Water vs. Wood

Water vs. Wood

  • damage prevention
  • decks
  • maintenance
  • moisture
  • water damage

How Water Affects Wood
By Graham Bailey
UBC Wood Products Processing Student

It’s well known that water can cause some problems for wood. But do you know why? Or how? Knowing what exactly water does to your wood product can be a powerful tool in the fight against warping, splitting, rotting and failed stains. In this article, we’ll discuss both the structure of wood and the environment it’s in, explaining how and why water affects wood.

Wood is designed by a tree for two main purposes: support and transport. The support part is obvious, but what about transport? Water, nutrients and chemicals need to move to all parts of the tree. This need causes the tree to develop tube-like cells (Figure 1), which run vertically and horizontally. We’re going to focus on the vertically oriented cells, which are called longitudinal tracheids or simply tracheids (trake-ee-eds).

Red Cedar Cells

Figure 1: A cross section of Western Red Cedar (Courtesy of Dr. Simon Ellis, UBC Wood Science)

Tracheids make up the majority of cells in a tree. They’re typically 3-4 millimeters long and 0.03 millimeters in diameter. To put that in perspective, you could fit about 170 tracheids inside a pin. So, these cells are really small. And they’re also hollow, and their walls are kind of spring shaped.

What does that have to do with wood-water interaction?

It turns out that if you take a bunch of really small tubes and bunch them together, they tend to absorb water. And if they’re spring shaped, they will actually get longer as they absorb water, and get shorter as water is removed from them.

So why does that matter? If that were true, you might say, then when wood gets wet, it’ll just get longer. You could buy an 8 foot 2×4 in July and by November it might be long enough to replace that split 10-footer in your deck, right?

Well, no not quite. The structure of wood is highly variable, and its cell-density can change drastically depending on where in the log a piece of lumber is cut from (Figure 2). Denser cells absorb more water and dry more slowly, while the opposite is true of less dense cells. Most lumber contains areas of differing density, and if this contrast is stark enough we run into problems. If the lumber is wet and begins to dry, those two regions will contract at different rates, and we get something called differential longitudinal shrinkage, though its more commonly known as a hockey stick or firewood. With that in mind, we can develop the following rule:

Wood Cell 2

Figure 2: An example of variable cell-density. The orange cells are denser. (Courtesy of Dr. Simon Ellis, UBC Wood Science)

Wood can only warp when its moisture content changes

Warped-Wood-Deck

On top of that, those changes only occur when the moisture content drops bellow roughly 20% in cedar. All moisture changes that happen above 20% cause absolutely no change in red cedar. The average “green” moisture content for red cedar is between 58% and 259%. Green lumber is any wood that isn’t kiln dried.

Another important fact here is that wood will equalize its moisture content with the humidity of the environment it’s in. For example, green lumber in the winter in Vancouver will eventually equalize at about 21% moisture content. In the summer that drops to about 10%.

But what does that mean for you? Basically, any green lumber you find is going to be soaking wet, and that isn’t a problem. Wood can only warp when it dries too much.

To wrap – or warp – up, we’ve looked at the basic structure of wood, and why that structure can change a boards shape. We also touched on what humidity has to do with moisture content, and that wet does not equal bad when it comes to buying green lumber. This knowledge will help you to better use wood products, and hopefully make your next project that much better.

Stay tuned for more articles on wood science, and make sure to leave a comment if you have any questions!