How Much of a Gap Should be Between Composite Deck Boards?

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how much gap should be between composite deck boards

Composite decking can be a great, durable material to use for your back deck. 

With its scientifically developed chemical makeup, it can last a lot longer than traditional decking. However, when you are installing it, there are many things you should consider that are different from wood. 

From the fasteners you use to its maintenance schedule, composite decking has some important differences from traditional wood decking. Just like wood decking, though, you will need to leave gaps between the planks. 

Knowing how much of a gap should be between deck boards can help you during construction. It will allow you to figure out the number of composite boards you need and their configuration.

Determining the Gap Between Composite Deck Boards: Basic Idea

In order to determine the gap you will need between composite decking, you need to consider certain factors. These include: 

  • Check With Local Building Codes
  • Check the Color 
  • Determine Your Ventilation Needs 
  • Consider Safety 
By taking all these things into consideration, you can build your perfect back deck. You can also make sure that your deck will last for many years to come and stay in good condition. 

Why Leave a Gap Between Composite Deck Boards?

why leave a gap between composite deck boards

Many people wonder why a gap should be left between their deck boards once it is installed

It may seem, at first, that you shouldn’t have gaps in your deck. However, they serve an important purpose when it comes to the deck’s longevity. 

If your planks had no gap, they would not be able to expand and contract with the varying temperatures. This is very important since your deck will be exposed to the weather elements all year long. 

You need to leave the gaps to allow the boards to shift and move as they heat up and cool down. If they are too close, they will splinter and crack as they expand. 

While composite decking won’t expand as much as wood decking will, it still needs gaps. Knowing what size these should be can allow you to give your deck the best chance at longevity. 

Generally, the gaps in your decking should be between ⅛” and ¼”. The size you choose will be determined by the various factors with your deck. 

Check Your Building Codes 

The first thing you should do before planning a deck is check with your local zoning laws and building codes. 

Every municipality has its own set of rules that you will need to follow when it comes to construction. Knowing what these are can help you avoid costly fees and violations. 

In your local building codes, there might be a stipulation on how big your gaps need to be. If you have one of these, you should follow that. 

There may also be other requirements for construction regarding your deck. 

These could be things like how high it can be or what kind of footings to use. Along with your gap size, following these regulations can help ensure that your deck is in compliance. 

Think About Color 

You should also consider the color of your composite decking when figuring out your gap size. 

Darker colors will absorb more energy from the sun and will expand more in the sun. If you have a darker composite decking color, you should include bigger gaps.

If you have lighter decking, such as a light gray or white, you can leave your gaps closer to ⅛”. However, any less than that will still not be enough no matter what color the planks are. 

Determine the Ventilation Needs

determine the ventilation needsThe amount of ventilation and drainage you need will also determine your gap size. 

If you live in a very moist area, you should include larger gaps. This will allow for ventilation and drainage when the weather turns poor. 

If you don’t have large enough gaps, the ground under your deck could have trouble drying. You may also have standing water issues on the top of the deck when it rains. 

Larger gaps will let more water through and also allow for better drying. This way, the soil under your deck won’t get mildewy or soft, which can be a structural issue. 

Think About Safety 

You should also be thinking about safety when it comes to the gaps in your deck. 

Larger gaps could present tripping hazards for the people walking on the top. The smaller the gaps are, the smoother the surface will feel. 

This is one of the reasons gaps might be included in your area’s building codes. 

Generally, as long as you keep the gaps between ¼” and ⅛”, they should be small enough to be safe. 

Frequently Asked Questions

faq how much gap should be between composite deck boards

  • How do you fill space between deck boards?

While it might be tempting to fill in the gaps, you should always leave them. 

As long as you have kept them up to regulation, they won’t interfere with walking. Plus, filling in the gaps will not be an attractive option. 

  • Can you caulk composite decking?

You can caulk your composite decking, but it is not recommended. 

If your gaps are too big, you might be tempted to caulk them. However, this will not only be unattractive, it will also not be effective. 

  • How do I stop my deck from leaking?

You can buy rubber flanges that go under your deck’s gaps if leaking is an issue. 

If, for example, you don’t need ventilation because your deck is on the second floor, this could work. They are easy to install and won’t interfere with the construction. 


Your deck should be able to last for many years. If you ensure that you have the right gap space, you are much more likely to achieve this. 

By considering the above factors, you can include the right size gaps and enjoy your deck for a long time. 

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Michael J. O’Connor is a writer and marketing specialist from the Bay Area of California. A graduate of Sonoma State’s Creative Writing program, he spent many years as a contractor and carpet layer, learning the ins and outs of flooring and general contracting. When he’s not typing away at his desk, he enjoys hiking with his dogs, woodworking and collecting rare books. See full biography here.

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