In our first year of using wood we did not have tarps and we used excess vapor barrier instead. I found the wood to stay much dryer and the thicker barrier keeps well too if it does not rip. We have tarps and the suck for keeping wood dry..what I should have is a solid structure but until then I bought 500 feet of thick vapor barrier for $5. Cheaper than most tarps so we will see how it holds up. I am just using staples on doubled up sheets to start with some items on top to hold it down. It is raining now so I know it works but heavy snow and melt will be the real test.