Roofs have always been a problem with Fortwo's. The 450 had glass roofs that would disintegrate into a million pieces if a flying stone hit it, as it was hardened glass instead of laminate. Luckly i went for the plastic solid roof of the base model (a Pure), and avoided such problems.
Then they thought a plastic clear roof was the answer, instead of glass, and the 451 had a plastic clear roof, and mine started to craze as the UV rays of the Sun created micro cracking that shimmered with the sunlight. It looked nice but was a bit worrying. Usually this happened outside of warranty as the roof took a few years to craze, so we all just had to live with it. It was a common problem.
Now with the 453, you can have a plastic roof with cloth style covering, or a clear roof again, and by the sounds of it there are problems again on some, as they age. I went for a 453 cabrio this time to avoid such problems. Just got to worry about the mechanism working, open and close, DOH
My neighbour who owns a 453 Fortwo coupe with the plastic roof with cloth style covering, went to buy and get a small glass tilting sunroof fitted (a Webasco Hollandia 100, the smallest in the range) and the sunroof fitter said it was too heavy for the plastic roof, and said it could cave in, and refused to fit it.
Its never easy with Smart car roofs, no matter which one you buy >
