Options for keeping materials stored under a deck dry include storing the items in sealable plastic bins or attaching a waterproof ceiling under the deck. Add a gutter system to collect the runoff from the ceiling and direct it to the ground to prevent splashes.
While these measures provide protection from rain or washing the deck, they may not be effective in the event of flooding. Avoid storing irreplaceable goods in this manner. Camouflage the under deck storage area by adding lattice to the sides. Use cup hooks to hang the lattice from the support structure of the deck for easy removal. Another option is to attach the lattice with hinges and use door hooks to hold it closed.