Thanks.
I did not use the foam gasket. I would not use any gasket in this application and the one provided is polyethylene foam which it seems to me is particularly unsuitable. If the roof was crowned enough to require something other than sealant it should be filled with something solid and shaped to the crown, then wet sealed without a gasket.
Gaskets generally work if the flange and the surface are stiff enough to retain some clamping pressure between the fasteners over the long term. The flange is certainly not stiff enough, and the roof is suspect!
