Isolating Pole Barn Poles From Concrete Slabs

The fear factor – comes up again and again in construction. Today’s fear is a concrete slab being poured against the poles (columns) of an existing pole barn will cause the columns to decay.

“We have a 25 year old pole barn with 12 main 8×8 poles sunk 8 feet into the ground.

We’d like to pour a concrete slab under the entire building (as it is dirt now).

I was just going to pour the concrete right up to, and around, the poles, but some folks have me thinking that the concrete directly on the wood will create a lot of condensation and moisture and eventually rot/weaken the poles right at the floor.

I hear of people using styrofoam or expansion joint felt as a wrapping for their poles, but all of those discussions relate to avoiding cracks in the concrete from pole movement. My concern is for the poles themselves.

What is an appropriate barrier material to clad the bare poles in, and then pour concrete right up to, that will prevent moisture from collecting on the poles themselves?

Yes, we will be doing a vapor barrier under the concrete – but there will be 12 8×8 holes punched into that vapor barrier which could allow moisture up right into the part of the concrete pad that we don’t want it – the part touching the poles…

All comments and suggestions appreciated.”

Mike the Pole Barn Guru responds:

Properly pressure preservative treated wooden timbers are not only designed to be buried in the ground (with the proper level of pressure treatment being UC-4B), but also are required by Code in cases where wood is in contact with concrete.

So, how is it concrete would cause condensation against the pressure treated wood? Concrete retains a significant amount of interstitial moisture for many years.  This moisture routinely moves in and out of a vapor phase, depending on the temperature and relative humidity.  Placing anything which is reasonably “barrier like” to moisture (which wood is for the short term) can cause moisture to collect under the material in a liquid phase.  This is one reason the wood needs to be pressure treated.

Another reason for the wood to be pressure treated  is concrete is aggressively alkaline.  In the absence of pressure treating (usually acidic), the wood will dry out and its cell structure will be damaged from the exposure to the drying/alkaline condition.  

The reality is there is really no reason to isolate the pressure treated columns from the concrete slab.

Other than fear (False Expectations About Reality).

For more reading on longevity of pressure preservative treated lumber, please read: https://www.hansenpolebuildings.com/2017/12/will-poles-rot-off/.