Pressure-Treated Wood (Should It Be Sealed?)


I can’t stress enough the importance of sealing pressure-treated wood decks. Despite being treated and designed for withstanding weather and decay, pressure-treated wood should be sealed regularly (once a year if possible). 

Pressure-treated wood is designed to be used outdoors, but for it to maintain its strength and beauty will need to be maintained. One of the key parts of maintaining pressure-treated wood is sealing it.

Sealing the wood helps to maintain a low water content preventing rotting of the wood. Sealed wood will crack and warp less with less expansion and contraction.  

So let’s go through the reasons to seal pressure-treated wood decks, as well as how often I seal it and how to do so.

What Is Pressure Treated Wood?

First, what is pressure-treated woodOpens in a new tab.?

Wood that has been treated to prevent fungi growth and rot of the wood.  Some of the most common outdoor wood treatments are.

  • MCA (micronized copper azole)
  • CA (copper azole) 
  • ACQ (alkaline copper quaternary) 
  • CCA (chromated copper arsenate) 

Source: US Forest Products LaboratoryOpens in a new tab.

There are different grades of pressure-treated wood – 12 of them to be exact. And these different levels of pressure treatment are for differing intensities of environments and building purposes.

It is best to build with a “ground contact” pressure-treated wood for a low backyard deck.Opens in a new tab. With a higher level of treatment, it will last longer when in contact with dirt. But this extra treatment can be advantageous even with a raised deck.

Why Pressure-Treated Wood Should Be Sealed?

There are plenty of reasons to seal pressure-treated wood, the most important being protection against rot. Although many pressure-sealed lumber grades have qualities to keep termites and fungus away, moisture can still penetrate the wood over time or due to cracks in the surfaces.

The more the wood cracks and warps, the more the overall surface area is exposed to water and other elements. When this happens, rotting can actually start and speed up in no time.

Another key reason to seal pressure-treated wood is that you give the surface extra protection from sun bleaching and UV rays. This is more of an aesthetic benefit, but it’s important to most deck owners nonetheless. 

Third, sealing pressure-treated wood is a smart investment. By sealing my deck regularly, you may save money in the long run since regular maintenance lowers the risk of needing to rebuild the deck entirely. Stain preventing rot and decay is less expensive than buying new wood and materials and starting from scratch. 

When To Seal Pressure Treated Wood?

The first thing to note is that don’t attempt to seal pressure-treated wood until the wood is fully dry. Pressure-treated wood often needs several months to dry outside, exposed to the elements, before it can get sealed.

If you seal your deck with an oil- or water-based stain immediately after building it, it may actually do more potential harm to the wood by trapping in excess moisture. 

After initial sealing, a pressure-treated wood deck should be reseal roughly every year. Annual sealing is the most effective to protect the wood from rot and mildew.

If the wood was previously painted, you are limited to what you can do. Paint will either have to be 100% stripped, which is very hard to achieve. If you don’t want to strip the wood completely, it would be better to repaint. Even if paint is not the ideal finish of a wood deck. Just make sure the paint has a high rating for exterior use, ideally for decks. Designed for a higher level of wear.

Brush on the sealant on a sunny day with no forecast for rain.  Before starting, confirm that the wood is fully dry. I like to test whether or not the deck is ready to seal by dropping water on its surface and seeing if the wood soaks it up. If it does, it’s dry enough to seal. If it doesn’t, wait until it’s fully dried. 

Ryan Nickel

A Red Seal carpenter, passionate about building decks to be enjoyed.

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