Some woods are naturally rot resistant---like black locust and the heart wood of redwood and cedar. Other more readily available woods can be made rot resistant by treating them with creosote---as has historically been done with telephone poles and railroad ties. Part of the ambiance of fooling around railroad tracks in the summer is the smell of creosote.
We also treat wood with copper in various chemical solutions.
Both of these approaches have their limitations over time depending on how far the treatment goes into the wood. Railroad ties have to routinely be replaced because of the decay that happens in the center of the ties. These ties often get re-sold as landscaping timbers where the decay and damage from wood destroying insects can continue until there is virtually nothing left except the exterior shell of the timbers.
At an inspection the other day, I found this retaining wall made of short sections of creosote treated telephone poles.
One can easily spot the deterioration of the center of the poles while the outer surface remains in good shape where the creosote was able to more fully penetrate.
If one goes to the big box stores or any lumber yard, one can buy what is known as “ground contact” pressure treated lumber. One would think that in buying this stuff you should be able to put it in: “contact with the ground.” In my opinion this is an egregious example of either “false advertising” or at the very least not telling the whole truth. The reality is that most of these “ground contact grade” pressure treated woods WILL decay over time---not the outer treated part---but the center of the wood where the treatment process could not get to.
Now there are grades of pressure treated wood where the treatment is “required” to fully penetrate the entire piece of wood. These “foundation- grade” pressure treated woods should not be confused with the readily available materials found at the big box stores.
As you can see in the following picture, this “ground contact” pressure treated 6x6 has completely rotted away at the center core leaving a very well defined area that actually got “treated”----the rest is compost.
These ground contact pressure treated materials actually hold up very well long term when they can dry out. But when buried in wet conditions---like when used for retaining walls---they will decay and or become infested with wood destroying insects.
Charles Buell, Real Estate Inspections in Seattle.
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WA State, Home Inspector Advisory Licensing Board