Very Nice Jörg......

Roof looks great..... I have never used chalks before, I use washes......
So for the roof I would use a black wash to get the detail showing and then I would dry-brush some markings like you did.
For the walls, you can take a light grey wash. I normally let it dry for 5 min or so and then wipe the bricks off with a wet clean earbud to get rid of the wash on the bricks.
It does take some of the colour off from between the bricks, but it works for me.
Then everything else I normally just dry-brush.....
Anycase, that is my process and it works for me. The secret is to try and play around until you find something that works for you.
Cheers
