Most experienced carpenters and construction professionals are well aware that wood moves. Here are a few helpful building and installation tips on how to get the best results when building an outdoor deck.
Hardwood decking is a great product to design and build decks with. You can build so many creative designs with natural products using typical carpentry tools. But wood moves – and every board will differently. How do you plan on dealing with this naturally occurring phenomena when building a deck? Let’s take a look at why and especially how and why wood moves first.
Why does wood move?
All wood moves. As it sheds moisture, all wood will shrink. As it picks up moisture, all wood will expand. When a tree grows, the grain of the wood runs vertically in the direction of the growth. Water is carried parallel to the grain to help the tree grow. Once a tree is cut down, it naturally begins to shed its water. When the tree is then cut into usable lumber, the wood loses even more water. If you look at wood cells under a microscope you can clearly see that, as wood loses its moisture, the cells start to collapse and wood begins shrinking. If that same piece of wood started to pick up 
moisture, it will then begin to “swell” and expand in size.
How does wood move?
There is very little movement of wood along its length (usually less than a ¼%). But wood moves a lot more in its thickness and width.
In this picture, you can see the wood grain in the radial direction (perpendicular to the growth rings) and tangential direction (across the growth rings) of a piece of wood. Tangential movement of wood is always the greatest, radial movement is usually about half as much as tangential expansion and contraction.
What about outdoor decking wood movement?
In the decking world, that means that flat-sawn boards (tangential width) will expand and shrink more so than quarter-sawn boards (radial width). In either case, the boards will change size as they pick up or drop moisture. Because outdoor wood experiences the harshest environment you can possibly subject a piece of wood to, great care should be taken to use each piece of wood to its best advantage.
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"Flat sawn" Ipe deck board. Growth rings are in the tangential direction along the board's width. Flat sawn wood boards tend to move more widthwise. |
"Quarter sawn" Ipe deck board. Growth rings are running vertically in the radial direction. Quarter sawn boards tend to move less widthwise. |
Most of the dimensional changes in wood occurs as the wood goes from the green state (about 45% moisture content) to a kiln dried state (about 12-14% moisture content). However, as the wood boards acclimate and reach equilibrium in their new environment, they will still move. Once the boards reach their equilibrium moisture content on site, the expansion and contraction lessens significantly, and you will see “seasonal’ changes in the wood dimensions with changes in humidity.
Using high quality hardwood decking and proper acclimation are the two best ways to achieve long lasting positive results with your new deck.
Choosing kiln dried hardwood decking is the best way to ensure more uniformly sized lumber. But the job is not done yet. Proper acclimation of the boards on site, before installation, is another critical success factor. If decking boards are allowed to properly acclimate on site before installation, an experienced carpenter can then choose the boards with similar widths for each row of decking before installing them. This way, the spacing between boards will stay a lot more consistent, even with seasonal changes and fluctuations in humidity.
Failure to allow the wood to properly acclimate on site may create unwanted results. If wood dries too quickly, it tends to react poorly by possibly checking, splitting, warping or cupping.
Using a UV inhibiting sealer is the final layer of protection initially
Pre-finishing your deck boards with a UV inhibitive sealer, such as Penofin, is a great way to help your hardwood decking acclimate more slowly on site. UV inhibitors protect the new wood’s surface in almost the same way that sunscreen protects a baby’s skin at the beach. The slower the acclimation, the better the appearance and performance of the decking over the lifetime of the deck. The UV blockers will help minimize the amount of, and severity of, any surface checking that may appear on the deck boards. Protect your deck properly and it will pay you back with many years of great performance and great looks.
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