Michigan frost depth: the 42-inch footing rule and what frost heave does

The rule, in the words of the code

The model residential code sets footing depth with a minimum of 12 inches below the undisturbed ground surface. Michigan replaces that number. Rule R 408.30522 amends the section so that all exterior footings and foundation systems extend 42 inches below actual grade, and R 408.30516 lists a frostline depth of 42 inches in Table R301.2(1). The same rule lets the building official modify the depth on evidence of freezing degree days, soil type, ground water conditions, snow depth experience, exposure or other specific conditions. So 42 inches is the default, and your city’s building official may accept something else for good reason.

Michigan is on the 2015 residential code as LARA lists it, which our code status guide explains. The 42-inch rule is written against that edition. Do not take a rule number on a site that shows a 2025 amendment history as proof the 2021 code applies; LARA says it does not, for now.

Why a footing goes below frost

Water in soil freezes and expands, and ground that freezes under or beside a footing can lift it. The base code answers in R403.1.4.1: foundation walls, piers and other permanent supports must be protected from frost by extending below the frost line, by a frost-protected shallow foundation design, or by being set on solid rock. It adds that footings shall not bear on frozen soil unless the frozen condition is permanent. That second sentence is why a contractor should not pour new footing on ground that froze overnight, and why an addition poured in January needs a plan for keeping the subgrade from freezing.

The code’s soil table gives a sense of which soils heave. Table R405.1 groups soils by the Unified Soil Classification: Group I soils such as clean gravels and sands drain well and change little in volume, while Group III, which includes fat clays and elastic silts, drains poorly and has high volume-change potential, with medium frost-heave potential for the clays and high for the silts. ML silts have high frost-heave potential. It is a code table, not a Michigan soil map, and it tells you why a clay-and-silt site needs more care than a gravel one without telling you what is under your house.

What 42 inches means for piers, underpinning and additions

The following is our interpretation of the code as applied to repair work, and not a statement by LARA. R403 is written for new footings. New footings added in a repair, such as the footing under an underpinned wall, a pier foundation for a porch, a deck footing or an addition, are new footings in practice, so a reasonable building official would expect them at 42 inches or ask the designer to justify another depth. The state does not say that, so ask yours.

Our piering and underpinning page explains what a pier does. Two limits apply. We can cite no government or extension source for push or helical pier design, so we describe none as proven, and none for how deep a pier should go in any particular soil. That question belongs to an engineer who has looked at your site. State law does not require a licensed engineer for planning and design of a detached one- or two-family home up to 3,500 square feet, but a city may still ask for engineered drawings, so find out before you hire.

If you dig for access, call MISS DIG first. The statute, MCL 460.725, requires notice at least 72 hours, excluding non-business hours, and not more than 14 calendar days before excavating; MISS DIG describes it as three business days. We could not confirm that on the statute text itself, so confirm the timing on the MISS DIG site.

What 42 inches does not mean

It is not a water line depth. We have seen it quoted that way, and the rule does not say it. It is also not proof your house has footings that deep. Your house was built under whatever code or custom applied when it went up, and we found no source that says what depth footings were set to in any Michigan era, so we do not guess by the year built. The Census says how many homes in your city predate 1980; our city pages give that number and nothing more. The only way to know your footing depth is to expose one or ask an engineer to read the records and the site.

What we can say about Michigan soils, and what we cannot

Regional context exists. The USGS ground-water atlas for Oakland County says former proglacial lake beds are evident in flat-lying, clay-rich sediments in southeastern Oakland County, with much lower permeability than outwash sediments. A USGS summary for the four-state area describes glacial till as unsorted clay, silt, sand, gravel and boulders with minimal permeability, 50 to 400 feet thick over most of the area. EPA ecoregion mapping places Detroit and several nearby cities in the Maumee Lake Plain, described as a glacial lake plain with clayey lake deposits and poorly drained soils; we ran that point-in-polygon check ourselves on a 2007 draft, so treat it as regional color. None of this says anything about your lot, where fill, a buried stream or a filled pit may dominate.

Three things we did not find, and so do not claim. We can cite no Michigan expansive-soil map; the USGS swelling-clays map exists as an image and we took no Michigan classification from it. There is no city water-table depth from EGLE, USGS or MSU. And no MSU Extension page on wet basements or crawl spaces could be read. The code defines expansive soil only through laboratory tests, so a claim that your clay is expansive needs a test, not a zip code.

Where to look for your own soil

The USDA Natural Resources Conservation Service runs the Web Soil Survey, which it calls the single authoritative source of soil survey information. Draw an area around your address, and it returns a soil map and descriptions. NRCS says onsite investigation is needed in some cases, including certain engineering applications, which is the honest limit: a survey tells you what is mapped, and a boring or an exposed footing tells you what is there.

How this should shape a quote

A contractor who proposes new footings or piers should be able to say what depth they plan, why, and who designed it. Ask who designed it, an engineer or the crew. Ask whether the city requires an inspection before concrete is placed; the base code’s foundation inspection is made after trenches are excavated and forms and reinforcing are in place and before concrete, though that text is written for new foundations. Ask for the permit in the contract. Our cost guide covers comparing quotes, and our engineer guide covers getting a written read from someone who does not sell the repair. We are paid by foundation companies, which is why we send you there first. The crack guide helps if your question began with a crack.