The video helps if you’re bored with reading.
Identify bad welds!
At first, I was just guessing where the leaks might be coming from but then I realized that if you compare sides of the van (and therefore supposedly similar welds on each side of the van) you can quickly see where some welding jobs look far less perfect than the opposing side.
I did this and ended up sealing a large portion of the area around the rear driver side of my roof. I also sealed halfway down the seam of the van on the driver side rear due to the weld looking dark (missing pieces) and patchy as compared to the perfect opposing (passenger) side.
Seal the weak points on your Transit roof.
There are a load of little caps “sealing” factory hole locations. These caps are merely painted over as their sealant from the Ford factory and therefore incredibly prone to leaking according to the internet…
When I got up on my roof and started inspecting and sealing my caps, I actually already found two of them cracking and therefore ready to admit water.
Just take some of your sealant and seal each of these caps. It’ll take no time and no cost and potentially save you huge headaches down the leaky road.
Here is a helpful forum for locating all your holes.
I started out by just sealing a little bit here and there every time I’d notice a leak. If I could go back I’d just seal anything that looks even potentially leak-worthy and be done with it.