While the slow drip drip of water in tents is a mainstay in most adventure stories, I’d prefer to skip that plot line.
So taking the advice of one of my friends I am applying fiberglass to the seams on the roof. The seams being where the pieces of plywood meet. (I’m also putting fiberglass over any cracks or knotholes.)

If I’m doing this correctly, it isn’t very difficult. The epoxy sets up very quickly so make tiny amounts of mix and make more as you go.

The resin is the expensive part of the equation. I got a 1 quart can and it looks like it will be more than enough.

For reference, I made sure that all the plywood was screwed securely to the joists before I applied the fiberglass. I wanted to seal any holes, including screw holes, not add more holes to the brittle fiberglass afterwards.