adding a Vapor Liner
The problem was that water was dripping down on the boat--much more from condensation than from the few leaks--and interfering in my work on the boat. What to do? After thinking about many options, and as always trying to keep the cost down, I decided to install an interior plastic liner. The key requirements were it had to be cheap, easy, and it couldn't have any holes. So I installed a 3/16" plastic cover wire cable I bought at Lowes with a turnbuckle on one end. I screwed eye-bolts into the bottom of the ridge pole as close to the ends of the shed as possible. Then I stretched the cable fairly tight. Next, I laid 4mm plastic over the cable and draped it 10' down on each side of the cable (the plastic came in a roll 100' X 20' which I also bought at Lowes. I cut it long enough to stretch from one end of the shed to the other plus a little extra so I would have enough to fasten the edges. Then I rolled the excess material around the edge and stapled it to the horizontal stringers at the appropriate height. To keep the top from sagging in I installed pine battens, that I ripped on my table saw from cheap 2X4s. I wrapped the top end of the batten in foam so it wouldn't puncture the liner and nailed the bottom of the batten to a horizontal stringer. The top 7/8 of the batten is free-floating so there are no holes in the liner except around the edge.
So far it works great. No water on the boat. Condensation drips down from the shed cover onto the liner then runs down the side and drips clear of the boat. This is certainly not a high-end solution but since the shed is temporary until the boat is finished there is no sense in spending more money on a solution than required to solve the problem.
Update to Vapor Barrier Liner 30 Sept 10
I got a little side-tracked today. The vapor liner has been sagging in the SRF for awhile. I installed last winter as a temporary solution to condensation forming on the inside of the single plastic cover (cold outside temps but warm air on the inside of the SRF). I constructed the vapor liner by draping cheap 4mm plastic sheeting over a plastic coated cable that runs the length of the shed just below the ridge-pole. The sides of the liner were held up by 1/4" strips of pine ripped from 2x4s with foam wrapped around the ends to protect the plastic sheeting. It worked fine last winter and spring but started sagging during the early summer and had finally become a droopy bag.
I did not want to spend a bunch of time or money to correct something that could be improved with a simple fix. I looked around to see what I had on hand. I ended up ripping about a dozen 3/8" thick X 1 1/2" wide strips from some 12' pine cut offs. I stapled and screwed them to the existing strips that were holding the vapor liner up. Now, instead of two independent pieces of wood supporting just the sides of the vapor liner I have bows that run all the way across the liner from one side of the SRF to the other. I should have done something like this last winter. But it's a slippery slope when you stop to work on a temporary requirement. Sometimes, perfect is the enemy of good enough. The liner is 100 percent improved.