Will Cedar Deck Boards Shrink and Should You Space Them?

Cedar decks are probably the most popular decks used throughout the US. Building your own cedar deck isn´t as hard as you might think considering you keep some very important things in mind.

For one, the screws or fasteners you use matter a lot. Cedar is known for eating certain types of screws. And the other big thing to keep in mind is that cedar tends to move quite a lot.

So you have to take into account that cedar can indeed shrink and expand quite a lot.

In this article, I will tell you everything that you need to know in order to build a cedar deck with the proper spacing to avoid any awful surprises later on.

First of all, cedar deck boards can shrink and expand depending on how high or low the humidity levels outside are. If it is especially dry outside then cedar wood will contract and shrink and if it is quite wet outside or it the humidity levels are high then cedar deck boards will expand.

How Much Does Cedar Wood Shrink and Expand

Wood is a porous building material which means it absorbs and disperses moisture from and to the environment. This will lead to the wood expanding and contracting.

So depending on the humidity that is surrounding the cedar wood it will either shrink when it is quite dry outside or the wood will expand when the humidity levels are higher outside.

Cedar wood shrinks on average about 1/8 inches and it can expand about as much. The amount of shrinkage and expansion depends on a lot of factors like the thickness of the wood, your location, the type of cedar wood, and more. So it can be quite hard to tell how much the wood will move unless you are willing to properly calculate it.

Contrary to popular belief the moving of the wood has nothing to do with how cold or warm it is outside. It’s all about the humidity.

Unfortunately, when it comes to calculating exactly how much the cedar board is going to shrink or expand it gets complicated really fast.

There are several things to keep in mind when calculating how much cedar wood shrinks but before we get to that we will firstly take a look at the formula.

(Width of the board (in inches) * annual change in moisture content) * dimensional change coefficient of the wood

This formula sounds very confusing right now but don´t worry! I will go over every part of it and I explain exactly what each of these things mean.

Width of the Board

This is probably the easiest to explain by far.

You simply need to measure the width of the board in inches.

Your Cedar deck board will shrink more or less depending on how wide it is.

Annual Change of Moisture Content

This is commonly referred to as the MC.

You will need the average moisture content of your cedar wood in order to properly calculate how much it shrinks and expands.

The problem here is that the average annual moisture content changes quite a lot depending on where you are in the US.

Luckily you can refer to a table from the esf. Simply follow this link and go to page 4. There you will find the average moisture content of wood that is exposed to the outdoor atmosphere ordered by States.

If you want very high accuracy, however, then I would recommend measuring the MC of your cedar wood with a moisture meter. You have to measure it in the winter and in the summer. Then simply take the average of your measurements and there you go.

Dimensional Change Coefficient for Cedar Wood

This number basically only tells you how well wood absorbs moisture and therefore how much it has the potential to shrink or expand.

This number actually changes by quite a bit depending on what kind of cedarwood you are using so make sure to pick the right number.

You can find a table of the dimensional change coefficient for cedar wood right here on another post of mine.

You can also refer to this post from popularwoodworking.com He has a table as well and he explains in even more detail how to calculate wood shrinkage properly.

Do the Math

Now all you have to do is entering all the right numbers and doing the math.

You will end up with a number that will tell you by how much inches your cedar wood boards will be able to shrink and expand on average.

You can now use this number to space your boards exactly at the right distance from one another.

Or you can simply take the average recommended spacing for cedar deck boards which is between 1/8 to 1/4 inch.

Is it Important to Space Cedar Deck Boards?

So you went through all the trouble of calculating the exact number by which your cedarwood is actually moving and it turns out it is not all that much.

And now you are obviously wondering if it is even important to space cedar deck boards in the first place.

It is important to space cedar deck boards even if the amount by that the wood expands is minuscule. Mainly because you want some space between the boards for water to run through and to give the boards some space to breathe. Both is important to ensure the longevity of your deck.

So to ensure the longevity of your cedar deck I would highly recommend leaving at least 1/8 inch space between the boards.

This will ensure that any water or snow can easily run through the spaces and does not sit on top of the board for too long. As this could result in rot.

Even though cedar wood is quite resistant to rot it is not completely immune to it meaning it will rot eventually. And if you don´t leave enough space between the boards then water residue may speed up the rotting process quite drastically.

You should also leave the space between the boards to give the wood some space to breathe. If you don´t leave that space then the wood will absorb or dispense the water unevenly to or from the environment. This will lead to warping which is definitely not desirable.

So make sure to give your cedar deck boards some room to breathe from all directions!

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