DIY Network

How To Install Underdecking

Learn how to create a rainwater diversion system with this underdecking.

More in Outdoors

  • Time

    Weekend

  • Price Range

    $500 - $1,000

  • Difficulty

    Moderate

Highlights:

Step 1: Install the Deck Joists

First, measure the length of the deck joists. Then, make a chalk mark from one end of the joist to the other with the pitch going away from the center of the deck. Line the joist rails up against the chalk mark. Attach the joist rail to the joist using 1" or 1-1/2" hot-dipped galvanized roofing nails or pan head screws. Use three to four nails (or screws) per joist rail. Be sure to leave about two inches between each piece of joist rail for drainage. Joist rails can be cut at the end of the joist to maintain the appropriate spacing.

Step 2: Add the Water Diverters

When all of the joist rails are nailed in place, slide water diverters between the rails. The purpose of the diverters is to route the water toward the gutters that will be installed on the opposite edge of the deck. Next, measure the length from the water diverters to the edge of the deck and use a box cutter to cut collector panels to the appropriate length. Slide the collector panels between the joist rails. Make sure that the matte finish side is facing down and the glossy side is facing up.

Step 3: Add the Joist Gutters

Once the collector panels are in place, cut joist gutters 1/4" shorter than the joist length to allow for expansion. Hold the joist gutter up to the joist; spread the gutter open slightly while pushing up firmly until the gutter snaps into the joist rail and nests tightly against the collector panel. Work your way to the end of the deck.

Step 4: Attach the Boundary Gutters

The boundary gutters are the final element of the water diversion system. These gutters collect and dispel the water that flows down the collector panels. Simply nail the boundary gutters below the collector panels at the end of the deck joists.

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