additional protective film under the mounting system to protect the flat roof from other damage.

2. Flat roof PV without roof penetration

Instead of firmly bolting the elevation for the photovoltaic system, it can also be fixed with concrete walkway slabs or paving stones. The ballasts also counteract the "caterpillar effect": This refers to the effect that the base rail expands when it is hot and contracts again when it is cold, thus changing its position over time. Due to the heavy weight of the ballast, the load-bearing capacity must be carefully checked.

Ballasting for pv system Mannheim

Ballasting of the substructure provides additional security, though the rooftop had to have its load-bearing capacity checked.

Maximum safety for elevated photovoltaic systems

Solar installations on flat rooftops are more susceptible to wind than on pitched rooftops. The modules may lift in very strong winds. For safety reasons, the flat-rooftop PV construction should therefore always be bolted from a sloping roof of approx. three to five degrees.

Sufficient edge distance should also be ensured. Fall protection is provided by the use of safety anchor, which are fixed anchorage points you can attach steel rope to for personal safety.

Existing lightning protection is not an obstacle to a flat-rooftop PV system. The solar experts at SENS can easily integrate lightning protection into it (usually a combination of aluminium round wire on stones and high lightning rods).

Combining photovoltaics and green rooftops

Just because companies already filled their flat rooftops with green spaces doesn’t mean they can’t be combined with rooftop PV systems. They can be seamlessly integrated and offer several advantages. By planting low-growing plants, for example, the roof heats up less, keeping the temperature of the solar modules lower and thereby having a positive effect on the electricity yield.

> Learn more about the advantages and installation of PV systems on green rooftops here

Advantages of flat-rooftop photovoltaics

  • The elevation offers more flexibility, so the solar modules can be individually aligned to create an ideal angle of sunlight.
  • The installation can be done quickly because there is no need to remove roof tiles or other objects beforehand, and there’s no drilling in the roof cladding.
  • The inspection, maintenance and cleaning of the modules is easier than on houses with sloping roofs.

Disadvantages of flat-rooftop photovoltaics

  • Elevated solar modules are more susceptible to wind, so they have to be optimally ballasted or securely anchored to the roof.
  • Sufficient distance must be ensured to avoid shading, usually meaning that fewer solar modules can be installed on a flat rooftop than on a gabled rooftop with the same surface area, for example.

Your expert for PV systems on flat rooftops

Would you like to learn more? Then contact us! When it comes to the construction of PV systems on flat rooftops and the special features that need to be considered, the solar experts from SENS have all the required know-how. They provide the planning and realisation of rooftop PV systems for customers from industry, trade and commerce with production and logistics warehouses. High safety and maximum yield, as well as efficiency and longevity of the photovoltaic system and the roof cladding are the top priorities

Images: SENS

Update: 07.11.2023

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