Spacing on Deck Boards in a Humid Area Leave a comment

How much space to leave when spacing on deck boards in a Humid Area? This question may be asked more than any other when it comes to installing a deck. Ever have a customer tell you the following when beginning a deck installation?

“I want no gap between the boards” or, “I want a 1/4″ gap between my deck boards” – Your reply should be, “when?”

Like a stopped watch that is only right twice a day, those deck boards are going to expand and contract as the seasons change, and no matter what you do, the gap between the boards will only be consistent for a certain window of time. This is why you find so much debate on how to set the spacing between your decking, because the answer truthfully is, “it depends.”

Spacing on Deck Boards in a Humid Area

Spacing on Deck Boards in a Humid Area The summer heatwave is in full effect across most of the country, and we continue to sell a lot of Siberian Larch decking. Installing a deck during summer assures you that your Siberian Larch decking (or whatever wood species you choose) is at one extreme of the wood movement spectrum. In other words, the wood will move, but only in one direction (contracting instead of further expanding). Across the Midwest, South, and East coast the decking is swollen with high humidity. All of those wood fibers are like a bundle of straws, and they have sucked up the moisture that is in the air, expanding the board width.

Each decking species will have different movement properties, but let’s use Ipe as an example. If we install a typical 1×6 Ipe decking board today using our weather conditions of 90% humidity and 4° while assuming the wood is 5.5″ wide, it will maintain that width through the heat and humidity; however, as we draw into the fall season, both the heat and humidity begin to drop. If we pick a day in early October and assume 50% humidity and 12°, then the Ipe decking will have shrunk approximately 3/16 of an inch. Now your starting 1/4″ decking gap is almost a half inch wide, and someone just got their heel stuck between the boards at that swanky dinner party.

Fast forward to the dead of winter, and the temperature is now 4° with the humidity around 25-30%. Your decking has now shrunk another couple of 16ths of an inch, and that half inch decking gap is closer to 5/8″. Now your Chihuahua just fell through the gap between the deck boards!

That widening gap will make any deck owner upset, but now let’s imagine the opposite scenario where the deck was installed with a 1/4″ gap in the dead of winter. By summer time those boards have expanded by more than 1/4″. In a perfect world we can assume even expansion in both direction, so each board would swell 1/8″ on both sides. If you are lucky, that original 1/4″ gap is gone. In reality, boards don’t always expand evenly, and it will vary depending on where the board was located within the original log. Usually the boards expand beyond that 1/4″ gap, and now the boards are buckling and possibly cracking since they have no place to go. This can result in serious damage and the need to replace decking as well as the need to soothe the anger of your customer.

Just to make things even more difficult, wood is anisotropic. This means that it moves differently in each direction, and this movement will vary based on how the wood was cut. Quartersawn boards barely move across the width whereas flatsawn boards will move the most (the examples above use flatsawn boards since this is most common). Now not only will the gap change, but it may not change consistently across the entire deck.

Choosing Deck Spacing Between Boards

What is the solution spacing Deck Boards in a Humid Area ? How to stop the madness? You can’t stop it. Wood moves, get over it. Embrace it and plan the gap between that decking accordingly.

In the above scenario where I installed the deck in the summer, I could install it with very little gap so that in the winter I would end up with a typical 1/4″ gap. You still want a little gap just to let rainwater drain through. With no drainage comes standing water and a whole new set of movement issues.

If the deck is installed in the winter, then you definitely need that 1/4″ gap and in the humid climates you might want to bump that out a 32nd or a 16th of an inch for insurance. So when your customer answers your “when” question and tells you they want a 1/4″ gap in May, then you need to run some numbers and figure out just how much that decking is going to expand and contract throughout the year.

Next time you begin a deck installation, take a moment to think about the potential humidity and temperature change throughout the year and what kind of sun and rain exposure the lumber will face. Wood movement percentages can be calculated with an app to help you determine just how much movement you could face.

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