The Ultimate Guide on How to Waterproof Under a Deck
Every outdoor space faces the unpredictability of the elements – rain, wind, snow and everything in between. That’s why adding a layer of protection around areas used for outdoor living is essential. Installing gutters, drains and waterproofing a deck are just a few ways to protect furnishings, outdoor electronics and wooden structural elements from rain and moisture that can cause costly damage and pose hazards to homeowners.
There are several things to consider before diving into a waterproofing project, such as choosing the best materials and practices for installation. Below are a few of the essential steps to keep in mind when waterproofing a deck.
Step 1: Understand Waterproofing vs. Water-Resistance
The first thing to understand is there’s a big difference between “water-resistant” and “waterproof” – and that many products in the marketplace are falsely labeled, so you really need to know your stuff.
- Water-resistant materials shed water but still absorb some moisture.
- Waterproof materials are completely impervious to water.
While both terms are often used interchangeably, you’ll want truly waterproof solutions for the most effective protection.
Step 2: Choose the Best Deck Waterproofing Materials
When it comes to choosing the best decking for your outdoor space, be sure to consider how they handle moisture. The following are a few of the most popular decking materials:
- Wood: One of the most common decking materials, wooden decking is water-resistant at best and will need extra treatments and sealants to last.
- Aluminum: One of the more expensive options, aluminum decking is durable and fully waterproof, but can be difficult to cut and maneuver, which creates issues during installation.
- PVC Decking: Made entirely of plastic, PVC decking is waterproof and a good choice for preventing water damage. The drawback with this fully synthetic material is that it can look “plasticky.” Also, in certain conditions, PVC boards can feel “springy” underfoot, making them seem less sturdy.
- Composite Decking: An increasingly popular choice for its long-lasting good looks and low-maintenance ease, composite decking is a blend of two materials, most commonly sawdust and recycled polyethylene plastic film. Due to the wood in the core, this material is considered water-resistant, but most of today’s offerings are capped with a protective layer for added protection against fading, staining, scratching and moisture penetration.
Choose the best material based on your needs, climate and desired maintenance level.
Step 3: Install a Deck Drainage System
If you’re interested in waterproofing the space beneath a deck, a key step is installing a deck drainage system to keep moisture away from the substructure. Using an integrated network of membrane troughs and downspouts, this type of system captures and diverts water away from a deck to protect the substructure from moisture and create dry, usable space below an elevated deck.
There are two main types of deck drainage systems to consider:
- Below-the-joist systems: These systems consist of metal or vinyl panels retrofitted to the underside of a deck’s joists, with the panels sloping to a gutter installed at the deck beam for drainage. They are installed under the joists and direct water away using panels and gutters. A disadvantage of systems that install below the joists is that they leave the wood and metal of structural joists exposed to the elements and, therefore, susceptible to water damage and corrosion. This type of system can also trap moisture in the core supports of the substructure, which can accelerate rotting and deterioration.
- Above–the-joist systems: For optimal protection and aesthetics, most contractors prefer drainage systems that install above the joists between the substructure and the deck boards. This method uses poly or rubberized membranes that drape over the joists and down into each joist bay from above the substructure, ensuring the entire substructure is shielded from moisture penetration. Furthermore, the membrane material (vs. metal, vinyl or PVC) accommodates a broader range of ceiling options for homeowners seeking a finished aesthetic.
In both cases, deck drainage systems are relatively simple to install and require minimal tools. Trex® RainEscape®, for instance, is an above-the-joist system that includes just five SKUs and can be installed using a stapler, utility knife and tape. Each trough is clearly labeled with staple markers to expedite the process, which takes no more than a few hours to complete, depending on the size of the deck.
Step 4: Install Under-Deck Ceilings
Under deck ceiling systems, commonly referred to as soffits, provide an additional layer of waterproofing and give an under-deck space a more stylish, polished finish. Aluminum and vinyl are most popular for a soffit because they’re inexpensive and easy to install.
- Aluminum’s primary advantage is its durability, as it won’t chip, crack, rust or warp. It’s also effortless to clean and maintain and doesn’t require repainting.
- Vinyl ceilings are thinner and tend to sag over time. They’re also more vulnerable to weather conditions like rain and snow, and temperature fluctuations can warp and fade the panels. Another drawback to vinyl systems is that they are not watertight, meaning they’re a no-go if you’re planning to install any electrical fixtures. Vinyl also requires more general maintenance and will likely need to be replaced multiple times over the life of a deck.
Step 5: Review Your Options
If you’re looking to add protection to your deck, but aren’t ready to install a deck drainage system, there are a few options to create a water-resistant deck that are less comprehensive, but still effective, to protect the space beneath your deck:
- Install Gutters: When strategically placed and angled around a deck’s perimeter, gutters will direct water away from the under-deck space. However, gutters must be free of debris. If ongoing upkeep is not feasible, consider adding gutter guards to prevent debris from accumulating and creating clogs or causing potential damage.
- Waterproof Deck Flanges: Instead of installing an entire under-deck ceiling system, rubber flanges fill the cracks between deck panels and act as a first defense against rain and snow. However, installing flanges may require altering the deck, which could impact decking material and open it up for water damage in the future.
- Apply Liquid Deck Coatings: If you are looking for a simpler solution, surface-level liquid deck coatings are easy to apply and give wood a fighting chance against rain and moisture penetration. Polymer-enhanced coatings are best for helping water to bead and “bounce” off deck boards. The major concern with these coatings is that they must be re-applied regularly throughout the life of a deck to maintain performance and aesthetics.
Step 6: Customize The Space Below Your Deck
Once protected, the area beneath the deck surface can be used like any room inside a house. Homeowners can add electricity, water and gas lines, as well as furniture, lighting, ceiling fans, appliances and entertainment components to create a fully functional outdoor living space. This area can also be used for storing everything from lawn and garden equipment, pool supplies and off-season patio furniture to bicycles, boats and ATVs. With a drainage system in place, the possibilities are limitless for how the space beneath a deck can be used.
No matter how you finish it or use it, the space beneath a deck can add valuable square footage to a home. To learn more best practices for waterproofing and outdoor space, visit TrexRainEscape.com.
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About the Author
Dave Kile is the owner of IBP, LLC, the company behind some of the decking industry’s most successful innovations, including Trex® RainEscape®, Trex® Protect and Trex® Seal. Over the past three decades, he has achieved incremental success by immersing himself in the industry, listening to customers and bringing new products to market that improve efficiency and effectiveness.
