Mind the Gap: Decking Tips

Most decking is installed when temperatures are at their high point for the year, often close to 90 degrees. If that’s the case for your decking installation, the good news is that you’re at the extreme end of the movement spectrum; from there, you can count on the fact that your decking lumber will move in only one direction. It won’t further expand; it will only contract, since the decking boards are currently swollen with humidity. When it comes to gap size, this scenario makes whatever gap size you choose now the smallest it will be. (Of course, if you happen to live in an area other than the East Coast, Midwest, or South, perhaps this may not be true for you.)

Since Ipe is by far our top-selling decking species, let’s consider a 1x6 Ipe board, as a case in point. Let’s assume that with humidity levels around 90% and temperatures topping 90 degrees, the board actually measures 5.5 inches wide right now, and the gap between boards is ¼ inch. As summer draws to a close, and autumn brings lower temperatures and humidity, shrinkage will occur. By the time October hits, humidity might be closer to 50% and temperatures hovering around 65 degrees. That same Ipe decking board will have shrunk by about 3/16 of an inch. The once ¼-inch gap is now closer to a ½-inch wide. Since outdoor parties might still occur in October, that ½-inch gap might be a bit much — especially if your client likes to host formal affairs where women wear high heels.

That same 1x6 Ipe decking board will continue to shrink as winter months arrive and temperatures fall below freezing. When it’s 25 degrees and humidity levels are around 25%, the decking board will have shrunk another 1/8 inch or so, making the gap measure closer to 5/8 inch. That’s really a bit wide: you could lose a cell phone or your keys through that size gap! We’re pretty sure that your customer who was pleased with the 1/4-inch gap during the summer won’t be pleased with a 5/8-inch gap during the winter.

But there’s an even worse potential scenario that a poorly planned gap can present. Let’s imagine that the same deck was installed, but this time the installation occurred during the winter and with a ¼-inch gap. By summertime, the boards would have expanded more than an 1/8 inch on either side, which would close the gap. (Of course, boards don’t always expand perfectly symmetrically, but that’s beside the point.) The boards would have no place to go as they continued to expand, making buckling and cracking the sad result. The damage caused would likely require replacing the deck.

Now, the example above was assuming that the boards were flatsawn boards, in which the greatest degree of movement is tangential, or across the width; wood movement also varies according to cut. For instance, quartersawn boards don’t move as significantly across the width. If your deck is comprised of a combination, the gap between some boards may change more drastically than the gap between others.

Continue reading with Part 4.