A number of companies make dry systems. The one shown here, RainEscape by the Trex company, is simple to install, as long as you are careful to seal every possible leak with the system’s special tape. RainEscapes installs on top of the joists, prior to install decking. This allows you to finish the underside of the joists (which is the ceiling of the patio below) in most any way you want. Other types are attached to the underside of the joists, after the decking is finished. That type pretty much finishes the underside of the joists by itself.
With RainEscapes, buy a downspout and a trough panel for each bay in your framing. Panels of various lengths are available.
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This article shows the installation basics for creating waterproof troughs that run between joists and empty into the system’s dowmspouts. If your deck has unusual angles, oddly-spaced joists, railing posts inside the framing, or other complications, you may have to cut pieces to fit, but the approach is pretty much the same: Install sheets and downspouts so that water flows downhill to the downspout, then completely cover all seams with the butyl tape.
First, install the downspouts. These are made to fit between joists spaced 16 inches on center if positioned in one direction; if your joists are 12 inches on center, simply face the downspouts the other direction. Check that the downspouts’ funnels will extend down at least 1 inch into the gutter you will install. The downspouts are 10 inches tall, so they fit well with 2x10 joists. If your joists are smaller, cut the funnels as needed, so they extend. If you have 2x12 joists, use extenders. Cut out a panel along the perforation, using a utility knife or simply ripping by hand. Position a downspout in each framing bay at the end where you will put the gutter, and staple them in place.
2. Unroll a trough panel and fold it in half lengthwise, pressing to create a crease; this will help create a trough for the water to travel along. Slip the panel under your ledger flashing as far as possible, and drive a staple or two to hold it in place.
3. Unroll the panel and staple it on one side with its edge flush with the edge of one joist. Pull the panel taut as you work. When you reach the downspout, tightly reverse-roll it to remove its roll “memory,” then cut it to length so its end falls near the center of the downspout hole.
4. Staple the other side of the trough panel. Follow the guide line on the panel; this will automatically create a slight slope for water to run down. Continue installing trough panels.
5. At an angle or another place where the downspout is not at the end of the framing, cut and staple short trough pieces. They should be sloped in their centers and cut so that water flows down onto the longer trough piece.
In some difficult-to-tape places, you may choose to use butyl caulk instead of, or in addition to, the tape. You’ll need to carefully apply a continuous bead under between two trough panels, or between the flashing and a panel.
6. Use a pair of scissors or shears to cut away any excess overlapping trough material. Starting at the house and working toward the downspouts, apply the special butyl tape provided with the system by removing paper backing as you unroll it. Work carefully to keep the tape from wrinkling, and press it firmly in place so it forms a tight seal at all points. Then apply the tape along the other end of the system, on the other side of the downspouts. Apply flat, then make any slits needed so the tape can lie flat. Press all the pieces smooth and tight. If you are unsure of a joint, apply more tape on top.
7. If you will install lighting, a fan, or other ceiling elements, you can do so now. The system will keep them dry.
8. Install your gutter system—any type is fine--- so that all the downspouts empty into it. The gutter should be sloped slightly for water runoff, and connected to gutter downspouts as with any gutter installation.