What the code requires
Three rules from Ordinance 50-833, the Wichita–Sedgwick County amendments to the 2018 IRC, shape every greenhouse foundation here:
- Frost depth is 24 inches. Exterior footings go at least 24 inches below undisturbed ground.
- Small freestanding greenhouses are exempt from frost protection. The exemption covers freestanding accessory structures of 400 sq ft or less with an eave height of 10 ft or less. It applies to both light-framed and other construction.
- Anything from 25 to 400 sq ft must be held down. It must be tied to the earth under the City of Wichita's "Non Vehicular Storage Structure Anchoring Standards," or attached to a permanent concrete foundation per R403.1.6. Resolution 080-2017 applies the same rule in the county.
Derby's Ord. 2623 uses the same 24-inch depth and the same exemption under the 2024 IRC. Our Derby page covers the rest of that city's rules.
The exemption drops away in three common cases. An attached lean-to is not freestanding. A 20 × 24 ft house is 480 sq ft, over the limit. And a tall gothic-arch house can exceed the 10 ft eave height. Those need footings to 24 inches.
Sedgwick County soils, side by side
The county's soils sort into three groups. The figures below come from NRCS Soil Data Access for the Sedgwick County survey (KS173). LEP (linear extensibility percent) measures how much a soil shrinks and swells as it dries and wets. Higher is worse for slabs.
| Soil series | Where it sits | Texture | Drainage | Hydrologic group | Max LEP |
|---|---|---|---|---|---|
| Shellabarger | West side, paleoterraces | Sandy loam | Well drained | B (C on 6–15% slopes) | 2.7% |
| Nalim | West side, paleoterraces | Loam | Well drained | C | 4.5% |
| Blanket | West and central terraces | Silt loam over clay loam (up to 42% clay) | Well drained | C | 6.0% |
| Tabler | Flat terraces, central and south | Silty clay loam over clay | Moderately well drained | D | 6.2% |
| Irwin | East side uplands | Silty clay loam over clay | Moderately well drained | D | 7.4% |
| Rosehill | East side uplands | Silty clay | Well drained, low permeability | D | 10.4% |
| Canadian | Arkansas River valley | Fine sandy loam | Well drained; no wet layer of its own | A | 1.2% |
| Waldeck (mapped with Canadian) | Arkansas River valley | Fine sandy loam | Somewhat poorly drained; seasonal water table 61–92 cm, Oct–Apr | B | 1.2% |
Rosehill is named after Rose Hill in Butler County, according to its official series description, and it swells roughly 9 times as much as Canadian by this measure (10.4 ÷ 1.2 = 8.7).
West side: Shellabarger, Nalim, Blanket
Shellabarger and Nalim are the easy ground. Ground anchors hold well and slabs stay put. Blanket is different under the surface: its subsoil runs up to 42% clay with moderate-to-high shrink-swell, so "well drained" does not mean it won't move. On Blanket lots we watch drainage and favor piers for glazed houses.
East and south: Irwin, Tabler, Rosehill
These are clay soils in hydrologic group D, meaning water soaks in slowly and runs off fast. They shrink in a dry August and swell in a wet spring. A thin slab poured on top can crack or tilt as the soil moves. On these lots we lean toward footings or piers that reach below the most active layer, plus drainage that keeps water away from the base.
Arkansas River valley: Canadian and Waldeck
Canadian soils are sandy and drain fast, but they sit on low terraces and high floodplains subject to rare flooding, per the series description. In Canadian–Waldeck map units, the Waldeck component is somewhat poorly drained, and NRCS data put its seasonal water table at 61–92 cm from October through April. That is 24 to 36 inches (92 ÷ 2.54 = 36).
A 24-inch footing on a Waldeck spot can sit right at that water table in a wet spring, with at most about 12 inches of margin. A sunken or earth-sheltered greenhouse 4 ft deep would sit below it. That is why we check the water table before designing anything below grade in the river valley. See our underground greenhouse page for that build type.
