Step 1, I pulled the door apart and recovered as much of the shiplap paneling as I could. It's cheap pine, unfinished, but it had aged to a nice brown and so I tried to recycle what I could.
Next, get some safe workable decking up on the intact joists while the hatch opening was large and available. The hatch is roughly centered on the 20' dimension of the shed, toward one wall, so there were two 8' by 8' areas to either end where the joists were intact. 2 by 6 joists, mostly 16" on center, and insulated.
The plywood/particle board that was up there was nailed in with one or two small finish nails per sheet, so no structural tie in. There were 2 by 6s screwed down the center of the attic to serve as cross-bracing for the rafters. I got four sheets of 19/32", screwed down half of the deck to brace the joists, then removed the 2 by 6 and screwed down the rest. 1-5/8" construction screws every 12 inches. The deck surface area is exactly 8' wide, but I had to cut the sheets in half in order to get them through the hatch.
Next, get some safe workable decking up on the intact joists while the hatch opening was large and available. The hatch is roughly centered on the 20' dimension of the shed, toward one wall, so there were two 8' by 8' areas to either end where the joists were intact. 2 by 6 joists, mostly 16" on center, and insulated.
The plywood/particle board that was up there was nailed in with one or two small finish nails per sheet, so no structural tie in. There were 2 by 6s screwed down the center of the attic to serve as cross-bracing for the rafters. I got four sheets of 19/32", screwed down half of the deck to brace the joists, then removed the 2 by 6 and screwed down the rest. 1-5/8" construction screws every 12 inches. The deck surface area is exactly 8' wide, but I had to cut the sheets in half in order to get them through the hatch.