When you have a basic understanding of wood movement, you won’t be surprised at the idea that whatever size gap you put between decking boards, that gap will change throughout the year. As moisture levels shift, the boards will absorb and shed moisture, making the time of year you’re installing the deck a major factor in the size gap you should consider.
Seasonal Installation Differences
There’s no doubt about it: summer is the most popular time of year for decks to be installed. Of course, since that’s also when temperatures and moisture levels peak, it means that your boards will be swollen with as much moisture as they will ever hold, making gap spacing simple to plan. The wood fibers will have acted like a bundle of straws, sucking up the moisture in the air and expanding across the width of each board; the boards will contract as moisture levels shift throughout the year, but they will not further expand.
As a result, you could install them with a miniscule gap, which would become a more significant gap during the colder, drier, winter months. (You would still want to include a small gap, though, in order to allow for drainage; otherwise, standing water can create movement issues that — trust us — you don’t want to have to think about.)
If your customer prefers that ideal ¼” gap in May, you’ll need to look up details about the specific species you’re using to see how much you can expect the boards to expand and contract throughout the year and plan accordingly, based on the timing of your installation.
Not only do seasonal shifts make a difference, but so does the relative humidity in your geographic region and job site in particular. Sure, the higher the temperature, the greater amount of moisture the air can possibly hold, but that’s just relative. In a drier climate such as Denver, Colorado, you’ll find that a ¼” gap in the winter will remain approximately the same during the summer months, due to the relatively low humidity levels year round.
While all lumber will expand when in environments and seasons of greater moisture levels, not all lumber species will move at the same ratio. The anisotropic movement of wood will vary, based on both species and cut. For instance, Quartersawn boards move very little across the width of the board, while flatsawn boards experience much more movement.
In addition to how wood is cut, the species of wood will definitely play a role in determining movement. One handy method of looking up species-specific movement is the Woodshop Widget. You can plug in information such as size, moisture levels, temperature, and species in order to determine potential expansion and shrinkage, allowing you to plan gaps accordingly.
While there’s no right or wrong when it comes to deck size, it’s important to communicate clearly with your decking customers, making sure they realize that whatever size gap they request, the gap won’t be exactly the same every month of the year.
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J. Gibson McIlvain Company
Since 1798, when Hugh McIlvain established a lumber business near Philadelphia, the J. Gibson McIlvain Company has become 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. Contact a representative at J. Gibson McIlvain today by calling (800) 638-9100.
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