Greenhouse foundation options
| Foundation | Best fit | Frost-protected? | Meets anchoring rule? | Watch for |
|---|---|---|---|---|
| Kit base on timber frame with ground anchors | Kits up to 400 sq ft, freestanding | No (exempt if ≤400 sq ft, eave ≤10 ft) | Yes, with anchors per City of Wichita standard | Timber rot; anchor pull-out in loose sand |
| Perimeter concrete curb | Polycarbonate and glass kits; gravel or soil floor | Only if carried to 24 in. | Yes (permanent concrete foundation) | Clay heave on east-side soils if shallow |
| Frost-depth footings (24 in.) | Attached lean-tos; houses over 400 sq ft or eave over 10 ft | Yes | Yes | More excavation; spoils handling |
| Concrete piers or helical piers | Clay lots; sloped lots; timber or steel-frame houses | Yes, when set below 24 in. | Yes | Pier spacing must match the frame |
| Slab on grade | Potting rooms, heated year-round houses, ADA access | Only if edges reach 24 in. (unless exempt) | Yes | Drainage, cracking on shrink-swell clay |
Derby's anchor schedule gives a sense of the hold-down involved. A 12 × 24 ft structure needs four anchors at 800 lb each, within 12 inches of the corners: 4 × 800 = 3,200 lb. Our wind-rated design guide explains where that load comes from.
How much concrete? A worked example
Take a 10 × 16 ft greenhouse. Its perimeter is 2 × (10 + 16) = 52 ft.
| Option | Dimensions | Volume |
|---|---|---|
| Perimeter curb | 52 ft × 6 in. wide × 12 in. deep | 52 × 0.5 × 1.0 = 26 cu ft ≈ 1.0 cu yd |
| Frost-depth footing | 52 ft × 12 in. wide × 24 in. deep | 52 × 1.0 × 2.0 = 104 cu ft ≈ 3.9 cu yd |
| 4-inch slab | 10 × 16 ft × 4 in. | 160 × 0.333 = 53 cu ft ≈ 2.0 cu yd |
At 160 sq ft, this house is exempt from frost protection, so the curb meets code at a quarter of the footing's concrete (26 ÷ 104 = 0.25). On Irwin or Rosehill clay we may still recommend going deeper. Concrete and excavation prices change by season and site access. For a national benchmark, Angi puts concrete footings at about $5 to $18 per linear foot installed (Angi). For the 52 ft perimeter above, that's roughly $260 to $940 before excavation. HomeGuide lists a concrete slab floor at $2.80–$3.60 per sq ft, installation not included (HomeGuide). Excavation is scoped per site, and we price each job on the actual lot. The foundation is one line in a full custom greenhouse build, and we design it together with the frame and glazing.
Drainage
Water is the bigger threat to a greenhouse base than frost. A few rules we design to:
- Set the base above surrounding grade and slope the ground away on all sides.
- On clay (HSG D) lots, add a gravel bed under the floor and a perimeter drain to daylight where the slope allows.
- Keep roof runoff off the foundation. A 10 × 16 ft roof sheds about 160 × 0.623 = 100 gallons per inch of rain. Gutters or a gravel drip edge keep that water from soaking the base. If you plan to capture it, see our irrigation water guide.
- In the Arkansas River valley, check the floodplain before choosing a spot. MABCD's location permit review checks for floodplain and floodway.
Radon: attached and below-grade greenhouses
A freestanding greenhouse with vents is not a radon concern. An attached greenhouse that opens into your house, or an earth-sheltered one you spend hours in, is different.
KDHE's Sedgwick County profile reported an average home radon level of 3.6 pCi/L in 2010, from 3,771 tests. 30.9% were at or above the 4 pCi/L action level. For attached and below-grade builds we plan a vapor barrier under the slab and a path for a sub-slab vent, and we recommend testing once the greenhouse is closed in.
Frequently asked questions
How deep do greenhouse footings need to be in Wichita?
24 inches, if footings are required. Freestanding greenhouses of 400 sq ft or less with an eave height of 10 ft or less are exempt from frost protection.
Can I set a greenhouse kit on pavers?
Not by itself if it is over 25 sq ft. It must be tied down to the earth under the City of Wichita anchoring standard or attached to a permanent concrete foundation.
What foundation works best on east Wichita clay?
Piers or footings that reach below the most active clay, with drainage away from the base. Thin slabs on Irwin or Rosehill soils can crack as the clay swells and shrinks.
Is a slab required for a greenhouse?
No. A gravel or soil floor inside a concrete curb or anchored frame meets the code for small greenhouses. A slab makes sense for potting rooms and year-round heated houses.
Does an attached greenhouse need a frost footing?
Yes. The frost-protection exemption only covers freestanding accessory structures, so an attached lean-to needs footings to 24 inches.
Get a quote
Tell us your address and the greenhouse you have in mind. We'll look up the soil series on your lot, check the permit line and recommend the base that fits. Get a Quote.