Markings or visual irregularities on the surface of roofs and side walls result from the industrial sandwich construction method and the associated different degrees of expansion of the materials used:
Wood and PU strips, insulation material, GRP or aluminium sheet.
The characteristics vary depending on the weather and time of year. Particularly in the transitional periods from summer to cooler autumn, from warm days to cool nights, these characteristics can be far more noticeable than is the case when temperatures remain constant. The GRP or aluminium sheet expands when heated and contracts when cold. The materials underneath, the insulation material and the wood do the same, but not to the same extent and not at the same speed. From a physical point of view, this is a completely normal process.
The markings are particularly noticeable on dark and metallic-coloured vehicles.