Photovoltaics on the flat roof

Many people associate photovoltaics primarily with sloping roofs. After all, gable roofs, for example, offer a good angle of inclination for catching the sun's rays. Due to increasingly efficient photovoltaic modules, the dependence on orientation and inclination is becoming less and less important. Renewable, environmentally friendly power generation at home also works on flat roofs. Very well, in fact! In this guide, you will find out what advantages photovoltaic modules have to offer on flat roofs and what you need to consider when installing a solar system on your flat roof .

Flat roofs and photovoltaics: What you should consider

Do you have an unused flat roof? Then there is usually nothing to stop you from using photovoltaic technology. Whether it is a house, garage, carport, barn or patio roof: photovoltaics work just as well on flat roofs as they do on sloping roofs. The following points are particularly important for optimal use of your flat roof:

• Alignment and inclination of the modules
• Statics
• Elevation
• Shading and row spacing
• Installation on the roof

What is the difference between PV on flat roofs and on pitched roofs?

Orientation and inclination

The biggest difference between photovoltaics on a flat roof and a sloping roof concerns the orientation and inclination of the modules. With sloping roofs, the existing roof slopes already determine the angle of inclination of the modules. An angle of between 30 and 45 degrees is ideal here. The direction of the modules is also already predetermined for sloping roofs: roofs facing south, east and west are best. But north-facing roofs with a maximum inclination of 20 degrees are also suitable for generating your own electricity with today's modern PV modules.

These conditions, which are literally given by nature, are not applicable with flat roofs: In order to achieve a suitable angle of inclination to capture the solar radiation, the modules on the flat roof are often installed with special support devices, so-called elevations , to suit the sun.

This is also the great advantage of photovoltaics on flat roofs: the modules can be individually aligned and tilted in order to achieve the optimum yield depending on the electricity requirement. If you want to achieve maximum electricity yield and take advantage of the intensive phase of solar radiation, you should align the modules to the south. If the most electricity is used in the morning and midday and in the late afternoon or early evening, as is usual in most households, it is also worthwhile to align the modules to the east and west in order to benefit as much as possible from the sun.

Depending on local conditions and space requirements, the angle of inclination for flat roof PV systems is usually between 15 and 35 degrees in order to achieve optimum use of space. The greater the angle of inclination, the greater the mutual shading and the wind resistance in storms. Therefore, in addition to checking the static specifications, particular attention must also be paid to storm-proof anchoring of the modules.

Modules laid flat on the roof do not require any support, offer the least wind resistance and do not shade each other. However, they have a less optimal inclination towards the sun, which can mean they are sometimes less efficient and more susceptible to contamination.

Statics

Compared to pitched roofs, photovoltaics on flat roofs require special attention to statics : not every flat roof is designed to bear additional weight. This is especially true if you are considering solar modules on a garage, carport or patio roof. In addition, the module stands may require additional weight so that they can withstand storms and do not blow away.

But even with a more solidly built house, it is essential that the load-bearing capacity is checked by experts . The weight of the solar modules is added by the mounting . In the winter months, snow can also remain on the flat roof, which does not slide off again sooner or later on its own - unlike with a pitched roof. Therefore, have all components and factors carefully checked for this before installation .

Space efficiency

While the modules can be placed close together on a gable roof and next to each other, the inclined positioning of the flat roof modules creates shading depending on the position of the sun . For this reason, when erecting modules, a certain area must always remain unused between the rows of modules before the next modules can be installed. If there is enough space on your flat roof, this is not a problem. If you only have a limited roof area, you can make up for the loss of space with particularly powerful modules.

How to calculate the required row spacing for raised beds:

To find out how much space you need to leave between the individual rows of modules on your flat roof, you need to do a little calculation. ¹ First, you need to know the length (L) of your modules and the angle of inclination (α) at which you want to set them up. Then you can calculate the height (H) of the module as follows:

 L × sin(α) = H

The required row spacing (RA) is then three times the module height (H):

3 × H = RA

An example : You use modules with a length of 105 centimeters and set them up at an angle of 28 degrees. This results in a module height of about 49.29 centimeters:

105cm × sin(28) = 49.29cm

Three times the height results in a required row spacing of 147.87 centimeters.

3 × 49.29cm = 147.87cm

This means that around one and a half meters of space must be left between the rows of modules. Expert solar installers coordinate the row spacing, roof area, direction, angle of inclination and performance of the modules to ensure they are ideally suited to one another. This way, you can get the maximum yield from your system. If you would like a customized offer for your PV system, please contact the EWE experts at any time. We will be happy to advise you!

