Sounds like the roof needs re-proofing as the water is not beading on it. You have not said whether it needs cleaning and maybe re-colouring first.
I used Renovo (3 products, cleaner, colourant and proofer) on the hood of my 968 cab when I first bought it - previous owner used Fabsil, but I needed to recolour it as well. If you decide on this, be aware that the colourant is thinner than water and gets everywhere, so mask off naywhere you don't want it to go, and if any gets onto the bodywork wipe it off quickly, it's a swine to remove once its dry (ask me how I know!!). Also do not fold the roof for at least 24 hours after you finish, or you'll get lighter coloured crease marks.
4 years on, the colour is still quite good and water still beads on the roof.
I've also seen good reports about Autoglym fabric roof stuff but have not used it myself.