Structures: Footings, Foundations, and Frost

I’m not going to lie. This one is pretty technical (not math wise), just technical dry. But if you’re really interested in learning these things, you’ll enjoy the knowledge.

Frost, is the reason why we have basements here in Canada and Northern United States. When building homes in these area, we have to take frost into account and build below what is known as the frost-line. The frost-line is how far down frost penetrates the ground; which is most commonly about 4′ down.

Basement is a term that we use as one of the common foundations configurations usually 6′ or deeper. In addition to basements there are crawlspaces, which you can think of as just short basements. Crawlspaces are used in holes where only organic material has been removed or the need to get to the frost dept. Crawlspace construction is virtually identical to building basements. The third common is the slab-on-grade. Slab-on-grade is basically a concrete floor poured right at the grade level; it is sometimes supported by perimeter foundation walls, piers or piles and grade beams, or grade beams directly on isolated footings. These foundations usually serve as the floor of the building and support for the dead and live loads.

Here are the 3 different types of slab-on-grade configuration:

Images courtesy of Carson Dunlop Horizon


Now, let’s get to how foundations are built. Something I’m intimately familiar with thanks to Dalma Forming Ltd. First they dig a hole (you know who they are, don’t you?). Then we have the engineers come in and place corner pins. Once the pins are in a worker will run the exterior wall line (it’s a string, tied from each pin to each pin in a big circle). This line is important; once we have this line we can start to form the footings from 2×6 or 2×8, and in many cases custom form sizes. The sizes of the footings range from 16″-many feet. Now, back to that line. To get the width of the footing we take, for example, 24″ and split it into 3 sections. One section is going to be the wall we’re going to build – that’s 8″, 24-8=16. That means there will be 8″ to the outside, and 16″ to the inside of the line. Here’s some of my artistry to help:

Ok, so what we’re looking at is a rough sketch of how we figure out where on the footing the wall will be. The orange line and red pins are the exterior wall side and the exterior corner pins left behind by our fearless engineer. So, 8″ over to the right (in this case) is the interior wall side in the light grey. The green line in the center is where the concrete setter will imprint a triangular line for a more water tight build and for the earth quake protection.


Now that’s only one type of footing (called strip or spread footings – when I was building them we always called them strip footings, so spread might be Americanese).

There are also piles and pad footings. Pad footings are really just small strip footings set down to support those metal poles you have in your basement that keeps you from making the perfect entertainment room – but you forgive it because it’s holding up your house… you haven’t forgiven it yet, have you?

Piles are used instead of pour foundations in areas where the soil is really bad. Piles are driven down to a point where the bear on bedrock or other sound substrate. They can also be driving in to soil far enough that the friction of the soil is enough to hold up the home. I’m not a particular fan of putting something that will rot in the ground to hold up my house (personal opinion).

We’ve also got pier foundations, brick foundations, block foundations, stone foundations, raft, mat, and preserved wood foundations. It’s a shame I don’t have any musical inclination, I could see this as a musical.

Now, I’m sure you noticed the last one there, wood foundations. I have one thing to say about wood foundations: “<sigh> No.” My professional opinion is that it is wood, it is in your basement (usually humid basement); it may be protected from water, but eventually it will rot. They only really work in warm dry areas of North America, not here in the Greater Ottawa Area. My personal opinion starts with a hearty laugh … laughing … laughing … still laughing… and a serious, definitive, end of conversation: “No.”

Not only that, more often than not, wood foundations are more often finished basements (top/bottom) and can be 100% concealed so no real inspection can be made of the foundation.

And that’s all I’m going to say about that.