One of the major selling points for pressure-treated lumber is the fact that it’s not supposed to shrink or should maintain its size and shape for a period of 40 years. That’s a heck of a boast and, for all intents and purposes, it’s true. However, there’s no such thing as perfect either.
The fact is, pressure-treated wood will shrink across its width by about ¼” in a few days’ time at most. When you place your pressure-treated deck boards down over the joists, it is important that you pack them tight up against one another.
It may seem counterintuitive to do so, after all, you want to give the dirt and mud a space in which to fall, along with a place for water to wash away and drain. However, placing the boards tightly together like this will lead to such an accommodation.
As the boards shrink their ¼”, the gaps you need will open up between them. If you place your boards down and leave gaps in between them from the get-go, those small, evenly spaced gaps may turn into large—ridiculously so—gaps that are unsightly.
What Causes Pressure Treated Wood to Shrink?
Pressure-treated or fresh cut from the tree, wood remains wood and acts as such throughout the seasons. What pressure-treating does, is it reduces the drastic effects of time and climate on your boards.
Pressure-treated wood is already wet when it is delivered fresh, however, that doesn’t mean that it won’t dry out and become infused with water again at some other time. As the humidity rises, moisture finds its way between the grains of wood, pressing outward and expanding.
As moisture departs, the opposite happens. Over time, this cycle will occur again and again. With pressure-treated wood, the process is not as severe but that doesn’t mean that it doesn’t occur at all.
The initial shrinkage from pressure-treated wood is the moisture that comes from the treating process departing the wood as it is vaporized. That’s why it is so important to construct your deck with your boards flush up against one another.
If you live in a place like Florida, where you never have to concern yourself much over whether or not the humidity will stay or go, you won’t have to worry about too much shrinking and expanding. It will happen, just not to the same degree.
Do Pressure Treated Boards Warp?
Shrinking is something that you can actively plan for, especially when it comes to how you place your deck boards down over the joists. Warping, however, is different altogether. Warping can occur on either end of the board and it can even twist the middle, turning your deck board into a sort of corkscrew.
There are two things to look for if you want to minimize the possibility of warping before or after you install your boards. Check to see if the lumber has been Kiln Dried After Treatment (KDAT) and you want your lumber to be Grade 1 pressure-treated.
With the cost of lumber rising over the last year, it’s difficult to spend that extra money, however, it can make all of the difference when it comes down to how your deck looks and even general safety when walking on it.
When you are building your deck, it’s important to securely lock the wood down. You want to use strong galvanized steel throughout, with your main concerns focused on the corners, where the wood can curve upwards, becoming a permanent tripping hazard.
You also want to avoid staining it until the wood is officially “dry enough”, which means it is not completely dry (which you want to avoid) but it’s dry enough that the wood will absorb, rather than repel, moisture.
You’ll know this by lightly spraying some water on the deck to see if it beads up rather than absorbs. Once it absorbs, it’s time to clean and stain your deck before it becomes too dry.
How to Deal with Shrinking, Expanding, and Warping
When it comes to warping, follow the above advice, and get your boards installed quickly. Even if they have dried out some, be sure to use galvanized steel to lock them down so any remaining moisture that’s present in the wood won’t rust the screws.
Dry Month Installation
As aforementioned, living in a more tropical climate keeps the humidity around for much longer than you would find in a more arid environment. However, there are dryer months, such as when the winter air cools everything down.
If you place your boards in less humid months, especially if your boards are already dry, you don’t have to be as steadfast about bunching your deck boards up, flush against each other. Leave about a ⅛” gap so that there is room to expand when humidity and/or the rainy season arrives.
If you’re pressure-treated boards are have not lost their moisture, then you need to install them flush, regardless of how dry or humid it is outside. So much of your decision-making during less humid times revolves around the condition of your pressure-treated lumber.
Humid Installation
Pressure-treated boards should go in with a ⅛” gap if they are relatively dry because the humidity is going to swell them up a bit over time. If they are wet, stick with placing them flush up against one another because at some point, things will have to dry out and they will shrink as they lose their moisture.
As talked about above, you will want to use galvanized steel as your choice when it comes to screws, brackets, and fasteners as they won’t rust away on you when they are exposed both to the outside moisture and the moisture within the boards.
Final Thoughts
Pressure-treated deck boards will shrink about ¼” in width, however, that simply means that if you’re prepared and lay down your planks flush against one another, you will have even gaps throughout and the deck won’t look like a disaster.
Pressure-treated wood can be finicky from time to time, so be sure to catch it in the moment that it’s not too dry and not too moist, then you can stain it and have a long-lasting, warp, and shrink-free deck.
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