Spider Trap
Quick answer: A spider trap is the fitting at the bottom end of a sunroof drain pipe, where it exits at the sill or wheel arch, put there to stop insects getting up the pipe. The form varies from car to car: some are a soft rubber flap, others a small nylon box with a plastic grate. Whatever shape it takes, it is a restriction at the far end of the drain, and it is one of the most common places a sunroof drain blocks.
Sunroof drains run from the corners of the sunroof cassette down through the pillars and out at the bottom of the car. At that exit, many manufacturers fit something to stop insects travelling back up the pipe. The trade name for it is a spider trap.
What it means
The name describes the job, not the part, and the part is not standardised. We have had one apart that was a small nylon box with a plastic grate across it. Our own video on the five most common causes of car water leaks describes a different one: a little rubber flap. Both do the same job in the same place, and both are equally capable of stopping water as well as insects. If you are looking for one on a particular car, look for whatever sits at the exit end of the drain rather than for a specific shape.
Why it matters
It is a filter on the far end of a pipe nobody can see, and filters block. Grit, silt and the fine debris that washes down off the roof all arrive at it and stay there, and once it is blocked the pipe backs up. The water then does what it does with any blocked drain: it fills the cassette tray and slops over the side into the headlining the first time the car pulls away.
Where the blockage sits is what makes it awkward. It is at the very bottom of the run, the hardest part to reach and the last part anyone thinks to check, which is why clearing a sunroof drain from the top so often fails. The obstruction is at the other end.
Why we often take them out
The spider trap solves a problem British cars do not really have. We have yet to meet a motorist whose sunroof drain was blocked by a spider; we meet a great many whose drains are blocked by leaves, grit and rain. A part fitted against the first problem, which reliably causes the second, is not earning its place. So where a car is clearly better off without one, we take it off and let the pipe drain freely.
That is what happened on a 2018 VW Tiguan: the fitting at the bottom of the drain blocks easily because of the way it is made, and there is no real need for it to be there at all.
Common mistakes
- Blasting compressed air straight down the drain from the top. It can blow the hose off its connector behind the dashboard, and a disconnected drain inside the car is a far worse problem than a blocked one.
- Flushing from the top, watching the water disappear, and calling the drain clear while the trap at the bottom is still partly stuck.
- Pushing a coat hanger or other stiff wire down a soft silicone drain pipe, which splits it or forces it off its connector.
- Fitting an identical new trap to a car that lives under trees, which buys a season at best before the same blockage returns.