What to use under deck to make a shed?

[spence]

I'd like to keep the space under our deck dry to use as a shed. It's going to have lattice all around, but I need something to divert rain running off the decking...Proper underdeck systems are pretty expensive.

I believe people sometimes use the plastic sheets of corrugated material attached to the bottom of the joists.

Anyone have any other brilliant ideas?

thanks,

-spence