Expansive Soils.
Much of the Denver metro is built on clay that swells when wet and shrinks when dry — hard enough to lift a slab and crack a basement wall. Here’s where the problem lives (and why the Evergreen foothills mostly escape it), what we look for during an inspection, and why a downspout extension is the cheapest foundation repair you’ll ever make.
Where expansive soils are on the Front Range
The culprit is bentonite — clay weathered from ancient volcanic ash — in the shales and mudstones under the plains and the hogback.
✓ The Denver Basin
Pierre Shale, the Denver and Dawson formations and their weathered clays underlie much of the metro. The Colorado Geological Survey puts their swell at up to 20% by volume, pushing with up to 30,000 pounds per square foot.
✓ Dipping bedrock along the hogback
Within a mile or three of the mountain front — parts of Jefferson, Douglas and El Paso counties — the bedrock stands on edge. Steeply dipping shale layers heave unevenly, which is the worst kind of movement.
✓ The usual suspects
The hogback front and the south metro — Ken-Caryl, Roxborough, Highlands Ranch, Parker, Castle Rock, Colorado Springs — have long histories of soil damage and mapped hazard overlays.
✓ The foothills are mostly granite
Evergreen, Conifer and most mountain communities sit on Precambrian granite and gneiss. Expansive clay is rarely the native soil up here, but valley fills and imported backfill can still contain it.
✓ Know your soils report
Colorado law requires the builder of a new home to give the buyer a summary soils report before closing, plus a swelling-soils guide where the risk is significant. Ask the seller for it; the county may have it with the permit.
✓ Swell potential in numbers
Geotechnical reports rate swell under a 500-psf load: under 3% low, 3–5% moderate, 5–8% high, over 8% very high. That one number drives the foundation design.
Signs of soil movement during an inspection
A home inspection is visual and non-invasive; we can’t see the soil, so we read the house.
✓ Doors and windows
Sticking doors, latches that no longer catch and windows racked out of square are early tells — especially when they change with the seasons.
✓ Drywall cracks
Diagonal cracks off the corners of doors and windows, and cracks that reopen after patching, point to movement rather than settling plaster.
✓ Floor slope and heave
A hump in a basement slab, a floor that runs downhill to one wall, or slab cracks with one side higher than the other. We put a level on it and note the direction.
✓ Masonry and foundation walls
Stair-step cracks in brick or block, horizontal cracks in poured walls, and any crack wider than about an eighth of an inch or with displacement goes in the report.
✓ Void forms and floating walls
We look for the features that manage movement: a gap at the base of basement partition walls, void boxes under grade beams, slabs isolated from bearing walls. A closed gap means the slab has heaved into the wall.
✓ Flatwork and exterior
Heaved sidewalks, driveways, patios and garage slabs; gaps at trim; separation between the house and the porch or chimney.
How Front Range homes are built to move
The design goal isn’t to stop the soil — it can’t be stopped — it’s to keep the structure from riding on it.
✓ Drilled piers and grade beams
Concrete piers drilled down into stable bedrock carry the house; grade beams span between them with a void underneath, so swelling clay pushes on air instead of concrete.
✓ Structural floors
In high-swell areas the basement floor is framed or suspended over a crawlspace void instead of poured on grade. A crawlspace under a basement is a good sign here, not a defect.
✓ Floating slabs and partitions
Slab-on-grade basements get isolated from the walls with expansion joints, and interior partitions are built with a gap — commonly 1–3 inches — at the bottom so the slab can rise without lifting the house.
✓ Post-tensioned slabs and overexcavation
Some builders stiffen the slab with tensioned cables, or dig out several feet of clay and replace it with moisture-conditioned fill.
✓ Perimeter drains and sumps
A drain around the footing feeding a sump pit is standard practice for below-grade rooms. Check that the pit has a lid, a working pump and a discharge that goes somewhere useful.
✓ Every jurisdiction differs
Floating-wall gaps, void sizes and drain requirements vary by county and era. A missing feature in a 1970s home isn’t a defect; a crushed void in a 2010 home is.
Drainage: the #1 mitigation (and the cheapest)
Almost every soil problem we see traces back to water reaching the clay beside the foundation. Control the water and most homes settle down.
✓ Grade away from the house
Code minimum is a 6-inch drop in the first 10 feet; Front Range geotechnical reports usually ask for more — on the order of a 10% slope in the first 5–10 feet of landscape. Backfill settles; regrade it.
✓ Extend the downspouts
Discharge at least 5 feet from the foundation, farther on clay lots, onto ground that keeps falling away. A downspout dumping at the wall is the most common finding in this category.
