Of course, you have to clean the glass roof now and then so it doesn't look filthy. I have to do that in the next few weeks as well..
To be completely honest....ufff.....I have never cleaned it, even though I was worried about it. The rain generally takes care of it. It also depends somewhat on the slope used. Even if you clean it, the cloth would be gray again after 2 weeks-
For that reason, my glass-covered terrace was one of the very few "really great" decisions when I built my house back then; also inexpensive if you keep it simple.