Screw the fiberglass panels that form the under-deck roof to the purlins.
walk out basement under deck designs - Google Search:
under deck landscaping | ... Fire Pit Area - landscaping ideas for off deck and walk-up basement
Swing under the deck
Firewood Storage Ideas
Traditional Under Deck Patio Landscape Ideas - Patio Design Ideas - 7603
how to build a floating deck on dirt - Google Search
Under-Deck Roof: The Family Handyman
The plastic on the left could be pointed in the direction of a gutter along the front of an outdoor storage bench with a spout sticking out the front pointing towards the backyard to avoid stuff getting contents wet!
The Hotdish Mom: Under Deck Landscaping and Fenced Kids Play Area