Hi - I suppose, if someone has washed the roof then, not knowing what product they used, it may have something to do with why its faded.
I'm not sure that the canvass roof is supposed to be 'washed' in the same way the body work would be! But after some years I suppose it would get fairly grimey.
I have cleaned mine recently with Renovo. See
here. The same people do a colour reviver. See
here. I've not used this but I've read reviews that it has bought the roof to looking nearly new again!
Hope this helps