Patios & Hardscape in Centennial, Colorado

Suburban lots, Denver Basin clay

Centennial is one of the newer cities on the Front Range — incorporated in 2001 — but the housing under it mostly isn't. The bulk of the stock is 1970s-through-2000s subdivision, which means three things for hardscape.

The original concrete is aging out. A lot of what we're asked to replace here is builder-grade flatwork poured on minimal base, and it's doing exactly what builder-grade flatwork on minimal base does after thirty winters.

The access is usually good. Wider lots, real side yards, and often a gate a machine can actually get through. That's a genuine cost advantage over the old Denver grid, and it shows up honestly in the price. See what a patio costs in Colorado.

The grade is deceptive. Centennial's lots read as flat and mostly aren't. They're gently sloped, water sheets across them rather than channeling, and a patio built without a designed fall will find the low spot and pond in it. Ponded water in December is ice in December, and ice against a foundation is a problem that outgrows the patio.

The ground

Standard Denver Basin conditions — expansive claystone soil and bedrock. The Colorado Geological Survey's work on Front Range swelling soils applies squarely here: smectite clays that expand as much as 20 percent by volume with water and can exert forces up to 30,000 pounds per square foot.

The wrinkle: Centennial sits in the transition zone between the Denver Formation to the north and the more variable Douglas County ground to the south, where the CGS explicitly warns that engineering properties vary significantly, both laterally and vertically — sometimes lot to lot. So we dig test holes. A base spec written off a subdivision name is a guess.

Permits — we call, we don't assume

Centennial's Building Division points applicants to IRC R105 rather than publishing a local amendment to the exemption list. The unamended model-code threshold for retaining walls is 4 feet, measured from the bottom of the footing to the top of the wall, with no exemption when a surcharge is present.

That's most likely the rule here. But we couldn't confirm the absence of a local amendment from a primary source, and we're not going to publish a threshold we haven't verified — several neighboring cities do amend theirs downward (Littleton to 2 feet, Lakewood to 30 inches), so the assumption is genuinely unsafe.

So we call the Building Division at (303) 325-8000 before we quote a wall. One phone call, and you get an answer instead of a guess.

Note also that fire code in Centennial is enforced by South Metro Fire Rescue, not the city — which matters if you're adding a fire feature.

How we build here

The good access means we can build the base properly without the labor penalty that hand-hauling imposes — so there's no excuse for a thin one.

Six to eight inches of compacted Class 6, placed in 3-to-4-inch lifts and compacted between each one. Geotextile over the clay. ¼ inch per foot of fall away from the house — built into the base, not faked with bedding sand. Rigid edge restraint on every free edge.

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