Is it normal for water to collect on the roof? No, but it can happen.
Due to the nature of sandwich construction, unevenness in the roofs is normal. Due to the material-specific properties of the 3 different roof materials: Sheet metal, insulation material and wood also have 3 different expansion coefficients. The material that absorbs the most heat from the sun expands more than the other materials at warm temperatures and also contracts more at colder temperatures. As there are load differences depending on the layout, there is a certain amount of sagging of the roof on one side, as we have self-supporting surfaces and do not incorporate any additional stiffeners into the roof.
This is not a justified defect. There is also no repair method that enables a permanently flat roof. Due to this situation, there is no disadvantage in terms of service life, quality or durability of the superstructure.