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The roof is your standard roof material, available as OEM or alternative suppliers, if maintained correctly will last a long time. They are typical lined with rubber in the centre, making them waterproof.
If left under the elements, its inevitable the roof will deteriorate quickly, mitigate this by using a weatherproof car cover.
If you find water leaking above the steering wheel, check the drainage channels are not blocked. This can be found under the 2 metal plates on either side of the rollbar, held down by 2 screws. A buildup of dirt over the years will force water under the roof seal.
Water gets between the roof and the header rail, it needs to be able to drain through the windscreen pillar, otherwise air pressure will drive it past the rubber seal into the car. Too much sealant when replacing the windscreen can block the channels.
Remove the plate and unblock using some soft thin bendable wire, then test by slowly pouring water down the channels, it should reappear by your feet under the car.
The area around the V shape of the frame, above the door button is a weak point and tends to weaken over time from constant flexing, this will slowly shred and wear, leaving you no option but to repair or start budgeting for a new roof.
The roof frame is rigid and not susceptible to failure apart from the frame bolts, which can shear and break off. Called a Pivot bolt, they are not available to buy separately but a precision engineering firm can replicate these.
The BMW toothpaste cleaner is very good at improving the appearance and remove light scratches. Quite fragile, it is good practice when folding to place a cloth or soft non abrasive material between screen and avoid contact with any other surface such as the space saver or CD Changer.
During winter months, keep the hood up and stretched. Additional audio equipment such as Amplifiers give off alot of heat which will adversely affect the screen.