Since there is little space on some flat roofs, maintaining the optimal row spacing can be a challenge. Depending on the power requirements and the type of roof, it may also be possible to orient the solar modules to the east and west . In this case, the modules are positioned slightly flatter, back to back, are less susceptible to wind, and the substructure is usually cheaper:

This way you can make the most of the sun's movement and produce the most electricity in the morning and afternoon. Whether such a two-sided orientation on your flat roof is worthwhile depends on many factors - for example, the amount of solar radiation at the location, the height of your flat roof and the associated possible shading from neighboring houses or plants.

How are photovoltaic modules installed on a flat roof?

There are two ways to attach a photovoltaic system to your flat roof: with or without roof penetration. Roof penetration means that the roof covering is damaged during installation: the stands of the solar modules are screwed to the supporting roof beams of the building .

The advantage of screwing the modules is that they are particularly stable and less susceptible to wind damage. At the same time, the damage to the roof membrane creates holes that require professional roof sealing and thermal insulation . This means that there are no problems inside the house, such as water damage.

Usually, in order to avoid long-term problems with the roof's watertightness and to avoid jeopardizing the warranty, the modules can be installed without penetrating the roof : To do this, the fitters glue the supports to the roof if necessary and then weigh them down with weights so that the modules remain firmly anchored to the roof even in strong winds. Concrete slabs or gravel trays are often used as weights. How heavy these have to be to securely attach the modules depends on the distance of the modules from the edge of the roof, the angle of inclination, the regional wind load, the module substructure and any wind deflectors provided.

Wind deflectors can be mounted on the open sides of the elevated modules between the top edge of the modules and the surface of the roof to minimize the load from strong winds. Again, the additional weight must be carefully calculated so that it does not exceed the load-bearing capacity of the roof .

Is photovoltaics possible on a green roof?

Plants or grass on a flat roof do not represent an obstacle to installing a photovoltaic system if the structure and fastening options are suitable. Quite the opposite: a green roof actually improves the performance of the solar modules, as it ensures lower temperatures. A cool climate on the roof can increase the performance of the PV system by up to 4 percent ² .

For this reason, it can be worthwhile to install a green roof around the solar system if there is no planting on your roof yet. However, remember that the additional weight caused by the green roof must also be factored into the static analysis. It is also important to note that the green roof can still be maintained after the PV system has been installed.

Particularly light-colored plants, such as silver leaf, white felt lavender or silver edge chrysanthemum, or a white painted roof can increase the effectiveness of your PV system. The light surfaces reflect sunlight better than dark colors and ensure that more radiation reaches the solar modules and the roof heats up less. Silver foliage plants are also very resistant to heat and drought.

Snow on the flat roof – no electricity?

If solar modules are covered in snow in winter, this will of course also affect electricity production. The good news: even thick layers of snow do not damage the module as long as the load is evenly distributed. The PV modules are of course designed to withstand the snow loads that are common in Germany - with the safety margins that are common in the industry. So don't worry: the PV modules can definitely bear the snow.

It is important that you, as the operator of a flat roof system, refrain from climbing onto the roof yourself to clear the snow from the modules in winter. The risk of slipping puts you in danger and you may damage the modules with the cleaning equipment. For more information, see our guide on cleaning photovoltaic systems .

The special features of photovoltaics on flat roofs at a glance

If we compare photovoltaic modules on pitched and flat roofs, there are some pros and cons. Since the roof construction is usually predetermined, the following aspects are more of an aid to implementation than a decision on whether to have a flat roof or a pitched roof.

On the one hand, you should keep the following challenging aspects in mind when it comes to flat roofs.

Challenges of photovoltaics on flat roofs

• higher susceptibility to wind
• Loss of space due to row spacing when raised
• possibly additional load and shading from snow
• with roof penetration: good sealing and insulation required
• without roof penetration: additional weight due to additional weights

Advantages of PV systems on flat roofs

On the other hand, the conditions on a flat roof also offer you advantages :

• possibly higher yield
• freely selectable orientation
• freely selectable tilt angle
• easier maintenance and cleaning
• Additional efficiency increase possible with light colors or light plants

Photovoltaics is by no means only suitable for sloping roofs: Flat roofs also offer the necessary conditions to produce your own solar power. Regardless of the roof, you can use a solar system to generate environmentally friendly energy and noticeably reduce your electricity costs from the public power grid. You can find out more about the costs and benefits of a photovoltaic system in our guide.

So that you can use the energy generated on your gable or flat roof even when the sun isn't shining, for example in the evening or early in the morning, it's worth installing an electricity storage unit . An electricity storage unit enables you to use the electricity produced in your household at a later time and make your own consumption even more independent .