Expert: Bruce E. Johnson - 10/26/2010

Question
HI: I am in Minnesota and want to build a slab home on grade. Do I have to dig a 6 foot deep foundation around the edge of the slab? I am going to build a 32X36 foot house. Or what is the code for this application? Thank You, Bill.

Answer
Hi Bill, there seem to be some "insulated shallow foundation" designs that have been accepted in Minnesota but from what I can see you still end up digging below the frost line.  Apparently there is concern that the ground, when frozen, may heave and disrupt a foundation that isn't deep enough.  Minnesota is divided across the middle the upper half of the state has a five foot frost line, the lower half has a 3.5 foot frost line so depending on your area you may get by with a shallower foundation than 6 feet.  But from what I can see, it looks like you will need to go deep with your foundation walls.  This is one of the reasons most northern homes have basements.  If you are going to have to dig six feet for your footings you might as well make the best of it and add a basement.  I find it odd though that the same building code that requires a deep foundation wall for the house allows a monolithic slab on grade for the garage.  Maybe they figure it's no big deal if the garage floor pushes up and heaves from deep freezing.  Anyhow, it looks like you will need to dig below the frost line for the foundation of your new home.  I hope this information helps, please feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com