The longitudinal waves/bubble formation on the left and right over the entire length of the roof on a width of approx. 20 cm on the roof are so-called expansion joints. Here the GRP is not glued. GRP and aluminium sheet both have the property that they expand and retract with temperature changes, which means that when it gets warm they expand and when it gets cold they contract again. For this reason, a strip, a so-called expansion joint, was not glued in the areas on the left and right of the roof over the entire length of the vehicle in order to give the respective material some "space" to work.
Gluing over the entire surface would lead to cracking noises, distortion and tearing out of furniture joints to the roof. This can be in the form of several individual raises as well as in the form of a continuous elongated raise. This has no effect on quality and durability and does not constitute a defect.
Due to small optimisations in the manufacturing process, expansion joints are no longer necessary since the model year change from 2018 to 2019 and are no longer installed in this way at Dethleffs.