If you have the problem of the roof not seating in the rain channel correctly, it may be beacuse the tension cable within the piping of the roof has snapped or corroded. If you have already tried replacing the elastic, of which there are numerous other threads on here on how to do, this may be your problem instead.
A new cable can be had for about £20, the part number is : 986 561 921 00
Open the roof about halfway, look at the base of the B pillar and you will see the bottom eyelet of the cable. Undo the screw and if you can pull the cable then its broken inside. It's a good idea to put a cloth underneath to avoid losing any screws etc down the drain hole!
To get to the top of the cable you will have to remove the front part of the roof from the frame as it is hidden inside behind the top of the window. Firstly remove the two roof latches:
Then the screws along the top rail:
Then the two at the corners that hold the side seal on:
Pull the seal completely away from the front and the side you're working on:
Under the side seal is a plastic retainer with four screws that needs to be removed, looks like this :
Under that are more phillips screws with one hidden under a piece of rubber seal that you need to pull off, took me a while to figure out why I couldn't undo it with all visable screws removed!
Now the fun bit, you need to carefully pull the roof back from the frame, there's a lot of adhesive but you will see underneath an aluminium plate with the top of the cable attached to a spring in shrinkwrap, which you will need to split :
Undo the top eyelet from the spring and pull it through the piping, here was mine, or at least half of it ....
Thread your new cable through the piping, attatch to the spring, and re wrap. I put the plastic back around it and then electrical tape to hold it.
The cable comes with the other eyelet unattacthed to allow you to thread it through so you will need to crimp it on. Then it's just a case of putting everything back together!
My roof now sits perfectly on that side so I hope this helps some of you with this problem if you have tried other options. It's a shame Porsche didn't make it from galvanised steel would save a lot of hassle!