[Shop-talk] Pole barn and deck footers

[Briggs John F]

Brian,
   
  It kinda depends on where in Michigan.  Building Code in Southern MI ( I'm in Livingston County) requires the the top of footings for pole building and deck footings to be at least 42" below final grade.  Northern MI would be deeper.  You can look up the normal frost line in your area on the internet.  The top of your footing can be at that level, but your county probably has a minimum depth.  You can make it less than that, but you should then expect things to move up and down with the frost, break,  and in some case to eventually "pop out of the ground".  A slab in Michigan requires a 36" (it might be 24" or so - it's been a while) deep "Rat Wall" below final grade.  The "rat wall" is to keep the moles, rats and skunks from burrowing under it and causing problems for your floor.  My pole barn (24' x 40' 2 Story) has the 42" deep footings for the poles and the 36" rat wall. The 5" to 6" floor is in 4 quadrants with welded wire mesh and is poured over a 2 foot tamped sand
 base.  There are three centerline poles to support the second floor.  I originally planned to do automotive work in it, but have since turned to building a few small boats.  Today I would put in a 3" floor with wire mesh or a wood floor. 
  I bought a one person gas powered auger and bored 48" to 50" deep 8" dia. holes below final grade and put in concrete to make the top of the footing at the 42".   The 4" x 6" poles went in after that hardened, dirt was tamped around the poles so that the bottom of the poles didn't move around and they were stablized with framing,  The rat wall and floor were poured on top of tamped sand as a single pour.  That fills in around the poles and everything is solid.  My wife and I built all of the barn (with some help from friends) except the concrete floor work that we had professionaly done.
  The only problem I have is because the service door is next to one of the poles and the floor moves up slightly in the winter due to frost causing the door to jam.  I may move the door so that both sides are on 2" x 4"s supported from the floor and that should solve the problem.
   
  If I were building the "same" barn today, I would pour an exterior wall foundation (48" deep x 16 or 18" wide) with interior pilings and install a suspended wood beam floor on the pilings.  The whole barn would be stick built rather than a pole barn.  It would have been a lot easier than dealing with the poles when they started to twist as the whole structure aged.  It's about 8 years old now.  In my area it is almost impossible to get 4 x 6 poles that are not twisted.  The wood floor would have turned out to be about the same cost as the concrete floor over sand and it would better suit my needs today for nailing braces for boat forms.  It would also be a heck of a lot easier on the feet.  

  Just my thoughts and how I did it.  Good luck with it.
  
John