The Downfalls of Capstock Decking

One of the chief problems with composite decking is the way it absorbs moisture.  This is the cause for symptoms like mold, swelling, and delamination. Capstock decking aims to solve this problem by covering the porous composite with a layer of durable PVC. The PVC acts as a shield, and in theory the solution is a good one.

There’s just one problem. The ends aren’t covered.

End Swelling

This is really unavoidable. Even if the capstock boards were end-sealed during production, the builder would just cut them open during installation. The exposed ends soak up water and expand, resulting in a flared look. The swelling can crack the PVC shell, further ruining the look of the deck. The uneven surface created by the swollen ends can pose a tripping hazard.

Unfortunately, once the board ends have flared, they never go back, even if the deck is completely dried out. This leaves the deck owner in the tough position of choosing between living with an ugly (and potentially unsafe) deck or paying replacement costs.

Structural Compromise

PVC is a fairly strong plastic, but it’s not impervious. When board ends swell, the surrounding PVC shell can crack, further exposing the composite core. More water gets in through these new cracks, and the swelling works its way through the entire board, cracking the shell as it goes. Once the shell is broken, the decking loses much of its strength and becomes more prone to breakage.

Environmental Health

One more thing. PVC isn’t a good thing. According to a study by the Healthy Building Network, “PVC is the worst plastic from an environmental health perspective, posing major hazards in its manufacture, product life, and disposal.”

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