Calculating ballast is very complex and almost as complex as determining the load capacity of the entire system.
The aim is to ensure structural integrity of the entire flat roof system for each individual project. Wind loads may cause that the entire flat roof system or parts of the system to shift and wind suction could cause the system to lift off the roof surface. Applying ballast is necessary in order to prevent this from happening. Using the necessary amount of ballast is crucial to counteract the potential effects of periodic wind load. Accordingly, first the wind load is determined and then the required ballast.
For each project, the K2 calculation software Base On determines site-specific maximum peak velocity pressure according to Eurocode specifications. However, when the wind blows over the system, it does not burden the system the same way everywhere. Depending on the specific geometry, the wind will affect areas such as exposed corners and edges more than the wind-shaded middle areas of the system. We have created special wind tunnel tests and wind reports in order to determine this.
On the basis of these wind tunnel tests, Base On determines the corresponding areas for each individual system and even for each individual module. In simple terms, the wind load is then divided into a horizontal vector and a vertical vector for each module.
(Fig.: Ballast plan in Base On)
In order to ensure structural stability, the ballast must be higher than the vertical vector wind load and the friction horizontal resistance must be greater than the horizontal vector wind load. The combination of both factors provides structural verification.
(Fig.: Test in the wind tunnel)
In addition to the general specifications from Eurocode, individual requirements for each project must be taken into account. The project location, building height and unevenness of the surroundings are particularly important. These factors are taken into account when formulating conclusions about site-specific wind conditions.
(Fig.: Entering the building height in Base On)
The individual geometry of each system alters the way the wind flows around the PV system.
For these reasons, a separate wind tunnel test is required for each system. On the other hand, this feature makes it possible to influence the wind load to a certain extent via wind design in the development of flat roof systems.
In the past, when many buildings were planned, installing a PV system on the flat roof was not a consideration. Load reserve for ballast was not taken into account in the planning of the building. As a result, the load reserve of these roofs is low and systems requiring only very little ballast are sought.
However, safety must never be neglected, as modules with incorrect ballast pose a major security risk. Therefore, the ballast calculation is very important.
The flat roof systems S-Dome and D-Dome are ballasted with slabs via the Porter (see images on the left) or the Speed Porter (see video below). We do not advise the use of sandbags, water bags or similar. If it is possible, a fixed connection to the roof may be an option.
A special feature of the S-Rock is its load-bearing ballast capability. It has an integrated module support element, where slabs are placed. This reduces the assembly time (see image on the right).