✓ Keep irrigation off the foundation
No sprinkler heads spraying within 5 feet of the house and no drip lines or flower beds soaking the backfill. Xeriscape the first few feet.
✓ Trees at a distance
Big trees pull moisture and shrink clay in drought, then let it swell in wet years. Extension guidance says 15 feet or more from the foundation.
✓ Window wells and flatwork
Window wells need drains or covers; walks and patios should slope away and be sealed where they meet the house.
✓ Consistency beats dryness
Clay moves when its moisture changes. The goal isn’t bone-dry soil; it’s soil that stays about the same moisture all year.
When we recommend a structural or geotechnical engineer
We report what we see and how concerning it is. Some findings need a stamp.
✓ Structural engineer
Evaluates the house: whether cracks and movement have affected the structure, what repairs are needed, and whether the foundation is still doing its job. This is the usual next call.
✓ Geotechnical engineer
Evaluates the ground: borings, lab swell tests and a soils report that tells the structural engineer what the foundation must resist. Needed for new construction, additions and major repairs.
✓ What triggers the call
Cracks over an eighth of an inch or with displacement, doors that no longer close, a slab hump, closed floating-wall gaps, or evidence of past repairs — piers, epoxy injection — without paperwork.
✓ Typical cost
In the Denver area a structural engineer’s visit with a written report commonly runs a few hundred to around a thousand dollars — small next to the repairs it rules in or out.
✓ Fixes range widely
From “regrade and extend the downspouts” to pier underpinning or slab replacement. An engineer’s letter also tells your lender and insurer the house is sound.
✓ Repairs need records
Any pier, helical anchor or slab work should come with a permit and an engineer’s letter. Undocumented repairs are a finding on their own.
Living on expansive soil
✓ Watch the seasons
Note which doors stick in spring and free up in fall. Seasonal cycling is normal; a door that gets worse every year isn’t.
✓ Don’t fill the floating gap
Finishing a basement? Keep the gap under partition walls, use slip connections, and don’t bolt cabinets across slab and wall.
✓ Maintain the sump
Test the pump each spring, add a battery backup if the basement is finished, and make sure the discharge line isn’t frozen or dumping at the wall.
✓ Water evenly in drought
In a dry year, light, consistent watering around (not against) the foundation prevents the shrink that opens gaps before the next wet year swells them shut.
✓ Re-check grading after landscaping
Every new bed, patio or sprinkler zone is a chance to undo good drainage. Walk the perimeter after every project.
✓ Keep the soils file
The builder’s soils report, engineer letters and repair permits. They matter at resale and to the next inspector.
Common questions
Is Evergreen affected by expansive soils?
Much less than the metro. Evergreen, Conifer and most of the foothills sit on granite and gneiss; the notorious bentonite clays belong to the shales under the plains and the hogback front — places like Ken-Caryl, Roxborough, Highlands Ranch, Parker and Castle Rock. Foothills homes still move from poor drainage, valley fills, imported backfill and frost, so we check the same signs everywhere; we just find fewer clay-driven ones up here.
Are foundation cracks normal in Colorado?
Hairline shrinkage cracks in poured concrete are normal almost everywhere. What we flag is width (more than about an eighth of an inch), displacement (one side higher or proud of the other), pattern (stair-step, horizontal, or diagonal off openings) and change over time. Those — especially together with sticking doors or a heaved slab — are when we recommend a structural engineer.
What is a “floating” basement wall?
A partition wall in a basement built with a gap, usually 1 to 3 inches, between its bottom plate and the slab, held with slip connections so the slab can rise with the soil without pushing the wall into the floor joists above. A finished basement should preserve the gap behind the baseboard. A gap that has closed means the slab has heaved; that is a finding.
Does a new house on the Front Range come with a soils report?
It should. Colorado requires the builder to give buyers of a new residence a summary of the soils report at least 14 days before closing, along with a guide to swelling soils where the risk is significant. Ask for it, keep it, and read the swell numbers and foundation recommendations. On a resale, ask the seller for the same documents; the county may have the original with the building permit.
What’s the single best thing I can do about expansive soil?
Manage water. Regrade so the ground drops noticeably in the first several feet, extend downspouts 5 feet or more, move sprinkler heads and beds away from the house, and keep moisture consistent through the year. Most foundation repairs we see could have been prevented for the cost of a few downspout extensions and a wheelbarrow of dirt.
This guide is for general education. It reflects what we look for during a home inspection, not a substitute for a licensed professional. If you have a specific concern, please contact a licensed structural or geotechnical engineer — and of course, we’re always happy to take a look during an inspection.
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