Sealing roof rail bolt holes to stop water leaks

The Problem

Aftermarket roof rails fitted with their supplied bolts allow water to leak through the fixing holes and track into the headliner. Silicone alone does not reliably seal a threaded joint: the raised thread lands prevent the silicone from flowing into and bridging the spiral gap, and the uncured bead cannot key to oily or galvanised steel, so a continuous waterproof skin never forms around the fastener.

Solution

1. Remove the existing roof rail bolts. On VW T5 and T6 the roof has factory captive nuts at the rail mounting points, so no headliner removal is required to access them. Other platforms (Transit, Sprinter) do not necessarily share this arrangement — confirm your van has captive nuts before you start, as some require interior trim removal to reach a free nut or backing plate.
2. Cut stainless steel M6 threaded rod into lengths roughly 10 mm longer than the original supplied bolts. Measure your own rails and holes and cut to suit — around 50 mm is typical for a standard six-hole rail, but verify against your fitment rather than assuming.
3. Thread a stainless steel M6 flat washer followed by an EPDM M6 x 16 mm rubber roofing washer onto each rod, with the rubber washer last so it will sit against the roof surface.
4. Insert each rod down through the roof hole into the captive nut and tighten a stainless steel flanged nyloc nut from above. Tighten by feel, not force: bring the nut down until the EPDM washer just compresses and spreads flat enough to fully cover the hole, then stop. Over-tightening dishes and splits the EPDM, breaking the seal; under-tightening leaves a gap the washer cannot bridge. Aim to flatten the rubber by roughly a third of its thickness — if you have a torque wrench, a low value in the region of 5–6 Nm suits an M6 sealing washer, but treat that as a starting point and verify the correct figure for your specific washer and rail with the manufacturer.
   • Optional: apply liquid PTFE to the thread before insertion, or inject a small amount of silicone into the hole first for extra insurance.
5. Slide the rail over the protruding studs, fit a stainless steel washer and nyloc nut to each stud, and tighten down.
   • If the slot in the rail is too narrow for a standard socket, grind a 1/4-inch drive socket on a bench grinder until it fits through.