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Will Deck Boards Shrink? Proper Deck Board Spacing

So you’re planning on building a deck in your property, and are busy researching all the requirements. Chances are you’re already confused about the spacing between the deck boards and why you need to leave them in the first place. Can a deck board made of solid wood actually shrink and expand?

Deck boards will shrink, and not only that, they will even expand. Fibers have the ability to hold moisture in them. So when the outside environment is humid, the fibers collect moisture and expand, leading the boards to expand. In the dryer seasons, they lose moisture and shrink as a result.

In this article, we will discuss this in more detail. We will discuss what the proper spacing should be when you’re installing deck boards. And we will also look at why the proper spacing is absolutely vital.

Do Deck Boards Shrink?

Deck boards can both shrink and expand. This is why it is necessary to leave appropriate spacing between them when building a deck.

Most deck boards are pressure treated, which means they will be somewhat wet when they’re new. When wet, the fiber in the woods holds moisture and, as a result, expands. Since deck boards are mostly installed outdoors, over time, enough exposure to the sun will dry them up and lead them to shrink a little.

But it doesn’t just end there. Due to the fluctuations in the humidity in the air, deck boards will continue to cycle between expansion and shrinking.

Why Do Deck Boards Shrink?

There are many factors that determine the shrinking and expansion of deck boards. The simple science is that wood is made up of fibers. And these fibers have the ability to hold on to moisture.

  • When these fibers hold moisture, they tend to expand, making the boards bigger.
  • And when they dry out, the fibers contract, making the deck boards shrink.

This factor is further complicated by the fact that deck boards are installed outdoors and are subject to the changing seasons. During the more humid months of summer, the wood fibers in the boards will soak in more moisture, leading them to expand. And during the dryer months of winter, the boards will once again contract as they lose their moisture.

Of course, this seasonal expansion and contraction of the boards depend on where you live.

If you live in a place where the humidity stays pretty much constant throughout the year, you won’t have to worry too much about your deck boards shrinking or expanding. But what if you live in a place with wild swings in humidity?

What Do I Do About the Shrinking and Expanding of Deck Boards?

The best way to deal with the shrinking and expanding of deck boards is to leave proper spacing. The process is pretty straight forward. But it depends on what season you install the boards in.

Installing Deck Boards During the Dryer Months

If you’re installing deck boards during the dryer months, you will have to consider that the boards will soak in moisture in the more humid months and, as a result, expand. So, in this case, you should consider leaving a gap between 1/4 and 1/8 in (0.64 and 0.32 cm) between the boards.

  • A 1/4 inch gap is more desirable with respect to other factors such as ventilation, drainage, and debris removal.
  • But the wide gap may also be a safety hazard, as it can lead to tripping.
  • A 1/8 in (0.32 cm) gap, on the other hand, won’t be as hazardous as a 1/4 in (0.64 cm) gap.

You must also consider that if your deck boards are pressure treated, they will come with some moisture in them and will eventually shrink down.

To account for this, you will have to install pressure treated deck boards flush. This way, the boards will shrink enough over time, leaving an appropriate gap.

Installing Deck Boards During the Humid Months

If you’re installing deck boards during the humid months, the process can get a bit more complicated. You might think that the process will simply be reversed. But instead, you will have to account for both the expansion during the remainder of the humid season and then the eventual shrinking that the boards will go through during the dryer months.

  • So if you are using dry, untreated boards, you will have to install them with a slight gap of about 1/4 in (0.64 cm).
  • This way, the boards will expand over the rest of the humid season and then shrink once the dry seasons arrive, leaving an appropriate gap.
  • If you’re using pressure-treated boards, you might want to leave a smaller gap of about 1/8 in (0.32 cm).

This means the boards will go flush during the rest of the humid season, and then once they lose the moisture from the pressure treatment and the additional moisture accumulated during the wet season, they will contract back in the dryer seasons.

If you want to avoid these complications, you might want to consider installing your deck boards during the dry season.

Here’s a helpful video about types of wood to use for a deck and proper board spacing:

Local Building Code

There are a few other factors to consider here. The first thing you need to consider and check is if your local building code covers deck board spacing. There are plenty of jurisdictions that follow the International Building Code.

And according to code, you will need to leave a minimum of 1/8 in (0.32 cm) gap between your deck boards. If you’re living in a place with this code, you could be required to uninstall your boards. So make sure to check it beforehand.

Why Appropriate Spacing of Deck Boards Is Vital?

Having an appropriate space between deck boards is absolutely vital for a number of reasons. We have already talked about the shrinking and expanding that they will be subject to during the seasonal cycles. But there are three other reasons why you will want to leave enough space between deck boards.

The first and most important reason is the proper circulation of air. With enough space, air can easily move through between your boards. The moisture caught in the boards can also evaporate more easily. This is important to prevent the boards from accumulating moisture for too long and, as a result, succumbing to rots.

Another reason why it’s absolutely necessary to have proper spacing between deck boards is to allow for drainage. Since decks are built outdoors, they will be exposed to rain and snow. Without proper spacing, instead of draining through, these will leave puddles all over your decks.

Not only will this make the surface unsafe to walk in, but it can also lead to a range of other problems like rotting or the buildup of mold and mildew.

The third and final reason you need to leave enough space between deck boards is to allow the debris to fall through. If they can’t fall through, they will accumulate, making the deck dirty and also causing rots.


Wood fibers can hold moisture in them. As a result, the fibers in a deck board will hold moisture and expand during the humid season, leading the board as a whole to expand a little. And this when moisture is lost during the dryer seasons, the fibers will shrink and so with the boards.

This factor is very important to consider when installing a deck. Without proper spacing, the deck boards can rot since they won’t have enough circulation of air between them. It also causes problems with drainage, and you will have puddles all over your deck.

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