Drainage

Convertible cars, both soft top and retractable hard tops, have drainage systems not found on their hard-top counterparts. The rubbers around the windows and doors may have special channels to guide water away and often the rear scuttle which the roof folds into will have gutters, channels and/or pipes allowing water to drain out of the bottom of the car.

These drainage systems are usually carefully concealed, and many people don't even realize they exist. But they are very important and need to be kept clean and free from dirt, debris and leaf litter. If the drainage channels become blocked, water leaks into the car causing wet carpets, damp smells, mould and mildew to form on the headlining. 

What it means

Drainage is the way a car is designed to deal with water that lands on it. Modern cars expect rainwater to get into places like roof gutters, scuttle panels, sunroof trays and around door and tailgate seals. Instead of trying to keep all water out of these areas, the manufacturer provides channels, trays, drain holes and tubes that collect the water and guide it down and out through controlled exit points. If this drainage system is clear and working, the water quietly leaves the car without ever reaching the carpets or trim.

Why it matters

• Stops water ending up inside: When drainage is working properly, water that gets past outer seals is caught and taken back outside instead of soaking soundproofing, wiring and carpets.
• Helps prevent rust and mould: Poor drainage leads to standing water inside body cavities, which encourages corrosion, mould growth and stale smells.
• Critical on convertibles and sunroofs: Soft tops, retractable roofs and sunroofs all rely heavily on trays and drains. A single blocked tube can flood a boot, footwell or control module.
• Guides proper leak diagnosis: Understanding how the drainage should work makes it possible to tell whether water is getting in through a bad seal, a split membrane or simply overflowing because a drain is blocked.

Where you’ll see it

You will see drainage mentioned on leak reports, inspection sheets and repair estimates, for example scuttle drains blocked, convertible roof drainage choked with debris or drainage holes behind bumper need clearing. It also appears in service bulletins and technical notes where manufacturers specify cleaning sunroof drains, door drains or air-conditioning evaporator drains as a maintenance item.

Context

Drainage runs all around a car. Common examples include the scuttle drains below the windscreen, sunroof cassette drains that run down the pillars, convertible roof gutters that discharge into the rear quarters, door and tailgate drain holes, and boot channel drains. Water is expected to land in these areas, then follow a stepped path out through grommets or slots in the body shell. Problems arise when leaves, mud, moss or trim sealant block the channels, when drain tubes split or disconnect, or when previous repairs have altered the intended path. Effective leak work therefore means understanding the original drainage design and restoring it, not just sealing over whatever looks wet.

Common mistakes

• Trying to seal every wet area with silicone without first understanding how the drainage is supposed to work.
• Poking sharp objects into drain holes and tubes, which can puncture pipes or grommets and make leaks worse.
• Assuming a wet footwell always means a failed windscreen or door seal, when the real cause is often a blocked scuttle or sunroof drain overflowing higher up.
• Ignoring regular cleaning of leaves and debris from scuttle panels, convertible roof gutters and boot channels until water starts pouring into the car during heavy rain.

Written by Danny Argent. Last updated 03/12/2025 15:13