Mind the Gap: Decking Tips

If you’ve ever travelled via London’s subway system, the Tube, you’ve heard the recorded voice over the loud speakers repeatedly warning passengers: “Mind the gap.” In fact, the wife of the actor whose voice was recorded for the caution in the 1960s requested that it be reinstated after his death. The voice and expression have, in fact, become so iconic for London, that you can even find T-shirts on Amazon with those words. Although the application is a bit different when it comes to decking installation, it’s also important that you, well, mind the gap.

Before we really get into gaps between decking boards, let’s do a little review of lumber movement in general. Wood moves constantly, and we can’t do anything to change that. What we can do is learn about precisely how it moves and anticipate its movement. While each lumber species has a specific ratio of movement, each species will demonstrate 3 types of movement: radial, tangential, and longitudinal. As you might expect, longitudinal movement refers to movement along the length of a board (which was the trunk of the tree, once upon a time).

Longitudinal movement is negligible, while the other two types can be fairly significant, depending on the species and the climate. Before your decking boards were harvested and milled, the structures inside them served the purpose of carrying nutrients throughout the tree. The wood fibers running up and down the tree acted like hoses; as those hoses would fill with moisture, they would swell; as they shed water from their ends, they would become thinner.

Your decking board still has those structures in place, and they still respond the same way to fluctuating moisture levels. The most significant shifts will occur as the boards come into an equilibrium with the moisture at your job site and each time they’re freshly cut, but they’ll also continue as surrounding moisture levels ebb and flow with seasonal changes. However, the hoses aren’t perfectly round, swelling identically in all directions.

Tangential shrinkage occurs across the growth rings of the tree, which translates into the width of the board. This side-to-side swelling and shrinking is the most significant movement you can expect your boards to experience. Radial shrinkage occurs perpendicular to the growth rings, or along the medullary rays that once brought nutrients to the center of the tree; now we think of radial movement to mean along the thickness of the board. While radial movement is less significant than tangential movment, it’s still significant enough to consider. The ratio of tangential movement to radial movement is referred to as the “T/R ratio,” and the closer the ratio is to equalling 1, the more stable we consider the species, and the less susceptible it is to warping and cupping.

Continue reading with Part 2.