Low-Cost Deck Drainage: Landscape membrane and off-the-shelf ...1 May 2007 ... Landscape membrane and off-the-shelf gutters keep the space below new and existing decks dry. ... Then determine how many pieces you need and lay them out alternating the wide and narrow ends across the 10-foot-4-inch-wide sheets of membrane (Figure 3). I can usually get about five pieces out ... At the ends of troughs that are more than about 6 feet long, you'll need to install baffles to catch any water that may shoot out past the gutter. Baffles are simply ...