A buyer under contract on a ranch-style home in Longmont got her inspection report back and called her agent in a mild panic. The report flagged hairline cracking in the basement slab and a radon reading well above the federal action level. She wanted to know what was wrong with this particular house.
Nothing was wrong with this particular house. That is the finding that surprises Longmont buyers more than any other: the crack and the radon reading are not evidence that something went sideways behind these four walls. They are close to the default condition of a Front Range property, and neither one can be predicted by which subdivision you picked or what your neighbor's inspection said last year.
The Ground Longmont Sits On
Longmont sits on the same expansive, clay-rich soil that runs through Louisville, Lafayette, and Erie across the flatter, eastern portion of Boulder County, the section of the county built out in typical Front Range subdivision construction rather than the mountain terrain to the west, according to a Boulder-area home inspection guide. That clay is largely bentonite, a mineral that absorbs water and swells when wet, then contracts and pulls away from itself when it dries out. Longmont's freeze-thaw cycles add a second layer of movement on top of that seasonal swing.
The practical result: foundation contractors serving Longmont report that homes here often need some form of foundation work within 10 to 15 years, driven by that repeating swell-shrink cycle rather than by construction defects or aging materials, according to Groundworks, which has worked in the area since 1999. Some Boulder County subdivisions built in the 1990s and 2000s carry known soil-related construction patterns that only became visible years after the homes were finished, per the same inspection guide. None of that is unique to one street or one builder. It is regional geology working on a predictable timeline.
That does not mean every crack matters equally. Inspectors and structural engineers look for pattern, not presence: cracks wider than an eighth of an inch, horizontal cracking, or a stair-step pattern in block or brick foundations point to real differential settlement, while hairline vertical cracks from ordinary curing are common and usually cosmetic.
Radon Doesn't Read a Zip Code
The same logic applies to radon, and this is where Boulder County's own guidance cuts against how most buyers think about risk. Boulder County's page on radon in real estate transactions states plainly that soil conditions can vary significantly from one building to the next, even within the same neighborhood, so the decision to test should never be based on a neighbor's results, a zone map, or a survey map. Every buyer should test, full stop.
Boulder County carries an EPA-assigned Radon Zone 1 designation, meaning the predicted average indoor screening level exceeds 4.0 picocuries per liter, the EPA's action threshold. County-level test data collected through Air Chek puts Boulder County's average indoor radon reading at 4.5 pCi/L, against a national average of roughly 1.3 pCi/L. A smaller set of Longmont-specific, user-submitted tests across the 80501, 80503, and 80504 zip codes averaged 8.0 pCi/L, though that figure is drawn from only a handful of submissions and should be read as a signal rather than a settled number. Statewide, about 48.3 percent of single-family homes tested in Colorado between 2005 and 2022 came back at or above the EPA action level, according to CDPHE data cited by Vertex Engineering. Half the state, roughly, tests high. The other half doesn't. Address by address, not neighborhood by neighborhood.
Here's what that means for a transaction timeline and budget:
| What gets ordered | What it tells you | Typical turnaround |
|---|---|---|
| Radon test (continuous monitor or paired passive devices) | Average concentration over the test period, placed in the lowest level that could become living space | 48 hours minimum |
| Standard structural engineer's report | Full assessment of foundation condition with repair recommendations | A week or more |
| Expedited foundation evaluation | A baseline read that keeps the due diligence clock intact | As fast as 48 hours in some cases |
A short-term radon test during inspection typically runs $150 to $250. If the reading comes back elevated, mitigation in the Longmont area typically falls between $800 and $2,000, with most standard basements handled by a sub-slab depressurization system in the $1,000 to $1,500 range.
What Colorado Now Requires Sellers to Put in Writing
Colorado tightened the paperwork around this in 2023 and refined it again this year. Senate Bill 23-206, effective August 7, 2023, requires every residential sales contract to carry a bold-type warning about radon's health risks, along with any test results, reports, or mitigation history the seller actually knows about, per the Colorado General Assembly's bill summary. The current Seller's Property Disclosure form, SPD19, effective for use on or after January 1, 2026, folds that requirement directly into the standard disclosure packet issued by the Colorado Division of Real Estate.
The part buyers tend to miss: sellers are not required to test. They are only required to disclose what they already know. A seller who has never tested has nothing to report, and that blank space on the SPD19 form is not a clean bill of health. It is a gap the buyer's own test needs to fill, which loops right back to Boulder County's core point: your neighbor's clean reading does not clear your house.
The law also touches investors and landlords directly. Starting January 1, 2026, the remedy that lets a tenant void a lease over undisclosed radon no longer applies to leases of one year or less, according to the Division of Real Estate's broker practice advisory. Anyone buying a Longmont property with rental income in mind should build that distinction into their lease terms now.
The Local Specialists a Longmont Contract Usually Brings In
A typical Longmont transaction that turns up soil movement or elevated radon draws on a short list of specialists rather than a single inspector's opinion:
- Colorado Engineering Services, a Longmont-based structural engineering firm, handles foundation inspections and repair recommendations for local homeowners and buyers.
- Groundworks has served Longmont foundation repair since 1999 and carries an A+ Better Business Bureau rating.
- Alpine Building Performance, led by Denver-based inspector Andrew Sams, a Certified Master Inspector and Radon Measurement Specialist named the Denver Metro Association of Realtors' 2022 Industry Partner of the Year, teaches continuing education courses on building diagnostics for Colorado real estate professionals.
- ColoRadon provides radon mitigation service specifically in the Longmont market.
Naming these firms is not an endorsement so much as a map. When a Longmont report comes back with soil movement or an elevated radon number, this is the kind of vendor a contract calls next.
What This Means If You're Under Contract
An elevated radon reading or a pattern of settlement cracking rarely justifies walking away from a Longmont home outright. Mitigation is fast and inexpensive relative to the price of the house, and radon contingency language commonly used in Colorado contracts sets a target reading, ties resolution to a certified professional's post-mitigation retest, and moves forward once that number clears. Structural findings deserve a slower look, since the fix and the cost scale with how far the settlement has progressed, which is exactly why pattern recognition, crack width, direction, and location, matters more than the fact that a crack exists at all.
The takeaway that should change how you shop: don't pick a Longmont neighborhood because you heard the soil is better on one side of town, and don't skip a radon test because the seller's disclosure came back blank. Order the test. Read the report for pattern, not just presence. That is how a Longmont transaction actually gets handled, house by house.
Frequently Asked Questions
Does a high radon reading mean the seller hid something? Not necessarily. Sellers are only required to disclose radon information they already know. If a home has never been tested, the seller's disclosure will simply be blank, which is why every buyer should order their own test rather than relying on the disclosure form alone.
If the house next door tested clean, can I skip testing mine? No. Boulder County's own guidance is explicit that soil conditions vary from one building to the next even within the same neighborhood, so a neighbor's result, a zone map, or a survey map should never substitute for testing your specific address.
Is a foundation crack automatically a deal-breaker? Rarely on its own. Inspectors and engineers look for cracks wider than an eighth of an inch, horizontal cracking, or a stair-step pattern as signs of real settlement. Hairline vertical cracks from normal concrete curing are common across the region and usually cosmetic.
If you're weighing a purchase or a sale in Longmont and want a clear read on what a specific property's inspection is likely to turn up, Jane Kraemer can help you plan the transaction around it before it becomes a surprise. Schedule Your Free Market Consultation to start the conversation.
