I was gonna open a new thread, but found this one describing exactly what I did today, so let me expand a little and give my thumbs up.
My 2007 Spider had two typical roof issues - rear window getting stuck on plastic embellishment of head hoops and arc corner sometimes slightly popping up after closure.
Both issues are solved, changing 2+2 gas struts inside the hood. Mine were totally shot, just as I suspect most of them are after couple of years (keep reading why).
First pair, upper one, rated at 100N, which seems to be responsible for tensioning the roof between 3rd and 4th arc. This keeps (heavy) rear window slightly up when going over the hoops. There´re also elastic straps, that go along each side of the window (photo below), which after couple of years loose elasticity - I pulled them slightly and made a knot, to tense them again.
Second pair of the struts, rated at 200N, keeps lower, fifth arc tensed and roof sealed to the cover. If the corner pops, or you need a lot of force to lock over-the centre struts, this most probably is the gas spring being dead weight.
My 100N springs were completely shot, no force whatsoever. The 200N, one had around 35N (causing shearing of the plastic cover on the hoop on that side), the other had around 120N.
For the replacement gas springs, you´re looking for Stabilus 2609NH and 2607NR. Avoid Alfa Service prices, they're double retail. Myself, I've fitted FA Krosno 23286 250N as lower springs, and Hahn Gasfedern 106729.2 + 2xPX22, charged to 150N (these are industrial quality springs, with valve to regulate or charge the spring, and steel ball mounts). FA Krosno you can get around here for 8-10 quid, Hahn are closer to 18-20 quid/piece. PM me, if you want some contacts here in Poland to get them, many places ship internationally. You basically want 155mm, 40mm displacement, 6/15 gas springs with fi10 ball mounting.
10 full cycles of opening/closing confirm that no issues whatsoever remain.
Removal/refitting is really easy, with just two rivets to drill (I've replaced them with small screws afterwards), and four screws to undo. PM me if you need a detailed procedure.
By the way, popping of the corner is also often caused by the rubber limiter on the over-the-center struts. Undo small allen screw that blocks it, and turn the rubber in.
Last but not least - why those gas springs break? When ordering my springs I spoke to the engineer that designs them, and apparently the biggest factor in gas springs loosing pressure is amount of vibrations, especially when extended.
Unlike gas springs in the boot, or any other places in the car, the ones in the roof are constantly vibrating with the roof up and the car at speed. This means that spring quality is paramount, but anyway they´ll die after 3-4 years anyway and should be replaced. (The ones in the boot can last even 20 years. My other Alfa proves that
So, it was a good day in the garage, now I need a slightly warmer days to take the Spider for a spin...