The longitudinal waves/bubble formation on the left and right over the entire length of the roof over a width of approx. 20 cm on the roof are so-called expansion joints. The GRP is not bonded here. GRP and aluminium sheet both have the property that they work with temperature changes, which means that they expand when it gets warm and contract again when it gets cold. For this reason, a strip, a so-called expansion joint, has not been 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.
Full-surface bonding would lead to cracking noises, tension and tearing of furniture connections to the roof. This can take the form of several individual bumps as well as a continuous elongated bump. This has no effect on quality and durability and does not constitute a defect.
Due to small optimisations in the manufacturing process, expansion joints have no longer been necessary since the 2018 model year change to 2019 and are no longer installed in this way at Dethleffs.