The Importance of Deck Board Spacing

By Keaton Smith, 11/18/19

Example Spacing for the Pacific North West

Installing a new wood deck can be a bit of an involved process, and you want to be sure that it’s done correctly the first time. That means you want to be sure you buy the right type, size and amount of wood, as well as storing it properly before and during installation.

But that’s just the start. It’s also critical to remember that wood is a natural material that slightly expands and contracts depending on the humidity and moisture levels in the environment in which the deck is installed. As a result, you need to ensure you leave the proper spacing between boards when you install your wood decking.

Here’s what you need to know about deck spacing, along with some tips to help you get it right.

What Does Deck Board Spacing Mean?

Spacing on a deck refers to the small edge-to-edge gap between decking boards, not the ends of the boards. While this gap is fairly small, it is still a crucial component to any good deck. Without the right deck spacing, the boards may expand, cup, and buckle out of place. As a result, you may have to replace broken decking, or reinstall the boards with wider spacing to prevent further cupping and buckling, which can be time consuming and expensive. That’s why it is essential to make sure you install the decking with the correct spacing between boards the first time.

There is no reason to put a gap between the ends (also known as butt ends or butt joints) of wood deck boards. Wood, both softwood and hardwood, is dimensionally stable in length regardless of temperature, humidity or moisture content. Keep the butt ends as tight as possible when installing wood decking. This rule does NOT apply to composite or PVC boards which can have a large thermal expansion; enough to create a serious roller coaster effect down the length of the PVC or composite boards if you DON'T leave room for thermal expansion.

Why Is Deck Spacing Necessary?

Account for Expanding and Contracting of Wood

The most important reason to have a spacing gap is to allow the boards the room to expand and contract naturally.

Wood, by nature, expands and contracts when there are seasonal humidity changes and the wood begins to absorb or expel moisture. When you don't take this into account and place boards too close together, they can expand which can cause them to buckle upwards off the deck, cracking boards and pushing the fasteners out of place.

Allow the Deck to Breathe

In order to minimize cupping and expansion / contraction of the boards throughout the seasons, it is important to maximize air-flow underneath and around the deck boards.

Permits Water Drainage

Rain can prove especially detrimental to wood decking if it doesn’t have a place to drain off the deck. When there are no gaps between the boards, this moisture remains trapped which can lead to issues like rot and mold.

Makes for Easier Cleaning

Another benefit to proper deck board spacing is that the spaces help make it easier to sweep away debris from the surface of the decking. While debris may not seem like a big deal, it will take away from the aesthetic of your deck. It can also add to the moisture levels and impede the deck’s drying process which, in turn, can also contribute to - you guessed it - mold and rot.

How Big a Gap Between Decking Boards?

Deck Spacing Tool in Action

The size of the gap you leave between decking boards can vary depending on the species of wood you choose, the moisture level, and the humidity and temperature in the climate you are installing in.

In general, you should always plan for high humidity levels and anticipate that the deck boards will expand completely in wet and humid weather. However, residents of drier areas of the country, like the Southeastern U.S., should also account for drought-like conditions when installing their decking.

For the best results, consider working with an experienced professional who will know what gap size is ideal for your climate and wood type.

For kiln-dried wood with a relatively low moisture content of around 10 to 12% and a relative humidity level of about 35 to 45%, the final deck gap should be about 1/4 inch. However, if the deck installation occurs during drier weather, with lower percentages of moisture and humidity, you may need to add an additional 1/16 of an inch, to account for additional expansion.

Air-dried decking can often be installed with a gap less than 1/4 inch (6.5 mm) because the boards will likely end up shrinking in width rather than expanding. When installing new air-dried decking the proper gap ranges from 1/8 inch (3.5 mm) - 1/4 inch (6.5mm) depending on the environment. If you are installing air-dried decking during a rainy or humid season you may want to place the boards with a 1/8 inch (3.5mm) gap. This is because the boards will likely contract over time as they release moisture creating a larger gap during the drier seasons.

