How To Keep It Dry Under A Deck

To ensure that your deck lasts for years to come it is important that you keep moisture at bay under the deck, failing to do this will lead to rotting beams which will drastically reduce the lifespan of your deck and could also lead to dangerous accidents.

Here are 4 methods you can use to keep under-deck moisture away:

1. Fit an Under-Deck Drainage System

By installing an under-deck drainage system, you can use the space beneath your second-story deck for something else.

decking outdoors

Simultaneously, it’ll protect your deck frames against moisture, which causes rot and shortens the lifespan of the wood.

How to Install an Under-Deck Drainage System

It’s advisable to install the deck drainage system during construction. The Trex RainEscape system, however, is the simplest to install even by yourself if it wasn’t set up at that time.

Here’s a step-by-step guide to installing an under-deck drainage system.

1. Get your materials ready:

  • Staple gun and staples
  • Caulk gun
  • Scissors (a utility knife will also suffice)
  • If you have one, a midspan nail will come in handy to keep the joists straight.

2. Acclimate the Trough Material

Acclimating the material is crucial in locations with extreme temperature swings. The best way to do this is by unrolling it flat on the ground.

Otherwise, simply unroll it and fold a soft crease down the center of the material.

3. Install the Downspout

Depending on the joist spacing, cut downspouts together with 12′′ or 16′′ sides on the pre-scored marked guidelines.

Next, install all downspouts along a single-rim joist or ledger board, cutting outside toward the open deck bay.

Cut or extend the downspout as needed to extend it one inch into the gutter.

4. Install the Trough

After that, unroll the trough using the guideline on the joist. On the inside joist, there should be a 1.5′′ score mark that can be used as a guide.

Check that the trough material’s smooth and shiny side always faces up. Also, avoid seaming or overlapping the troughs to extend them.

Repeat the step for the other joists.

5. Tape All the Seams

Make sure the tape completely covers all seams and any screw holes in the deck drainage system.

2. Install an Under-Deck Ceiling

Another way to keep the underneath of your deck dry is by installing an under-deck ceiling.

This is an ideal solution for waterproofing any decking that is located above a living space as it will prevent water from dripping down through the decking.

How to Install an Under-Deck Ceiling

Before installation, take a look at the space above the under-deck ceiling.

This gap between the deck boards and the ceiling should be watertight to avoid potential damage in the future.

Here’s how to install your under-deck ceiling.

1. Prepare the Tools and Materials

Here’s what you need:

  • Quick square
  • Combination square
  • Tape measure
  • Miter Saw
  • Pressure treated 2x4s
  • Two ½ inch ceramic deck shoes
  • Mason’s twine chalk line
  • 1-inch galvanized wood screws
  • Gutter screws
  • Circular saw
  • Fiberglass panels
  • 4 or 8-foot level and torpedo level

2. Plan the Purlin Locations

Begin by laying out the purlin locations. Then, use a tape measure to mark the bottom of each joist every three feet.

If you plan to add gutters, factor in the best place to place them as you’re measuring. Remember that your ceiling panels should have about an inch of overhang from your gutters.

3. Cut the Purlins

To determine the length of each purlin, measure the width of your joist at each mark. Keep in mind that the first purlin should be three feet away from your house.

Cut the purlins to 3/4-inch by 1.5 inches on a table saw. Then, using a miter saw, cut to length. Continue to add 3/4- inch every three feet. Finally, confirm the length of each piece across your joists and cut it.

If you’re adding gutters, make sure to leave room for them when you attach the last purlin.

4. Install the Panels

Measure the length of your deck to determine the size and number of corrugated fiberglass panels you’ll need. Then, attach the panels with one-inch galvanized wood screws with neoprene washers to seal the holes.

Afterward, screw the panels into the purlins. To do this, overlap the panels with a corrugated ripple, securing each one in place with a screw.

5. Install the Gutters

Lastly, install the gutters. Make sure your gutter has a little slant in the direction you want it to drain. Every 10 feet, it should have a half-inch slope toward the end of the beam where you intend to add or tie into an existing downspout.

Note that the gutter screws should be installed every 24 inches.

3. Fit Waterproof Decking Flanges

The third method on our list is using waterproof decking flanges. This dry deck system fills the gaps between deck boards with rubber flanges. They will then function as water channels that lead to the deck’s edge.

Dexerdry is a waterproof spline that works as a mini-gutter. It’s a popular product available in various sizes that match the edge grooves found on leading manufacturers of synthetic decking.

To install waterproof decking flanges, simply insert the rubber flanges in the spaces between deck boards. Then, pound the flanges into the decking edges with a rubber mallet.

Contractors are usually required to install them because certain flanges may need to be trimmed to fit the size of the deck boards. If they’re not properly installed, they might lead to various issues later on, such as inadequate waterproofing.

4. Use Waterproof Deck Matting

Our final option is to use a waterproof deck mat. There are many different types of outdoor rugs on the market, but the wrong one could ruin your deck.

We suggest using polypropylene rugs because they’re made of materials designed to withstand moist, humid, and rainy environments. Moreover, these rugs are durable, easy to clean, and dry quickly.

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