Anyone installing a roof rack (e.g., on a VW T6/T6.1 , but also on other vehicles) usually thinks about load capacity, crossbars, accessories, and appearance. However, what often determines long-term reliability is a detail: the sealing of the threaded fixings to the vehicle roof .
Because water can penetrate the car body at precisely these points – not just anywhere , but through the threaded points where the roof rack is screwed on. And this happens more often than many people think.
What are "threaded points" on the roof – and why are they critical?
Threaded points (e.g. threaded inserts, rivet nuts, factory-installed threaded mounts) are the screw points where the feet/adapters/brackets of the roof rack are attached.
These areas are constantly exposed to stress:
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Rain, snow, condensation
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Splash water from the road
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Car wash / High-pressure cleaning
Important: A thread is not automatically watertight just because the screw is tight.
How water gets into the vehicle through threaded fasteners
Even with a properly tightened screw, microscopic gaps remain. Water often finds its way in precisely there:
1) Capillary action in the thread
A thread acts like a fine channel. Water can be drawn inwards along the threads – especially during prolonged exposure to moisture and temperature fluctuations.
2) Under screw head / washer / contact surface
If the surface does not seal properly (e.g. due to minimal unevenness, settling, slippage), water runs under the contact surface – and ends up back at the thread.
3) At the base of the threaded insert
Depending on the roof structure, water can enter cavities/layers as soon as it reaches the threaded area – often initially unnoticed .
Rule of thumb: Water doesn't need a visible gap – a micro-gap is enough.
What can happen if it leaks "just a little" through the thread?
Thread leakage is insidious because it often develops gradually. Typical consequences:
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Corrosion around the thread point (and later poor grip)
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Damp insulation → odor, mold
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Water migrates into components/connections → Electrical risks
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Stains on the headliner , deformations, complex drying process
Important: The problem often only becomes apparent weeks later, when moisture is already "working" inside.
Why this quickly becomes a major construction site, especially with the Multivan
In some simple interior designs, access to areas under the headliner is relatively straightforward.
In the case of the VW Multivan (and generally vehicles with complex technology under the headliner), a leak at a single threaded point can mean:
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more disassembly (headliner/trim panels),
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more troubleshooting,
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More time for drying and reconstruction.
In short: What is still manageable with a "simpler" version quickly becomes a real mental challenge with the Multivan.
Airholder recommendation: Always seal threaded points
When installing a roof rack (e.g., Airholder on VW T6/T6.1), we strongly recommend:
✅ Every threaded fastening on the roof must be sealed – not just “around” the outside, but in such a way that water cannot find its way along the thread.
This always involves two levels:
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Sealing the thread path (to prevent water from migrating along the threads)
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Sealing the contact surface (so that water doesn't even reach the threaded point)
Important: Sealing only works on a clean, dry, and grease-free surface. Otherwise, the material will detach later – and leakage is inevitable.
Typical mistakes we see again and again
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"Tightly tight = airtight" (unfortunately not)
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Seal only the top; the thread path remains open.
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Mount dry and then "re-lubricate" on the outside later.
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Unsuitable sealants that detach over time
Important: Subsequent "external sealing" rarely solves the problem if water is already migrating through the thread.
