Deck-Building 101: Planning for Gaps, Part 2

waterfront wood deckAfter reading Part 1, you’re probably wondering how in the world you’re supposed to plan for gaps! After all, you need to keep your customers happy and the decks you build safe and intact throughout the year. Relax. There is an answer.

The first step is to realize that wood moves and that is a reality of life we have to work with. Of course, you’ll have to help your customers to understand this concept, as well. When they tell you the spacing they’d like to have between boards, you can introduce the idea by asking them, “When?” If they don’t get it, you can either try to explain the concept of seasonal movement to them or refer them to the article linked above — or Part 1.

If you look back at the example of a 1-by-6-inch Ipe decking boards, 5.5 inches wide, you could install the boards in the summer with hardly any gap at all — just a tiny gap big enough to allow drainage for rain water — so that in the winter, you’d have an ideal ¼-inch gap. (If you didn’t allow for drainage during the summer months, you’d end up with standing water and related movement issues you really don’t want to have to address!)

However, if your customer insists on having a ¼-inch gap during the summer months, you’ll want to crunch some numbers to find out exactly how much their species-of-choice in their geographic area will likely contract throughout the year. Luckily, there is an app for that! The Woodshop Widget can save your precious gray cells from uncomfortable expansion by doing the math for you.

fresh ipe deckFor those who don’t live in areas with high humidity during the summer, the issue of movement isn’t nearly as significant. While temperature and humidity both affect wood movement, moisture is the primary culprit. At the same time, warmer air has greater water-holding potential. The relatively dry climate of Denver, Colorado, means that despite the 70-degree difference from summer to winter, only an 1/8-inch of movement is likely. So an 1/8-inch gap during a summer install will translate into a 1/4-inch gap in the winter, which is pretty much ideal.

Of course, it’s not just decking that’s subject to movement. You should really consider the temperature and humidity changes throughout the year, as well your project’s potential level of exposure to the elements, whenever you’re planning any type of woodworking project. You can easily look up the wood movement percentages on the J. Gibson McIlvain website, according to species (look here for Ipe).

In the end, there’s no “right” sized gap, except one that doesn’t buckle or split in the summer or allow your customer’s cat to fall through in the winter. The important thing is to understand related movement issues and educate your customer to make an informed decision.

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 ( 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.

From the McIlvain blog:

Image credits: Top by Mario Beauregard/Fotolia

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