Decking

After 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.

For 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 (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.

Image credits: Top by Mario Beauregard/Fotolia

When it comes to the gaps between boards in a new deck, customers often have a definite opinion. They might tell you they want no gap, a ¼-inch gap, or another specific idea of exactly what they want. The problem is that regardless of the amount of gap at the time of installation, that particular size of gap will only be that way for a small portion of the year. As an organic material that responds to the environment around it, wood expands and contracts with seasonal shifts in humidity.

As uncomfortable as it may be to install a deck in the heat of summer (when most deck building takes place), the good news is that your lumber is at one end of the movement spectrum and will only move in one direction. For much of the country, high humidity accompanies the higher temperatures; that humidity causes the wood fibers to swell with moisture, expanding the board width to the widest it will be.

The most popular (and highest quality) species of exotic decking lumber is Ipe, so we’ll look at its properties as an example of how much movement you can expect. A typical Ipe decking board is 1-by-6 inches. If the temperature is 96 degrees, and we’re at 90% humidity, it will retain its width throughout the hot summer months but will shrink as the temperature and humidity become lower in the fall. Let’s say the board is 5.5 inches wide. By October, we may be seeing temps around 65 degrees, with 50% humidity. That 5.5-inch board will have shrunk approximately 3/16 of an inch, making the once ¼-inch gap closer to a ½-inch wide. Now, let’s consider that same deck in the dead of winter, when the temperature is closer to 25 degrees and humidity is around 25%, causing the decking to shrink an additional 1/8 inch or so, making the gap swell to 5/8 of an inch. As you might imagine, your customer who was once happy with the ¼-inch gaps would be a bit frustrated with the situation.

Worse than larger-than-expected gaps, of course, would be buckling and cracking boards. Let’s say you followed that same request for ¼-inch gaps between boards, but you installed the deck in the dead of winter (unlikely, I know, but just go with it, for now). By summertime, those boards would have expanded by more than 1/4 of an inch, resulting in serious damage, compromising the integrity of the individual boards and the deck as a whole.

The amount of movement depends on even more than the species and size of the board, but also on the precise cut. Lumber cut from closer to the center of the log will respond to climate changes differently than boards cut from the perimeter. Since wood is anisotropic, it moves differently in each direction, making quartersawn boards move much less across the width than flatsawn boards (as used in our example above).

In part 2, we’ll give some specific recommendations for lumber spacing.

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.

Image credits: Top & Bottom by Robert Cocquyt/Fotolia

Despite the increasing popularity of composite decking in recent years, these manufactured products possess some major flaws. Particularly when you compare them to the organic materials they’re meant to replace, composites simply don’t measure up. While these products might appear similar to the tropical hardwood lumber they replace, the similarities don’t even go beyond the surface level.

Scratch Resistance

The material typically used for the outer shell of composite decking is polyethylene, the same plastic used for disposable water bottles; it’s a pretty weak plastic. Sometimes used instead, polypropylene is a tougher substance, but it is still plastic. This water-resistant outer shell, or cap stock, shields the vulnerable wood flour core from mold and decay; however, this shell is extremely thin. When the delicate surface is scratched, appearance is far from the main concern: the inner core is exposed to moisture and mold. For this reason, most manufacturers warn against using metal snow shovels on composite decking.

Fire Resilience

Unlike Ipe, which has a Class A fire rating, composite decking is quite susceptible to fire damage. Like all plastics, it reacts to intense heat by melting, which causes the release of toxic gases into the air. Replacing deformed boards means an unevenly sun-bleached deck as well as disposal of the unusable boards into landfills, where they will remain for hundreds or even thousands of years. Not only are tropical hardwoods more resilient to fire, but they can be easily replaced and finished to match the surrounding decking. What’s more, the damaged boards will easily biodegrade in about a decade.

Irregular Movement

While you may contest that scratching or fire damage can be avoided, seasonal shifts in temperature and moisture levels are inevitable. Seasonal movement of traditional lumber is unavoidable but predictable. Wood moves noticeably across the grain, but almost imperceptibly along the grain. Based on lumber species, we can even estimate the amount of movement to a high degree of accuracy. By contrast, composite decking lacks grain patterns to allow for predictable expansion. Instead, both the plastic shell and inner core of composite decking expand in all directions when exposed to heat. As the plastic shell is stretched with the core’s expansion, it retains the larger size even once the wood flour shrinks back to its original size. The result is separation of the core from the shell, producing uneven warping along the board lengths, with the ends swelling to a greater degree than the interiors. When cut ends occur throughout the structure instead of only at the ends, this issue is even more noticeable. The swelling and moisture traps will end up requiring widespread board replacement, again landing many pieces in landfills.

A simple way to avoid these and other pitfalls of composite decking is to take a look at the attractive and durable species of tropical hardwood decking. J. Gibson McIlvain highly recommends Ipe and Cumaru and carries a wide variety of board lengths and widths for decking purposes.

J. Gibson McIlvain Company

Since 1798, when Hugh McIlvain established a lumber business near Philadelphia, the McIlvain family has immersed itself 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.

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