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