What Causes Pressure Treated Wood to Rot Out?

One of the main reasons why pressure treated wood is used in areas of construction that may be more susceptible to moisture exposure than others, is that it is much more resistant to rot than other wood used to build things, whether on the framing side or the finish side.

Even this being the case, we field calls from perplexed homeowners here and there where someone has had pressure treated wood rot out on them and they do not have the foggiest idea as to why.

Their confusion is certainly well-founded.

After all, if pressure treated wood is viewed as a rot resistant building material, then how the heck can it rot out??

Great question!

Pressure treated wood is a type of wood that has gone through a process where high pressure has been used to inject a preservative into the cell structure of the wood.

These preservatives can vary (some even make the wood fire retardant!) but their whole intention is to disallow decay, insect damage, mold, and water damage as best as possible.

All wood is subject to rot when certain types of fungi are able to penetrate the wood and feast on it over time, as the fungi enjoy their meal, the wood gradually breaks down, softens, and rots over time.

So, the question remains, if pressure treated wood is made the way that it is with the intention of preventing this type of thing from happening, then why does it happen?

The simple answer is that not all pressure treated wood is created the equal.

There are different grades of pressure treated wood.

Pressure treated wood is marketed with the lumber grades Premium, Select, Number 1, Number 2, and Number 3.

The higher the grade, the less challenges – including potential rot – you are likely to have down the line.

Lumber is tagged or stamped to provide a variety of info, including its grade.

Though these markings can seem like a foreign language, the information will be readily available as to what grade of pressure treated wood one is purchasing.

If you have a project which is requiring lumber to have some type of ground contact or will have long term exposure to moisture (such as the flooring on a deck), the commonsense thing to do would be to utilize as high a premium grade as possible whose rating is such which notes that it should be the type of wood utilized in such a situation.

One of the biggest challenges is that the more highly rated types of pressure treated wood may not be available at every place one shops for wood and that most folks are unaware that there are varying grades of pressure treated wood to begin with.

I often find local lumberyards to be a tremendous source of knowledge and supply for these very types of situations.

If you ever have a question of whether a piece of pressure treated wood was a high enough grade of wood and rated for ground contact, the reality of the situation is your chances of coming across a knowledgeable professional who can honestly help you out are much higher at the local lumberyard than at one of the box stores.

No matter where you purchase your pressure treated wood, the secret is in the grade of the wood.

Although you may certainly find pressure treated wood that seems “good”, choosing the wrong type may actually lead to an unpleasant discovery somewhere down the road and finding yourself on the phone sounding baffled as can be,

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