However, if the wood has been fully acclimated to the local climate, which can take up to 8 weeks for air dried material, it’s still best to stick with a 1/4 inch (6.5mm) gap to ensure the boards have plenty of room to expand. We recommend using a full 1/4 inch gap even with kiln dried or fully acclimated 1x4 and 5/4x4 as well as 1x6 and 5/4x6.

The ideal spacing can vary based on the season; for example, in the Pacific Northwest, the summers are dry but the winters are very wet, or in the Northeast, the summers are generally humid and subject to significant rain but the winters tend to be below freezing and, therefore, extract water from wood. If you live in a varied climate, you might look into a resilient (spring loaded) deck clip, like our ExoClip that expands and contracts with the wood, and automatically maintains an appropriate gap.

Using a Moisture Meter

The best way to decide the appropriate spacing for your boards is to check the moisture content of the wood before installation. The amount of moisture in the wood will let you know whether the wood will expand or shrink right away.

As the humidity increases, the moisture content increases which causes the wood to expand. As the humidity decreases, the moisture content decreases which causes the wood to shrink.

Read more about the movement of wood in relation to moisture content: Wood Handbook: Wood as an Engineering Material. The relevant table is listed below.

Recommended Mosture Content

Use a moisture meter to find out how much water is in your wood. Be sure to check multiple pieces of wood and take an average moisture count, instead of relying on just one piece of wood that could be an outlier. Calibrating your moisture meter to the particular wood species is also a critical step to ensure that the readings you get are accurate - follow the recommendations from the manufacturer.

If you have a high moisture content, leave less space between the boards as they will shrink as the moisture is released. If you have a low moisture content, leave more space to accommodate the expansion of the boards as they gain more moisture. Typical moisture contents for exterior wood products are between 10% and 14%, depending on the local climate and humidity - anything above 15% is considered high moisture content and thse boards will likely continue to shrink.

Tips for Proper Deck Board Spacing

Once you receive your decking, it is best to let the wood acclimate for at least 1-2 weeks in a dry area outside in order to adjust to the local climate.

Once your decking has properly acclimated, we recommend prefinishing the boards using a penetrative oil-based finish such as Nova’s ExoShield Premium Wood Stain.

During installation, make sure to keep your decking material in a cool, dry place where it will not be exposed to excess moisture and expand, which can affect the spacing between the boards.

Use spacers, a sixteen penny nail, or a pencil to help ensure even spacing between all the boards.

Consider using a pre-spaced hidden fastener system, such as Nova USA Wood’s ExoDek QuickClip. The ExoDek QuickClip was designed to ensure proper spacing, allow for the wood to expand and contract, and allows for a much faster installation.

Before fastening the boards into place, perform a dry run to ensure the final results will look good and are evenly spaced.

Either use two stainless steel screws through the face of the board at each joist or ExoDek QuickClips to fasten the boards to the deck frame.

The first board you install should be about 1/8" (3.5mm) away from the siding, to account for drainage, expansion and contraction.

Predrilling is typically necessary when fastening any type of hardwood decking.

Gaps are not necessary at the butt-joints (where the end of two boards come together) when installing hardwoods.

Don’t Forget to Mind the Gap

Deck Spacing at the Coast

Choosing to install a real wood deck is a big investment and one that will leave you with a beautiful statement piece for your backyard. But, with what you spend on materials, you want to be sure you do it right. One critical step that can be easily overlooked is the board spacing.

Leaving a gap between boards is critical for a well-built deck that will have space to drain, dry, expand and contract without causing the wood any damage. But, you also want to be sure you leave the right size gap: Go too small, and there won’t be enough space; go too big, and the gap can become a safety hazard.

If you still aren't sure about how big a gap to leave for your specific wood decking, then we strongly encourage you to talk to a decking professional. They will have the ability to check the moisture content of the wood, tell you the best spacing and help with installation to ensure it gets done the right way.

For more installation tips, contact Nova USA Wood Products to speak with an expert.

By Keaton Smith, 11/18/19

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