Every year we get called to look at an addition somebody else built, usually in spring, usually because a crack has opened at the joint between the new work and the old house. The homeowner wants to talk about drywall. The problem is almost never drywall. It is four feet below grade, and it has been there since the day the concrete was poured.
An addition is a second building standing next to your house. It has its own weight, and it needs its own way of getting that weight into the ground. If the two structures rest on footings at different depths or on soil with different bearing capacity, they move differently through the freeze and thaw cycle, and the joint between them is where that difference shows.
Frost depth is not a suggestion
Water in soil expands when it freezes. If the bottom of a footing sits within the zone where the ground freezes, that expansion lifts the footing, and the structure above it, by a surprising amount. When the ground thaws, it comes back down, but not always to the same place. Repeat that a few winters and you have differential settlement.
In Bozeman we build to a minimum of 36 inches to the bottom of the footing and we go to 42 inches on exposed benches, in Bridger Canyon and at elevation. That is not us being conservative for its own sake. It is the number that keeps the footing below the zone that freezes in a hard Montana winter.
There is an engineered alternative called a frost-protected shallow foundation, which uses horizontal rigid insulation wings to keep the soil under a shallow footing above freezing. It works well on heated slab-on-grade structures, it is in the code, and it is genuinely useful on some additions. But it is a designed system with specific insulation extents, not an excuse to stop digging at two feet.
Footing size is about soil, not about habit
The depth question gets asked. The width question usually does not, and it matters just as much. A footing spreads a concentrated load across enough soil area that the soil can carry it without compressing.
The code gives presumptive load-bearing values for different soil types. Silty clay and clayey silt, which describes a great deal of the Gallatin Valley, comes in at 1,500 pounds per square foot. Sandy or silty gravel is 2,000. Well-graded gravel is 3,000. Solid rock is a different conversation entirely.
| Point load at the post | 1,500 psf soil | 2,000 psf soil | 3,000 psf soil |
|---|---|---|---|
| 8,000 lb | 24 x 24 in | 24 x 24 in | 20 x 20 in |
| 14,000 lb | 36 x 36 in | 32 x 32 in | 26 x 26 in |
| 22,000 lb | 46 x 46 in | 40 x 40 in | 33 x 33 in |
| 30,000 lb | 54 x 54 in | 47 x 47 in | 38 x 38 in |
Scroll the table sideways to see every column.
Indicative square pad sizes for the required bearing area. Real footing design accounts for thickness, reinforcement and eccentricity, and needs an engineer's stamp.
Look at the spread across that table. The same 22,000 pound point load needs a pad nearly two feet wider on each side in weak soil than in good gravel. That is why a soils report, which costs a fraction of a foundation, is money well spent on any addition carrying significant point loads.
The load path is the actual subject
Footings are the last link in a chain, and the chain is what we draw first. Snow and roof dead load land on rafters or trusses. Those bear on a ridge beam or on exterior walls. Wall loads collect over openings into headers. Headers deliver a concentrated load into trimmer studs or posts at each end. Posts have to be continuous, floor by floor, down to something that can take a point load. And that something is a footing.
The most common failure we find is not a small footing. It is a post that lands on nothing. A beam ends over a floor joist, the joist flexes, and the load quietly redistributes into framing that was never designed for it. In a basement you can usually see it: a temporary jack post someone added, or a sag in the joist directly below a beam end.
Two more details that get skipped
- A capillary break between the footing and the stem wall, so ground moisture does not wick up into the framing above
- A perimeter drain at the footing, bedded in washed stone and taken to daylight or a sump, not simply buried in backfill
- Damp-proofing or waterproofing on the outside face of any below-grade wall, applied before backfill rather than after
- Backfill placed in lifts and compacted, because a trench that settles pulls the grade toward the house and sends water where you do not want it
None of this is exotic. It is standard practice that gets skipped when a schedule is tight or a price has been cut, and it is invisible the moment the trench is filled in. Which is exactly why it is worth photographing. On our projects, every footing gets photographed with a tape in the trench showing the depth, and the photograph goes into the client's folder before the concrete truck arrives.
