Revisions log for Answer #1135

Yes, there is a DIN that writes something about 8 mm. Basically metal always needs some air to expand.

As I understand it: ...it is often installed in such a way that the guide rail rests on the window sill rail (as seen in your case) and this is also _not supposed to be wrong_, because it can be ensured that the window sill is easier to clean. From a thermal point of view, this gap is not relevant.

Corinthians pull themselves up by the 8mm and then may be annoyed later that dirt spots form on their window sills because they cannot clean under the smaller/equal 8mm. That is the difference between theory and practice! This is my layman's half-knowledge on this subject.