Why Good Decks Go Bad - Part 1 - Spacing Between Boards

"I've been installing decks for over 20 years. I don't need to read your instructions." Red faced and fuming, Joe was really pissed off when I told him you can't install a hardwood deck 4 inches off the ground with no spacing between boards. I thought he was going to throw a punch, but he was really just massively embarrassed. A decent general contractor with lots of experience, he just "forgot" about the nuances of installing wood decking after years of installing PVC's and composites.

Tangential and Radial Shrinkage in Wood

Deck spacing between boards is absolutely critical when it comes to installing wood decking. Let's start with the quick technical background. With changes in moisture content, wood swells and shrinks in thickness and width only, not significantly in length. You can always butt your boards tight without worrying about them swelling or shrinking in length. When you look at the end of tree you see the concentric growth rings – radially, from the center (or pith) out, the wood moves only half as much as it does tangentially to the growth rings. This difference between radial and tangential shrinkage is critical to understand and it varies significantly between the different species of wood.

The Wood Handbook, Wood as an Engineering Material, USDA, Forest Products Laboratory, 2010
https://www.fpl.fs.usda.gov/documnts/fplgtr/fpl_gtr190.pdf

Why Do We Recommend ¼" Spacing?

And now, let's get back to the business of deck board spacing. Wood shrinks linearly in relation to the moisture content from the fiber saturation point all the way down to bone dry, aka over dry. With typical movement of 6-10% in the tangential direction and 3-5% in the radial direction, we are talking about total movement on a 5-1/2" board from monsoon wet to desert dry of 0.55" maximum! That's a lot!! Of course, these are the extremes.

Courtesy of Lignomat USA, manufacturers of outstanding moisture detection equipment

In real life (IRL), we should expect typical humidity changes from 20% to 80%, depending on the time of the year. Based on the EMC (Equilibrium Moisture Content) Table, this works out to a moisture content range of 5% to 15%. With linear movement and fiber saturation at 25% MC (Moisture Content). Now the math part: 10% divided by 25% = 40%, now multiplied by 0.55" – we now have a worst case movement IRL of about 0.22", or just under ¼". This is absolutely consistent with what we see in the real world. Decks with ¼" spacing will occasionally close up to almost no gap at all in the middle of the Winter – and you probably already knew that meant there were lots of flat sawn boards in the deck. With pure quartersawn material, the swelling would be about half that much, just under 1/8".

Maintaining Your Deck – Yet Another Reason for ¼" Gaps

You've got to have a way to clean between boards. The leaves, needles, tree droppings, and dirt that will undoubtedly fall on your deck must have a way to fall through the gaps and be washed or blown away periodically. These ¼" gaps allow for debris to fall through the gaps during most times of the year.

The Fastening System Matters

Some fastening systems like the Marksman Pro by CAMO, make it extremely difficult to ensure proper and consistent spacing. The tool claims to have the spacing tool built in, but the lack of grip and depth of the tool directly contradicts those claims. The difficult to use tool leads to uneven and improper spacing of the deck boards.

Improper or inconsistent deck board spacing can lead to debris build up, rot, cupping, cracking, buckling, and damage not only to your deck boards but to your deck structure as well.

ExoDek QuickClip – Guaranteed ¼" Spacing

The easy-to-use ExoDek QuickClip has built in ¼" spacing – which guarantees your spacing will stay consistent during installation AND after the wood expands and contracts.

If you don't want to see your good deck go bad, stop using fastening systems that aren't designed to protect your deck. Use a system that was carefully engineered to ensure the longevity and integrity of your deck. The ExoDek QuickClip.

Now that he's using the ExoDek QuickClip, Joe doesn't have any more issues with deck board spacing! Happiness has returned to our friendly contractor.

By Steve Getsiv, 08/24/23

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