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Specifying Decking for High-Moisture Exposure Applications

Georgia's Port Royale Marina is installing high-performance composite decking throughout its facilities, including in this children's spray park. PHOTO CREDIT: Port Royale Marina

Moisture exposure is a leading cause of deck deterioration, whether from precipitation, waves or water splash from pools and hot tubs. The volume of moisture exposure can be huge. If you build a modest-size commercial deck in a city on the Gulf Coast, every year it will be exposed to more than 200,000 gallons of rain – that’s nearly six full rail cars of water pouring on the deck annually. The same size deck far inland would still receive more than three rail cars of rain each year.

Since the lifespan of a treated wood deck is only about 10 to 20 years, depending on the wood species, treatment and the exposure, building professionals are increasingly specifying higher-performance alternatives like composite decking to extend the lives of their projects. When selecting a composite decking, it is important to evaluate the product’s moisture resistance to ensure its long term performance.

Moisture resistance factors for composite decking

Composite decking is a combination of wood fibers and plastics that are specially engineered for high performance. Because composites include wood, any exposed wood fibers are still susceptible to moisture absorption, which can lead to the twisting, warping, or cracking that is common with traditional wood deck boards.

The most effective way to ensure durable and long-lasting composite decking is for the manufacturer to fully encapsulate the wood fibers in water-resistant plastic. Yet, this is technically difficult to do. So, when evaluating composite decking, it is important to ask the manufacturer the degree to which the wood fibers are encapsulated and if their decking has experienced any field failures.

Because of the manufacturing challenges of achieving full encapsulation, some composite manufacturers developed another approach to managing moisture: adding a plastic cap to their decking boards. Although caps offer several benefits, they still fall short in defending composites against water damage. The problem is threefold: 1) protective caps are almost never included on the ends of boards; 2) many capped composites only wrap three sides of the board, leaving the bottom undefended; and 3) screwing or nailing the boards penetrates the cap leaving the core of the board unprotected.

Each of these areas provides an entry point for moisture. Therefore, as with uncapped composites, it is crucial to check if the composite core is made with fully encapsulated wood fibers. Although caps alone aren’t sufficient to prevent water infiltration, their benefits can include enhanced stain, fade, scratch and slip resistance.

In addition to confirming that the wood fibers are fully encapsulated, when specifying capped composites, it’s also important to ask the manufacturer how they attach the cap to the composite core. In some cases, improper adhesion can lead to the cap separating from the core, directly exposing the wood fibers to moisture. Some specialty boards integrate the cap with the core for a stronger and longer-lasting bond that continues to protect the core from exposure to the elements.

Though it means asking a few questions to find the right product, many commercial facility owners and operators have found that the increased moisture resistance of high-performance composites with fully encapsulated wood fibers makes for more long-lasting decks, docks and boardwalks.

One extreme example of moisture resistance is composite decking installed on a submerged canoe and kayak launch at the Blue Run of Dunnellon Park on Florida’s Rainbow River. The 15-foot section of the ramp that goes into the river stays underwater year-round, yet the decking remains strong and good looking, unlike wood which would quickly deteriorate in these conditions.


The next time you’re planning a deck, dock or boardwalk for your facility, picture a train of rail cars full of water coming down the tracks to drench the surface and ask yourself if the option you’ve selected is durable enough to withstand this onslaught year after year. If you’re not sure, consider tougher options like moisture-resistant composites with fully-encapsulated wood fibers.

Brent Gwatney is Senior Vice President for Sales and Marketing for MoistureShield composite decking. Contact him at

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