Under-Deck Roof Family Handyman Convert the space under a second-story deck into a dry, spacious patio by installing this simple, under-the-deck roof and gutter system. How To & repair . Rune screwed treated 2×4 purlins to spacer blocks to support the panels and provide the necessary slope (Figure A). Gutters aren't Rune chose pvc gutters because they're inexpensive and easy to cut and install. If your deck is already built, you can cover the bottom of the joists with a system like 's DrySpace.