Flat-rooftop photovoltaics

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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

Which flat roof is suitable for PV?

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.

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 flat rooftops

There are typically two types of mounting available, each with advantages and disadvantages:

• The PV substructure is firmly screwed to the roof. This option ensures stable positioning and prevents the modules from coming loose in strong winds. The disadvantage is that the screw connection creates holes in the roof cladding. Since this is paramount for good thermal insulation, protection against moisture and statics, it’s essential that the PV installation is carried out professionally. Using an additional protective film under the base rails to protect the roof from other damage is another recommendation.
   
• The elevation for the PV system is weighted down with ballasts. Usually, concrete paving slabs or paving stones hold the modules in place. These ballasts counteract the “caterpillar effect”, which refers to the base rail expanding when it’s hot and contracting again when it’s cold, thus changing its position over time. The disadvantage here is that there is more weight on the rooftop. The load-bearing capacity must therefore be carefully checked.

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

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