Thickness of OSB block is 1/2".....please note this info is for a Ford RB van.
Interestingly, I was up in Fresno last week and stopped in at the Sportsmobile factory. Jonathan was very helpful answering lots of my questions when I mentioned that I have a PH and would be installing it in a van. We went on a tour of the plant floor and checked out a few vans with holes in the roof being prepped for the PH.
One useful bit is that the butyl sealant tape SMB uses to seal the canvas to the roof is from McMaster-Carr (looks like it's P/N 76385A16).
On the ford RB body, the front edge where the canvas attaches to the roof has roof corrugations under it. The valleys just have a double layer of the gooey butyl tape (at least on the PH that I removed from a 2003 RB).
One thing from the factory tour threw me a bit of a curve ball......One new vans, and according to Jonathan and the worker building the van on the production line, they remove a roof cross-member support from the roof before making the cut and they epoxy and screw it behind the roof cross member at the B pillar, so the cutout is one cross-member width behind the welded in cross-member at the B pillar (the cut out starts at the rear edge of the epoxied in cross member). I asked how long they have been "doubling up" the roof cross-member at the B pillar and the answer was "forever".
Well, "forever" apparently doesn't go back to 2003.......
Yesterday I drove down to San Diego and carefully re-measured the roof-hole in the van that my top came from and it does not have a doubled-up cross-member at the B pillar. I'm pretty sure that if I did it the new way ie doubling up the roof support, then cutting the hole, the top would be about 4" or so too far rearward. I'm assuming that SMB either relocated the holes in the top or the rails to correct for the more rearward hole cutout.
FWIW: I have a thread going about refreshing my PH viewtopic.php?f=24&t=11764