It’s a hotly debated part of wood deck building, so how do you know if you should gap pressure treated decking where you live?
Here’s the short answer
Yes, you should gap pressure treated decking. There are exceptions, but not many.
Here’s the deal with pressure-treated lumber:
- Pressure treated lumber is wet, upwards of 40% moisture content when it gets to retail and/or to your jobsite. It literally has no other physical option than to change size–shrink–as the moisture leaves it.
- All wet material shrinks.
Why Is Pressure Treated Lumber Wet?
The majority of pressure treated lumber is Southern Yellow Pine (SYP) trees because they are uniquely capable of accepting the chemical. While other species can be treated (Douglas Fir, Red Pine), this article is about SYP.
Water-Borne Chemical.
After milling, SYP boards put into enormous pressure cookers that force the chemicals (formulae include MCA, CA, ACQ) into the lumber at the microscopic level. It renders the material useless to the microorganisms that “rot” wood. This is why I always use Rated for Ground Contact deck boards. Those twigs, branches and leaves that fall in between your deck boards this year are a microbe factory next year.
Tip: Use a painter’s multi-tool to get that organic matter out of the crevices between the boards.
So after you screw down your deck board, the water evaporates, leaving only the chemical behind. It only takes about 3 weeks. The board shrinks across the grain leaving inevitable approximate ¼-inch gaps as it stabilizes.
It is a miracle of the Supply Chain.
But I Was Taught To Gap Them.
Go ahead. Just understand that however much you gap them, expect it to grow another ¼-inch. You’ll be able to see through the deck boards to the ground, usually just mud. It also serves no purpose. I call this look Railroad Ties.
What About Miters?
Wood decks are built by the bucket load with top caps on the guard rail mitered. And they can look great the day you install them. However, when wet lumber dries, it changes shape. No different for a deck board than a miter and you can jump up and down all day long yelling at me about glue and biscuits and BS, I don’t care. The miter will open at the bottom and pinch at the top.
