Moisture-Resistant Decking: How Composite Materials Beat Georgia’s Humid Heat

moisture-resistant decking

If you live a wet or humid climate, you don’t have to worry about moisture affecting your composite deck, as long as you’re using fully-capped bamboo-based decking.

A few years ago I had the great pleasure of traveling through Georgia and the Carolinas, staying with old friends and helping them with their building projects. Originally hailing from a much rainier part of the country (the Pacific Northwest), I was amazed to see in the South the same kind of deck damage that I regularly replaced and repaired back home. Then, while working on a college rental house in Athens, the humidity got particularly bad and I suddenly understood! If my own body felt completely encased in moisture, the decks were, too. A quick early morning investigation of the bottom of the deck revealed the simple diagnosis—moisture was being absorbed into the wood from below.

In the South, decks and porches are beloved, ubiquitous, and almost necessary. They’re especially useful in escaping the heat and humidity found in the depths of summer. But high humidity can put nearly as much strain on a deck as heat, even for composite decking. Composite decking material that is fully capped is highly effective at keeping this moisture at bay, leaving you more time to simply relax and take refuge from the heat.

Water on the Underside of the Composite Deck

Homeowners and contractors prudently go to great lengths to protect the exposed surface of their composite decks from precipitation, however, decking boards are much more prone to being damaged from below. This is so because not all composite decking is protected on the underside. Instead, the top of composite decking is often coated with a plastic capping material that isn’t used on the bottom of the board.

This is important to consider because, while rain isn’t impacting your deck from below, there is still plenty of moisture under a deck. This is due to condensation. As the air cools at night, the moisture in the air condenses and sticks to whatever surfaces are nearby. The cooler underside of a deck creates the ideal conditions for the accumulation of water condensation, and the more moisture there is in the air—I’m looking at you, Georgia—the more condensation will form on your boards. With unprotected composite decking, this moisture gets absorbed into the boards and can cause them to swell, crack, and even crumble.

Capped Vs. Uncapped Composite Decking

Keeping condensation off the underside of wooden deck boards is difficult, in part because attempting to apply deck sealer from below is generally a messy and ineffective proposition. I’ve even known some folks who have taken the trouble of removing their decking boards one by one in order apply a sealer to the bottom of the boards. It’s this type of extensive maintenance that has made low-maintenance composite decking popular and appealing. But not all composite decking is equal. Here’s how composite decking has evolved over the years, and what it means for you:

  • First Generation Composites (not capped): The very first generation of composite decking actually had no capping material surrounding the decking boards, which proved to be problematic. The board’s interior (wood fiber and plastic) was exposed to the elements, causing fading, discoloration, swelling, cracking, and crumbling due to sun and moisture exposure.
  • Second Generation Composites (partial to fully capped): Subsequent composite decking added a plastic coating, or ‘cap’ on the top and sometimes on the sides as well, to protect the core of the board from wear and tear and to prevent the wood fiber from absorbing water. Some brands were capped all the way around, with the moisture resistance improving the more sides were capped.
  • Second Generation Composites with Advanced Materials (fully capped): Not all capping is created equal. The best capping material I have encountered has been manufactured with technologies imported from the tire industry. Like a tire, it’s durable and even a bit rubbery, so that it feels good under the feet—even bare feet—and provides good traction, which is always an important consideration (especially so in rainy and humid environments like Georgia). Rather than making the boards look shiny and plastic, like some capping does, this material gives decking a matte appearance, and makes it look and feel quite a bit like a stylish Brazilian hardwood.

Composite Decking Filler That Doesn’t Absorb Water

In addition to a full cap, I often suggest that my clients who live in humid environments use composite decking boards that replace sawdust filler (found in most composite decking) with bamboo flour as the filling agent. In Georgia, even a tough and rubbery cap may allow some moisture get in, which is why I recommend using composite boards that incorporate bamboo, as boards with bamboo leave fewer channels for water to enter.

During the production process, sawdust has a tendency of clumping, which opens up channels for water to enter into the board, saturating the moisture-loving clumps of sawdust. Once that moisture starts to collect in the board, all the regular damage from water starts to manifest, like swelling and warping. Since bamboo doesn’t clump together during the production process, water has far fewer channels into which it can enter the board should the capping allow any in.

Every now and then I get back to Athens for some quality time with my friends, and the composite decks I put in are looking as good as the day I finished them and apparently have not required any substantial maintenance work—even with heavy use at the hands of students. When my friends in humid climates ask for decking recommendations, I point them in the direction of a high-quality fully capped bamboo composite decking material. Produced by Fortress Decking, it’s both highly functional and aesthetically pleasing, and I also can’t recommend enough Fortress Building Products’ larger catalog of building products for the rest of the job. Their durable, corrosion-resistant fencing, railing, and decorative hardware are a good choice for any area of the country—especially hot, humid states like Georgia.

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