My TT has a "no walk" roof typical of a light weight TT. It is eight years old and in the five years I have owned it, I cleaned it once, last year, with a ladder on the side, a brush and soapy water.
Other than removing the top layer of dirt, I really do not see any benefit to doing what a days drive in a hard rain would do.
I wash the sides because the dirt and grime does not look good but I do not recall anyone commenting on my roof being dirty.
At this point, in think I will limit my roof activities to seal inspection. No reason to risk damaging the roof with some ham handed activity on my part.