Inspiration

[Whis]

I would create a roof like this by, indeed, painting it light gray and then use dark gray chalk (powder) to get the distinctive patterns. My experience is that you can create perfect gradients from light gray to dark gray, or even black, by using this method. I believe dry brushing is more effective for highlighting details and such then it is for creating smooth color gradients.

Hope this helps and I'm very curious for the results, whatever method you will use