As we discussed in Part 1, when a customer requests a certain size gap in their decking, you need to ask them when they would like that size gap. Depending on the season and climate in which you install the deck, that perfect ¼-inch gap may widen to a ridiculous size in the coming months or — even worse — cause buckling and cracking.
Add to seasonal shifts in moisture the fact that how wood moves varies based on the species and cut, and you have a complicated problem on your hands. Like we said at the end our last post, though, there is good news: You can learn how to plan for these kinds of things.
Spacing According to Season
The first thing you need to keep in mind is that you can’t keep your decking boards from moving; however, once you embrace this reality, you can plan accordingly. For instance, in the summer-installation scenario we looked at, installing with a barely perceptible gap would mean that in winter, there would be a nice ¼-inch gap.
Why have a gap, at all? You want rainwater to be able to fall through, instead of sitting on top of your deck; standing water causes all kinds of movement issues you really don’t want to have to deal with.
For a winter installation, you’ll probably want a little more than a ¼-inch gap (maybe 1/16 or 1/32 more) just to be safe. With an app like The Woodshop Widget, you can plug in the dimensions and species of the boards to calculate projected expansion or contraction.
Spacing According to Climate
If you don’t live and work in an area with high humidity, you have it a little easier than the rest of us. Wood movement is generally tied more to humidity, rather than temperature, so if you never have high humidity, the shift in the gap from season to season may be extremely small. An added consideration, though, is how much sun and rain exposure your deck will encounter.
Spacing According to Species
The two major types of shrinkage are radial and tangential. Radial shrinkage is the amount of movement that wood undergoes, along the radial or medullary rays, transporting nutrients into the tree’s interior. Radial shrinkage is typically not as significant as tangential movement, which occurs along the growth rings; the result of tangential movement is side-to-side swelling of wood fibers, which act like a bundle of straws.
Each species has a ratio of tangential movement to radial movement; the T/R ratio of a species relates its level of stability. Even the most stable species, though, won’t move in a uniform way; the ends dry out more quickly than the middle will.
Regardless of a species’ inherent stability, we can lessen the wide swings of moisture by keeping lumber in a well-ventilated area and out of direct sunlight. You can also sticker the wood and make sure you’re using kiln-dried lumber.
J. Gibson McIlvain Company
Since 1798, when Hugh McIlvain established a lumber business near Philadelphia, the McIlvain family has been immersed in the premium import and domestic lumber industry. With its headquarters located just outside of Baltimore, the J. Gibson McIlvain Company (www.mcilvain.com) is one of the largest U.S. importers of exotic woods.
As an active supporter of sustainable lumber practices, the J. Gibson McIlvain Company has provided fine lumber for notable projects throughout the world, including the White House, Capitol building, Supreme Court, and the Smithsonian museums.
Contact a representative at J. Gibson McIlvain today by calling (800) 638-9100.
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