Protecting Your Deck from Moisture is One of the Best Ways to Extend the Life of Your Deck

While there is no true way to waterproof your deck, there are ways to protect it from water damage. Rain, snow, and humidity can all lead to moisture seeping into both the decking and the substructure beneath. This moisture intrusion can cause rot, corrosion, and structural instability, potentially compromising your deck’s safety, appearance, and value while significantly reducing its lifespan.

If you want to know how to make your deck as watertight as possible, begin with protecting the wood framing that supports your decking. Whether by applying water-resistant butyl tape to the top of your joists, rim joists, and beams, or protecting the deck substructure from moisture with an over-the-joist drainage solution, this is where water management starts.

Next, consider the decking surface. If you opt for wood decking, moisture protection requires ongoing efforts.

• Untreated wood decks can deteriorate rapidly, with warping, cracking, or rotting occurring in as little as 5 to 10 years. Regular applications of a waterproof sealant are necessary to extend the life of wood decking.
• Pressure-treated lumber, already treated to resist decay and pests, also benefits from the additional protection of sealants to help lock out moisture, potentially extending its lifespan to 20 years or more.
• Naturally resistant woods like cedar and exotic hardwoods like Ipe already boast a longer natural lifespan of 20 to 25 years, but applying water-repellent finishes can push their longevity even further—often surpassing 30 years when properly maintained.

The maintenance cycle required to get the most from wood decking can be demanding and time-consuming, which is why composite decking is a popular choice for so many homeowners today.

• Composite decking options, such as those offered by Trex®, are made to withstand moisture without constant upkeep. These materials can last for over 25 years—with some warrantied for 50 years—offering a truly hassle-free alternative to wood decking.

While water protection from shielding the wood framing to sealing wood decking or choosing a composite costs more upfront, it’s a valuable long-term investment. Water-resistant decks not only survive the elements more successfully, but also retain their aesthetic appeal and structural integrity, which is well worth the added cost.

Why Composite is Better Protected Against Water Damage

One of the main advantages of composite decking is its inherent resistance to water damage. Composite materials are engineered from a combination of wood fibers and plastics with a non-porous surface that prevents rot, warping, and splintering. If you're wondering how to "waterproof" your deck floor effectively, choosing high-quality composite decking is one of the best ways to do so.

The plastic components in composite decking offer excellent resistance to water, as well as mold and mildew—common culprits in the breakdown of natural wood. This resilience ensures that composite decks won't swell, shrink, or degrade, making composite decking a durable choice for areas frequently exposed to water, including regions with high precipitation or humidity, pool sides, and coastal homes.

If you’re interested in an easier, no-hassle solution, composite decking offers significant advantages. Not only does it withstand water better than wood, but it also requires far less upkeep. For tips on how to keep composite decking looking its best, check out this guide on cleaning a composite deck and this one on low-maintenance decking.

How to "Waterproof" Under a Deck (Wood Substructure)

Protecting the deck substructure from moisture is just as important as protecting the deck surface. If you neglect the wood substructure, water can seep into the joists and other structural components, leading to rot and weakening your deck’s supportive framing.

The best time to install water protection under your deck is during the initial build or major renovation when the deck boards are removed. There are two main ways to protect the wood substructure of your deck from moisture:

1. Joist Tape: This is a simple, affordable solution that protects the deck framing. The self-adhesive tape is applied to the tops of the joists, beams, and rims to create a barrier that prevents water from soaking into the wood. It’s effective, easy to install, and a small investment that increases your deck’s lifespan. For details on applying joist tape, refer to this Trex Protect guide.
2. Under Deck Drainage System: For elevated, second-story decks, an under-deck drainage system like Trex RainEscape is a great option. The system is designed to maintain a dry storage or living space below the deck, but it has another added benefit: As an over-the-joist system, it creates a waterproof barrier that collects and diverts the water that drains between the deck boards before it ever contacts your deck frame. The substructure is kept completely dry and protected as the water channels away by the gutter system.

