Some woods are naturally rot resistant—like black locust and old growth 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 a while back, 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 one 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 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.
This is especially true if the wood “checks” deep enough that water entering the crack can get to the untreated core. This is common on beams and support posts than are more than 1-1/2″ thick.
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. Some species of wood (like Yellow Pine) are much better at accepting treatment than are any of our West Coast species.
As you can see in the following picture, this “ground contact” pressure treated 6×10 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.
If you have uses that will require the materials be exposed to wet conditions continually, foundation grade materials must be used.
It is best practice to not assume the regular ground contact pressure treated lumber used in your deck will last forever. Routine inspection is recommended and just because structural members look good on the outside does not mean they are OK on the inside. As the fungal growth on the side of this deck beam indicates, the center of the beam is rotten just like the picture above.
By Charles Buell, Real Estate Inspections in Seattle
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