If your home has a second-story deck, or you are thinking of building one, the outdoor space beneath offers unlimited potential for entertaining, relaxing, and creating a storage area. However, nothing should go in this space that you wouldn’t put outdoors unless you waterproof under the deck.

There are several deck waterproofing systems, but only one fool-proof option. Let’s review your choices for creating a dry outdoor living space beneath your deck.

Decking Flanges

Whenever it rains or snows, water will drip — or pour — through the space between your deck boards. That’s usually an effective way to avoid prolonged moisture exposure for your deck. Those spaces are perfect for diverting away rainwater.

However, the spacing isn’t ideal if you want to do anything dry beneath your deck. One option for waterproofing is to fill the gaps with rubbery flanges. Using spacers between boards can prevent water dripping. Some flanges even serve as mini-gutters to channel water toward deck edges. Usually, contractors pound flanges into decking edges with a rubber mallet.

Such flanges may require you to cut your boards to accommodate their shape. If the product breaks down over time because of improper installation, the waterproofing will not be effective.

Under-Deck Ceiling Systems

Another approach is to install an under-deck system or to add guttering beneath the decking boards to draw away the water.

An underdeck ceiling system provides all-in-one drainage and a finished ceiling. These ceiling systems often use corrugated plastic or aluminum panels to catch the water that drains through decking material and channels it away from the area below.

Unfortunately, these systems all allow water to drip through the deck boards, over the wooden joists and into gutters below. This is an under the joist drainage system, and it can be very damaging to your deck. Why? When the deck joists repeatedly get wet, they will eventually rot.

Below-joist drainage systems cause damage and wood rot in three ways:

• First, they allow the wood to get wet over and over again as rain drips through the deck boards, over the joists, and into the drainage system below. The water hits the pan and runs out, but not before drenching the wooden joists. 
• Second, this type of system can trap moisture. If installed ceiling panels are beneath the deck, there’s no air coming in to dry out the wood. The only ventilation is through the quarter-inch or less space between the top boards. 
• Third, in the hot months of summer, the temperature further cooks this trapped, wet wood and accelerates the rotting. Over time, this is the perfect formula for wood rot: moisture + heat – ventilation = destruction.

Watch this video to see what happens to wood with under-joist drainage systems. 

Under-Deck Drainage Systems

The best way to waterproof under your deck is with a deck drainage system installed during construction. A drainage system diverts water away from the joists and beams.

An over-the-joist deck drainage system such as Trex RainEscape provides 100% protection from the damage caused by wood repeatedly getting wet. This type of deck drainage system covers and protects the entire wooden substructure. It can be installed with a composite or wood deck.

If you’re a homeowner who enjoys doing your own home improvement, choose an over-the-joist system that’s easy to install. Use this handy chart for comparison. 

Under-Deck Drainage Installation

The tools needed to DIY your Trex RainEscape system are probably items you already have handy:

√ Scissors or utility knife

√ Staple gun

√ Caulk gun

It’s also helpful to have a midspan nailer on the underside of the joists to keep them straight.

Installing your watertight gutter system requires just four steps. The highlights are given below:

Step 1:  Install the Trex RainEscape Downspout

Cut downspouts along with one 12″ or 16″ side on score mark, based on joist spacing. Install all downspouts along one rim joist and/or ledger board; cut outside toward open deck bay. The downspout should extend 1″ into the gutter: cut or extend as necessary.

 Step 2: Install the Trex RainEscape Trough

This is the most involved step. When working with the Trex RainEscape trough material, ensure that the smooth/shiny side always faces up. Do not seam or overlap the troughs to extend its length.  

 Get complete instructions and watch a video to ensure a quality installation. 

 Step 3: Follow Step 2 for Remaining Joists

 Step 4: Tape All Seams

Make sure the tape completely covers all seams and anywhere a screw will penetrate the Trex RainEscape system.

To estimate the cost of your under-deck drainage system, check out this materials estimator. If you’re not up for a DIY project, call a trusted deck builder.

Trex RainEscape is not recommended for an existing deck unless the deck boards are removed.

Tips For Installation

There are also some quick dos and don’ts to keep in mind. The main idea is that you’re creating a waterproof system, so you’ll have to take precaution not to puncture the system in a way that would create a leak.

Do

• Make all joists straight and square.
• Add outlets to gutter every 12′ – 14′.
• Cover all joists and blocking with trough material, then tape.
• Cover the Trex RainEscape system with deck boards.
• Ensure the troughs and downspouts are dry and clear of debris before applying tape.

Don’t

• Do not staple Trex RainEscape products anywhere other than into or over floor joists, ledger, rim joists, or headers.
• Do not use spiked hidden fasteners or any fastener without a flat base to provide pressure to butyl tape. Call a Trex RainEscape Specialist at 1-877-348-1385 for recommendations.
• Do not overdrive double thread screws.
• Do not stand anywhere other than the top of the floor joists, ledger, rim joists, or headers. The Trex RainEscape system cannot support body weight.
• Do not use nails to attach the deck boards above the Trex RainEscape system. For non-standard deck sizes or special features, please reference the installation guide or contact a Trex RainEscape Specialist.

Add the Protection of Wall Flash

During deck construction, you have an opportunity to add years of life to your deck. To do this, think about any wood that can be exposed to the elements.

Easy-to-install, self-adhesive Trex RainEscape Wall Flash seals the space between your house and ledger board to keep water out. Wall flash is best used during new deck building and is another small investment with big returns. 

Trex RainEscape® Wall Flash comes with a 20-year warranty from Trex, the world’s #1 composite decking brand. See how Trex RainEscape® Wall Flash protects in this video.

 Ensure a waterproof deck, and create dry space — for deck storage, entertaining, or relaxing — with a Trex RainEscape® drainage system and wall flash.