Photovoltaics on a bitumen roof

Generating electricity on bitumen roofs: what you need to consider

Flat roofs are widely used in industry and are ideal for photovoltaic systems. These roof surfaces are often covered with bitumen sheets - a material that is very weather-resistant, but also has some disadvantages. These need to be considered if a photovoltaic system is to be installed on your own bitumen roof. For example, the roof waterproofing and solar installation must be well-matched to prevent damage and moisture ingress. Fire protection also deserves special attention. Find out here what you need to consider when installing roof systems on bitumen.

Advantages of bitumen on flat roofs

Bitumen is a material that is extremely resistant to water and weathering. This is why bitumen membranes are often used for waterproofing flat roofs on large industrial buildings. The membranes are usually applied to the roof in two layers and welded or glued at overlapping points. This ensures a tight connection so that no moisture can penetrate. In addition to high durability, the advantages of bitumen roofs include cost-effective installation and comparatively simple maintenance.


When installing a PV system on a bitumen roof, the distances to light strips and rainwater drains must be observed.


Disadvantages of bitumen roofs

Bitumen is a thermoplastic material. This means that its aggregate state depends on the temperature: While the material is solid when cold, it becomes softer and more viscous at higher temperatures. A temperature difference of just a few degrees is sufficient for this. This elasticity can become a problem with bitumen membranes on flat roofs: In bright sunlight, the membranes can heat up so much that they slip. When it cools down, the sheets then contract again. As a result, they become increasingly brittle over time and develop cracks through which water can penetrate. The fact that bitumen roofs have low reflectivity further increases the heating effect.

Bitumen membranes can be repaired by gluing a new layer of bitumen membrane over the cracks, but this is not a permanent solution, as after the third or fourth repair layer, water can drain away less and less easily. In addition, the greater the thickness of the layer, the greater the risk of fire and the structural strength may not be sufficient.

What needs to be considered for fire protection?

Fire protection is always an important issue when installing photovoltaic roof systemsFire protection measures should be considered as early as the system planning stage and the highest possible safety standard should be selected. The PV modules and all other components should be tested and TÜV-certified. Professional electrical installation is also crucial in order to minimise the risk of a fire starting or spreading.

Additional fire protection requirements apply to a bitumen roof. This is because bitumen is highly flammable. Special fire protection measures are therefore necessary to ensure maximum safety. These can be fire-retardant coatings to protect the surface from fire or fire barriers to prevent the spread of fire on the flat roof. Fire-retardant insulation materials under the bitumen roof also increase safety as they slow down the spread of fire.

In order to find the best solution for the respective situation, a fire protection expert should be involved in the planning. Information on suitable fire protection measures can also be found in VdS Guideline 6023 "Photovoltaic systems on roof surfaces with combustible building materials". It applies to both new and existing systems.


When does PV on bitumen roofs make sense?

Whether your flat roof is suitable for a solar installation depends on various factors. The following requirements in particular must be met and should be checked by experts, such as structural engineers or roofers:

  • High load-bearing capacity of the flat roof, as the mounting system and ballasting place more weight on the roof than a system on a pitched roof
  • High compressive strength of the insulation material in accordance with the specification for "utilised roof surfaces"
  • At least 20 years remaining service life of the roof waterproofing so that the solar system can remain installed for a long time.

SENS offers its customers this type of suitability test as part of the design of photovoltaic systems on flat roofs. It ensures that the flat roof can withstand the additional load of a solar installation and that the bitumen membranes are in sufficiently good condition.


Example project from SENS in Zeusnik, Germany: planning and installation of a nearly 650 kWp high-performance PV system on bitumen


When does a PV installation not pay off?

If cracks are found in the bitumen sheeting during the inspection of the existing flat roof construction, this can usually be repaired by adding an additional layer before the photovoltaic system is installed. This must then be statically approved.

It is also important to professionally assess the remaining useful life of the roof waterproofing. This is because if the roof leaks after the PV system has been installed, the solar system would have to be completely dismantled again in order to renovate the roof. This is very costly, so that a worthwhile return on investment is no longer possible.


How is a PV system mounted on a bitumen roof?

The installation of a photovoltaic system on a flat roof requires precise planning. There are two basic types of flat roof mounting: either the structure is firmly bolted to the roof or it is ballasted to the flat roof. The second method is usually chosen for bitumen roofs because this type of fastening does not require any roof penetration. This is the best way to avoid possible leaks.

The elevation for the photovoltaic system is ballasted using stones, which are placed in the rails for the substructure. The ballast fixes the frame and also counteracts the "caterpillar effect": This refers to the effect that the base rail expands when it is hot and contracts again when it is cold, thereby changing its position over time.


Fastening the substructure to the bitumen roof with stones in the rails of the substructure


The weights on the substructure fix the frame and counteract the "caterpillar effect"


However, it is also possible to screw the substructure for the photovoltaic system firmly to the bitumen roof. A special mounting set should be used for this: Sealing plates seal the hole, distribute the load over a large radius and thus protect the bitumen flat roof as much as possible.

What needs to be considered during installation?

If a solar installation is mounted on a bitumen roof, as with all roofs, care must be taken to ensure that it is structurally sound: The support system must not slip, even under heavy wind or snow loads. In addition, the functionality of the roof waterproofing must not be impaired. This means that rain and melt water must be able to run off unhindered. This is particularly important for bitumen roofs. The bitumen membranes should be laid in multiple layers, whereby a combination of bitumen and polymer bitumen is ideal. This ensures high resistance to perforation up to a total thickness of 10 mm. Depending on the desired elongation, tear and perforation resistance, the carrier insert can be made of reinforced glass fibre, polyester fibre, woven glass threads or a combination of the different materials.


Is your flat roof suitable for a photovoltaic system?

Thanks to their angle of inclination, flat roofs are ideal for the installation of solar modules. This applies to all types of flat roofs - whether gravel roof, bitumen roof or foil roof.

PV systems can also be easily installed on a green flat roof. The combination of photovoltaics and green roofs even has a positive effect on the performance of the photovoltaic system because the roof heats up less. The green roof also protects against UV radiation and mechanical damage, which benefits the service life of bitumen roofs. However, in order to green a bitumen roof, the roof must be waterproof and root-resistant. An additional root protection film is essential.

No matter what type of flat roof you have: Have it inspected by experts before planning a solar energy system! The SENS team will be happy to assist you. We have been designing and realising energy-efficient solar system solutions for customers from industry, trade and commerce for years. Of course, this also includes checking the condition of the existing roof structure - whether pitched or flat roof - as well as providing advice on all aspects of roof waterproofing.

Find out more about how to realise your PV roof system step by step or about the special features of different roof types in connection with PV in our free white paper.



Images: Bernd Retzlik

Published: 18 March 2024

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