Sunroof joints for BMW Serie 3 - E21

A sunroof is not watertight; it is designed to drain water

Contrary to popular belief, a sunroof does not prevent water from entering. A small amount of water always seeps in between the panel and the bodywork; it is collected by a gutter running around the frame and then drained away via four drains – one at each corner – which run down the pillars to the underside of the car.

The seal therefore serves to limit water ingress and, above all, to eliminate wind noise, not to make the whole assembly watertight. This has a crucial practical implication: if water appears in the passenger compartment, the seal is not necessarily to blame. A blocked drain causes the gutter to overflow, and water will then seep inside even if the seal is brand new. You should therefore check the drains before ordering anything, by pouring a little water into each corner of the frame and checking that it runs out from under the vehicle.

Rubber and velour: two different types of seal

• The rubber perimeter seal runs all the way round the panel and presses tightly against the frame when closed. It is this seal that prevents water ingress and reduces wind noise, and it is this seal that hardens over time.
• The velvet seals, or slide felts, line the rails along which the panel slides. They guide the panel, cushion the movement and prevent squeaking. When worn, they allow the panel to vibrate and creak when opened, without causing a leak.

A panel that leaks is down to the former; a panel that squeaks is down to the latter. During a complete refurbishment, both are replaced at the same time, as the removal process is the same.

How to recognise a seal that has reached the end of its life

• The rubber feels hard to the touch and does not spring back into shape when squeezed between two fingers.
• It retains the imprint of the frame permanently, rather than springing back into shape when the roof is open.
• Cracks appear in the corners, which are the areas under the most strain.
• A whistling sound at a steady speed, which stops if you press on the panel from the inside.
• The panel no longer sits flush with the roof, or is flush on one side only.

A hardened seal does not recover: conditioning products restore a decent appearance but do not restore the lost elasticity. They are useful for preventative maintenance on a seal that is still flexible, not for repairs.

Fitting and maintenance

Many sunroof seals come in lengths that need to be cut to size and joined. Always position the joint atthe rear of the panel, never at the front where it would be exposed to headwind, and cut both ends square so that the joint is invisible and watertight.

Before permanently bonding the seal in place, position it and close the roof to checkthat it lies flush: a seal that is too thick will prevent the panel from lowering, whilst a poorly positioned seal will cause it to bear the weight on one side only. Then, carry out maintenance. A rubber care product, applied once or twice a year to a clean seal, effectively delays hardening. However, avoid using ordinary greasy substances, as these cause certain types of rubber to swell and become permanently deformed.