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Installing a DIY Under Deck Drainage System

If your deck is elevated or on a second story level, you’ll want to consider installing a DIY under-deck drainage system, which diverts water away from the deck and into gutters so the area beneath the deck is dry and useable.

After installing an under deck drainage system, you’re free to use the space below the deck for a patio giving you a lovely retreat during inclement weather. You can also use the space to create a waterproof under deck storage.

Trex RainEscape by Trex Company, named among the 2019 Remodeler's Choice 100 by Qualified Remodeler Magazine, is simple to install and affordably priced. Trex RainEscape is installed before you lay the deck boards. It is an over-the-joist deck drainage system. This type of deck drainage system allows you to finish the underside of the deck (which is the ceiling of the patio below) with utilities for electricity, lights, ceiling fans, entertainment components and more.

Decks Ceiling of Outdoor Patio

Tools & Materials Needed for a DIY Under Deck Drainage System

  • Utility knife or scissors
  • Staple Gun
  • Caulk Gun
  • Trex RainEscape Downspouts
  • Trex RainEscape Trough
  • Trex RainEscape Butyl tape
  • Trex RainEscape Butyl caulk
  • Optional:  Trex RainEscape Soffit Lights, ceiling fan, TVs or other elements

How To Waterproof Under Your Deck with a DIY Deck Drainage System

This article shows the installation basics for waterproofing the space under your deck using troughs that run between joists and empty into the system’s downspouts. If your deck has unusual angles, oddly-spaced joists, railing posts inside the framing or other special cases, you will may need to modify the troughs and downspouts to fit the space but the approach is pretty much the same: Install downspouts and trough so that water flows to the gutter, then completely cover all seams with butyl tape.

Trex RainEscape is installed over the joists before the decking boards are installed, ensuring no water touches the joists. Protecting the deck foundation from water is essential for preserving the longevity of the deck.

Some homeowners ask how to waterproof under an existing deck. This is not recommended, as it would require a system installed under the joists. An under-the joist deck drainage system allows water to run through the deck boards over the joists and into gutters under the deck. Repeatedly exposing the joists to water will result in wood rot and decay.

Deck Installation DIY Project

Cut your downspouts and install them along one rim joists and/or the ledger board

Cut your downspouts and lay them out

1. 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 in 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 tear by hand. Position a downspout in each framing bay at the end where you will put the gutter, and staple them in place.

Tuck the trough under the flashing to the wall

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 staple it in place.

Roll out the trough. Staple and cut to length.

Roll out the trough. Staple and cut to length.

3. Unroll the trough panel and staple it on one side with its edge flush with the edge of one joist. Pull the trough taut as you work. When you reach the downspout, tightly reverse-roll the trough to remove its roll “memory.” Then, cut it to length so its end falls near the center of the downspout hole.

Staple and roll out more trough

Staple and roll out more trough

4. taple the other side of the trough. Follow the guideline on the panel. This will automatically create a slight slope for water to run down. Continue installing trough panels.

Fill in the small areas at the angles

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 between two trough panels, or between the flashing and a panel.

Apply the tape at the seems and joints

Apply the tape at the seams and joints

6. Use a utility knife or scissors to cut any excess overlapping trough material. Starting at the house and working toward the downspouts, apply the butyl tape provided with the system by removing the 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. Make any slits needed to help keep the tape flat. Press all the pieces smooth and tight. If you are unsure of a joint, apply more tape on top.

Nice and dry!

7. If you plan to install Trex RainEscape Soffit lights, a fan or other ceiling elements, you can do so now. Since this is an over-the-joist deck drainage system the space below the deck is dry.

Install your gutters

8. Install gutters for under deck - any type is fine - so that all the downspouts empty into the gutter. It should be slightly sloped for water runoff and connected to gutter downspouts, as with any gutter installation.

Install your decking

9. You can install decking using face screws or most types of hidden fasteners. The Trex RainEscape butyl tape forms a watertight seal around screws that penetrate the tape.

You can learn more about waterproofing your deck in this article. With a deck drainage system, you’ll easily learn how to keep the area under the deck dry. Additionally, here is a reputable place to buy Trex RainEscape.