Expert: Daniel Humphrey - 2/11/2009

Question
I want to build a cabin on our lake lot (northern MN) with a concrete slab foundation. We are on a tight budget. How thick should it be and what do I need for material under the slab? Thanks.

Answer
Dear Chrissy,

The minimum slab for a residence is considered to be 3-1/2 inches thick--the depth of a 2x4--with reinforced, thickened edges of at least 6". Since you are in a frosty environment, the simple slab is not generally the way to go, since the edges will be prone to frost heave, and some building codes will require you to dig down below the frost level for footers and a stem wall, which will support your slab.

To help prevent frost heave, especially with a simple slab, lay down a base of rock about a foot deep and larger than the footprint by four feet on all sides, then sand or gravel compacted and leveled out, then a layer of plastic for moisture barrier, then the slab. Sometimes, a layer of foam insulation is placed directly under the slab (trapping ground warmth) so that the floor will not heave when the heat is turned off. Foam also keeps the floor a bit warmer when the interior is being heated.

Done correctly in a cold environment, a slab is not the most economical floor. I think the quickest and most economical cabin foundation is a sturdy, insulated wood floor set atop pilings, 24" above the ground.

Daniel