Expert: Ted Barnhill (Principal, Design45 LLC) - 11/9/2007

Question
We are planning to build a 16x20 cabin in northern MN (Gunflint Trail area).
The area has a lot of shield rock, but until we start digging it's hard to know
what we'll encounter, and at what depth. We're getting conflicting advice from
folks up there about footings. One fellow tells us that pier foundation (sono
tubes) are a bad idea because when we build above the ground on piers, we
are exposing the ground to colder temps since it will no longer be insulated
by snow in  the winter. He says that he's seen buildings where rocks heave up
out of the ground over time and punch through the floors. He recommends
an insulated slab with in-floor tube heating, saying that we need to keep the
in-floor heating system running all winter (with a small water heater or
something to pump warm water through the tubes).

We won't be using the cabin in the winter, and live 5 hours away so we can't
easily run up there to check on it. I am not wild about running any appliance
unattended all winter (we do have electricity, but it goes out frequently up
there). This is also really expensive for a simple little cabin. We've seen cabins
on pier foundations, and even just on stacked cinder block which presumably
isn't buried very deep (and certainly not to frost footing depth). What kind of
foundation do you recommend for this small structure?

Thanks for your advice!

Answer
Teresa,

You are building is a beautiful area - my family spends time each summer on the Gunflint Trail.  I wish you the best in your planning and building process.

There is nothing wrong with building on piers to frost up north.  While snow at times does insulate the ground, it is unpredictable over time and distance.  Some winters are very very cold with little snow cover.  The established frost depth accounts for the deepest frost is likely to penetrate, snow or no.  As for heaving rocks, you will leave at least 18" of space underneath your cabin - I find it hard to imagine a rock heaving that far!

Another possible foundation system is a shallow frost-protected foundation (see http://oikos.com/esb/43/foundations.html).  These can be used for heated or unheated buildings (but are built differently, depending).  If you'd like to consider at some point using the cabin as a year-round structure, you may want to consider this method.

Good luck with your cabin.