Here at J. Gibson McIlvain, we truly believe knowledge is an ally in the lumber world. We find that informed customers are happiest, largely because their expectations match their lumber. Especially when customers are shelling out big money for tropical decking species such as Ipe or Cumaru, they deserve to be happy. So let’s make sure they’re informed.
Question: Why is tropical decking rougher than other species?
Answer: Most exterior lumber is quartersawn, serving the dual purposes of highlighting the beautiful consistency of the vertical striping on the face and achieving optimal stability. There’s a tradeoff, though: quartersawn lumber exposes medullary rays, the mechanisms used to transport nutrients from the exterior to the interior portions of a standing tree. These dense structures do not plane easily, so the surface of a freshly milled quartersawn board is typically in need of a little TLC.
It’s not a sign of defective wood, just wood that needs a little sanding in order to achieve a desirable smooth surface. The same issue can actually occur with any species, but because tropical decking species are so hard, the distinctions are more obvious.
Question: If tropical decking is so stable, why are my decking boards warping?
Answer: If you’re having this issue, it’s probably due to a problem with your deck’s design, drying, or installation. For starters, an exterior wood species of any kind will necessarily have a higher moisture content than lumber used indoors. Unlike kiln-dried products, decking products are usually air dried. As such, they typically have moisture levels of between 14-18%. If the boards were, instead, kiln dried to moisture levels less than 8%, the thirsty boards would seek to acclimate to the surrounding atmosphere during the first rain or a humid season. By keeping the moisture content higher, we can avoid rapid movement caused by large swings in moisture content.
If decking lumber is stored out of direct sunlight and stacked to allow for proper air flow for 2-3 weeks prior to installation in the same climate as your job site, it should come into equilibrium with the local environment.
As long as proper moisture levels are retained and the wood is allowed to go through a reasonable acclimatization period, the wood will be permitted to expand or contract before it’s screwed to a rigid sub-structure.
After installation, movement throughout the lifetime of your deck can be lessened through proper spacing and side ventilation. We also recommend a wax-based coating called AnchorSeal, which slows absorption and loss of moisture from the ends of the boards. Without such a sealant, those ends act like bundles of straws, allowing for the kind of rapid shifts in moisture which allow for checks and cracks.
We hope that as you understand more about the valuable natural resource called lumber, you’ll be more enthusiastic about your next project and future building jobs. When you’re ready, check out Part 2.
