If you think we’ve exhausted all the deficiencies of composite decking compared to Ipe decking, think again (see Parts 1, 2 & 3). Despite conventional wisdom to the contrary, composite decking is not environmentally friendly; and that issue makes the fact that it will not last as long as Ipe become an even more worrisome issue. In many ways we’ve barely scratched the surface — which leads us to the next issue.
This issue is somewhat connected to the hardness issue, but some of that depends on what kind of plastic is used. While polyethylene is more typical, a tougher plastic, polypropylene, is sometimes used. Regardless of the plastic, though, composite decking with the cap stock construction described in Part 3 relies heavily on the outer shell to keep moisture from the inner core, which is comprised of wood flour. Because that outer shell is extremely thin, though, it can easily be scratched. The scratches are not only unsightly, but they risk exposing the inner core to moisture, creating movement as well as mold. For this very reason, composite decking manufacturers often warn against using metal snow shovels on their products; clearly, that makes plow trucks on a beach boardwalk out of the question.
To some degree, the strength of composite decking will vary according to manufacturer. But if you take a look at the installation manuals, you’ll almost always find the recommendation for 12-inch center joists rather than typical 16-inch centers; but most people won’t read that fine print and will use 16-inch centers instead. Those recommendations are to keep boards from producing bounce — as well as deformation. Since plastic has a “memory,” once it is deformed it will not bounce back. The weak inner wood flour core has no stiffness on its own, making a compromised outer shell even more problematic. Of course, the heavy foot traffic that a public boardwalk will endure leads to even more potential deformation. However, if you do install your composite deck on the recommended 12-inch centers (instead of 16-inch centers), you can eliminate those issues; but you will pay much more for the added lumber.
Wood movement can be a frustrating aspect of using real lumber in general and for decking in particular. So we totally understand the appeal of composite decking — after all, it would allow you to avoid that pesky issue of movement, right?! Not really. The wood flour core means that while the core will absorb moisture, it won’t do so in the kind of predictable fashion — across the grain — as a solid wood board will. While we can’t stop wood from moving, we can predict which way it will move and by how much, based on the species and environment. Not so with composite materials.
Continue reading with Part 5.