Tools & Materials Needed for a Trex RainEscape Under-Deck Drainage System

• Tape measure
• Marking pen
• Utility knife or scissors
• Staple gun
• Caulk gun
• Trex RainEscape downspouts
• Trex RainEscape troughs
• Trex RainEscape butyl tape
• Trex RainEscape butyl caulk
• Trex RainEscape rubber wall flashing
• Trex RainEscape 4x4 post flashing
• PVC, aluminum, or steel gutter
• Hacksaw or jigsaw
• Cordless drill/driver
• Self-drilling screws
• Gutter hangers
• Gutter downspout

Optional: Trex RainEscape soffit lights, ceiling fan, or other accessories

How to Install the Trex RainEscape Under-Deck Drainage System

Follow these installation basics to waterproof the space under your deck with the Trex RainEscape trough-and-downspout system. For decks with unusual angles, uniquely spaced joists, or special cases like railing posts inside the framing, you may need to modify the troughs and downspouts to fit. The goal, however, is simple: Direct water to the gutter and seal all joints with butyl tape.

Install the Downspouts

1. Funnel-like Trex RainEscape downspouts fit between joists spaced 16 inches on center. For joists spaced 12 inches, rotate the downspouts 90 degrees.
   
   
   • For 2x10 joists, the downspouts fit without modification.
   • For smaller joists, trim the funnel outlets to extend 1 inch into the gutter.
   • For 2x12 joists, attach a section of PVC downspout to the funnel outlet using self-drilling screws.

Cut a crescent-shaped section from one side of the funnel outlet along the indicated cutline for proper flow.

Position a downspout in each joist bay along the rim joist, and secure with staples.

1. Install the Trough Panels
   Unroll the trough panels and let them acclimate, then fold each panel lengthwise to create a crease for water flow.
   
   With the shiny side up, slip one trough end under the wall flashing at the ledger board and staple it in place, avoiding the flashing itself.
   
   
2. Attach and Cut the Trough Panels
   Staple one edge of the trough panel along the top of a joist, keeping it flush with the joist edge, stretching the trough material taut as you staple.
   
   Fold the material tightly at the downspout to remove roll memory, then cut it to end near the center of the downspout hole.
   
   
3. Secure the Troughs and Verify Slope
   Staple the opposite edge of the trough to the adjacent joist, following the guidelines on the panel to create a consistent slope.

Continue installing trough panels in all joist bays, pouring water into each trough to confirm proper drainage.

1. Address Angles and Cantilevers
   For sections that cannot accommodate a downspout, make short trough sections and back-pitch them so that water flows to the longer trough and into the downspout funnel.
   
   
2. Seal with Butyl Tape
   Trim excess trough material with a utility knife or scissors, ensuring no bare wood is exposed.

Apply Trex RainEscape butyl tape over all seams, starting at the ledger and working toward the downspouts. Smooth the tape to avoid wrinkles and ensure a tight seal.

Seal the edges of the downspouts with additional tape, slitting it as needed for a flat fit.

1. Once the Trex RainEscape system is in place, the area below the deck remains dry, allowing you to customize the space. Install optional features like Trex RainEscape Soffit Lights, ceiling fans, or other ceiling elements, following the manufacturer’s guidelines.
   
   

Install the Under-Deck Gutter
Attach a PVC, aluminum or steel gutter beneath the system's downspouts. Ensure the gutter is aligned to catch water flow and slightly sloped—1/8 inch per foot is typical—so the water runs toward to the gutter’s downspout. Attach the gutter using hangers, with each downspout’s outlet extending 1 inch into the gutter.

Install the Decking

1. Decking boards using screws or hidden fasteners; the Trex RainEscape butyl tape creates a watertight seal around screws to protect the joists from water penetration.

Waterproofing a Deck, Top to Bottom

Waterproofing your deck is essential for protecting its structure, maintaining its appearance, and extending its lifespan. From choosing water-resistant materials and maintaining your decking to applying joist tape and installing an under-deck drainage system like Trex RainEscape, each method contributes to keeping your deck protected, functional, and visually appealing. Trex RainEscape does double duty, shielding the joists and substructure from water, while transforming the space below into a clean, usable area. With the right waterproofing measures in place, you can enjoy a deck that remains strong, functional, and beautiful for years to come.