DEAR POLE BARN GURU: I have read much debate about setting the poles in concrete with no “footing” beneath them vs. setting the poles on a concrete footing and back filling with gravel and earth. Seems to be an issue of rot. What do you recommend? MYSTIFIED IN MAGNOIA
DEAR MYSTIFIED: It seems you and I have been reading the very same posts on advice boards all over the internet!
Let’s start with the “no footing” concept. Typically the soil is not able to resist applied vertical loads when those loads are transferred through the post alone. Therefore, the post is set on some type of a footing, which in the case of post-frame construction is usually concrete.
Footings must be adequate in area (radius squared x pi) to prevent the building from settling under not only the weight of the structure itself, but also the load from snow or minimum live load requirements. They must also be thick enough, to prevent the column from punching through the footing.
Considering even a fairly small span building, with minimum loads requires a footing diameter of 18 inches or more, it is fairly impractical to think of a material other than concrete which could be placed affordably beneath the columns.
On to the “rot” issue. Chapter 23 of the IBC (International Building Code) requires wood in contact with concrete to be either “naturally durable” or pressure-preservative treated. This, in itself, tends to take away credence from the “concrete rots treated wood” faction.
I know every major post frame building company in America. I have yet to have any of them report of having a single “properly treated post” rot off.
The clincher in this is “properly treated post”.
When the first IBC was published in 2000, Section 1805.7.1.2 stated, “Wood poles shall be treated in accordance with AWPA C2 or C4. This language remained the same in the 2003 IBC. The AWPA C2 and C4 standards have been withdrawn, therefore are no longer applicable or referenced standards in later editions of the Code.
In the 2006 IBC, however, things changed. Section 1807.7.1 states, “Wood poles shall be treated in accordance with AWPA U1 for sawn timber posts (Commodity Specification A, Use Category 4B)”. In the 2009 IBC (and repeated for the 2012 edition), the language remained the same, however the referenced section of the Code is now 1807.3.
What this means to you or the average consumer who is shopping for a new pole barn? Everything!
Take a visit to the local lumberyard or big box lumber store. Take a walk through the pressure treated lumber department. Every piece of pressure preservative treated lumber has a tag on it. This tag identified who the pressure treater was, as well as the level of pressure treating. Sadly, most of the pressure treated posts will be treated only to UC-4A…which does NOT meet with the Code requirements for use in pole buildings!
What this means is there are a plethora of buildings which have under treated columns, which is responsible for the rot issues, not the proximity to concrete.
Every time I have someone try to foist the “concrete rots treated wood” story on me, I ask them to show me a single laboratory test, which proves their point. None exist.
The best solution – and my recommendation – use a properly treated post, backfilled with pre-mix concrete in a monolithic pour. Place a minimum of six (and better eight) inches of concrete below the column and eight or more inches up the post (this is known as a bottom collar).