Trying to get chickens lined up to do a 40x72 pole barn,heated & insulated. I will either be using the horizontal lumber on 16" or 24" centers. Found a local bandsaw guy that sells wood for close to 1/2 the cost of lumber yards. And it would be nice to get the full 2" thickness. Well a possible dilemma is my bottom cord if it is too close to the ground may rot eventually. If I used regular lumber yard wood it would be no problem since I could use pressure treated for the very bottom. But my wood will be an extra 1/2" thick. Any ideas on the best preservative for close to ground contact ? I have used alot of old motor oil in the past on shed projects when the wood is close to the ground or water splash up with excellent results. Thought about getting the bottom cord wood earlier before the project starts and start slapping the old oil to it. Thanks

Could you just double 5/4" deck lumber, that should be 2"?

X2

Tow points or one point and one question. The first is, put pressure treated lumber on the bottom where there is a chance of rot. Making the wood thicker is not going to solve rotting.
You are not going to be able to match oil or creosote or heating it to penetrate the wood to anything acceptable.

The question is, is this wood dry, kiln dry or what exactly are you getting. I am not asking for an answer, but wood is not all the same wood. Is this softwood or hardwood, that makes a difference as I ran into a batch of oak in a project and it literally was snapping star screws off. If the wood is not completely dry, you are going to have shrinkage, and that means you will have screws popping off, and that includes your metal sheeting.
I'm not saying that the local wood is not acceptable, but if it is green or has not completely cured, you are not saving anything.

I would also recommend the closer centers on lumber, for wind, ice or snow.

make sure code in your area will allow for un-graded wood.

Concrete footer and two layers of block. Then wood plate and studs......jk

I like the five quarter for a base plate idea you can always put a 2 x 4 on top of that

Are you planning to have poles in the ground or sitting on a concrete foundation. Lots of questions and work on this large project.

I was thinking double thickness 5/4 decking also. Originally was going with poles in the ground,then thought of footings and studs. My buddy who is a contractor said poles are a lot cheaper vs footings,frost wall and studs. Settling on sono-tubes to above grade and laminated 6x8 post with PT lumber on the bottom 4 feet or so.

How about metal roofing on the bottom 3’ with a flashing on top of it and then your 2” thick boards. You could put plywood under the metal roofing for strength. The metal roofing would withstand the weather and being lose to the ground

I am NOT someone to listen to on this, but the thought occurs to me...can you get black locust (Robinia pseudoacacia) for the plate?

Please understand, I know nothing about proper building techniques.

Dont Scrimp...you'll be sorry.

Yeah! Remember, those old barns built of rough sawn green lumber don't seem make much more than 100 years.

In other words. The green lumber, or air dried, from your sawmill guy is just fine, as long as you use the proper techniques for the materials you are using. I don't dare try to answer your actual question without knowing a whole lot more about the actual frame construction. Wood shrinks, bends, and twists as it dries, so, don't be mixing methods of dry and green construction.

There is actually a mill that has locust about 40-50 miles from me.

I don't think using hemlock that may be a little air dried and not shaky as skimping. Most of the lumber that the little local guy sells is pretty tight/small and very few knots. And using a full 2x12 at the top will hold quite abit on 8 foot post centers. My dads pol barn was built out of rough cut in the early 80's is still fine. Only difference is his bottom skirt is heavily creosoted and still good shape.