Help with Dormer Windows! :(

I’m getting crazy with these. Each time I change the roof height, my dormer window is filled with roof again! (<— yes, I know, it’s awfully phrased… I’m french, remember! ;))

I build my attic, add the main roof, add windows to the attic, add roofs on the window attic. Untill then, everything is fine. The dormers have been “cleaned” from the main roof. Then, I edit the height of the roof, because the main roof and the window one can’t be both fine with the same height, and tadah, it’s borked. :(

How can I do this properly? PLEEEEASE?!

EDIT: I got it! My main roof was only one-tile wide, but it seemed to work when it’s two-tiles instead! Thanks for your suggestions! :)