Concrete Slabs: Pads, Pole Barn Floors and What Goes Under Them

Concrete Pads: Shed, Hot Tub, AC and Generator Slabs

A pad is a small slab with one job: hold one thing level. HomeGuide's slab guide describes most shed slabs as about 4 inches thick and says they don't typically need reinforcement. That's a fair starting point for a garden shed, and the whole cost of the job is in the parts nobody sees: stripping the topsoil, a compacted gravel base, and forms set level with a slight pitch so rain runs off instead of under the shed door.

Hot tubs and equipment pads are different, because someone else's warranty is riding on the slab. A filled hot tub puts a lot of weight on a small footprint, and the tub maker's installation sheet usually spells out what it wants underneath. Read it before anyone forms anything; it's the spec that matters if the shell ever cracks. Air conditioner condensers and standby generators often sit on precast pads, which HomeGuide notes can cost less than forming and pouring for small, simple jobs. The equipment installer usually supplies or specifies that pad, so ask them first.

Where Small Pads Go Wrong

Pads poured against a house sit on backfill, the loose soil pushed in after the foundation went up, and they settle toward the wall. A pad poured right on topsoil heaves and tips in its first spring. And a pad tied to the house with pins or poured against the siding has nowhere to move. Wisconsin's dwelling code addresses that last one directly; see the frost section below.

What Goes Under a Concrete Slab: Subgrade, Base and Vapor Retarder

Most slab failures start below the concrete. The drawing shows the layers on a typical floating slab with a thickened edge, the way many Wisconsin garage and shed details draw it.

Subgrade: Wausau-Area Soils

The Marathon soil series, named for this county, is described by USDA as silt loam formed in loess over glacial till or grus weathered from granite, with unweathered bedrock within 40 to 60 inches in some places. Two things follow for a slab. First, topsoil and soft silt come out; a slab poured on organic soil keeps settling. Second, silt holds water, and frost heave needs cold and water to happen. A slab on wet silt with no drainage under it is the one that lifts at the corners in March. Wisconsin's dwelling code makes this a rule for house floors: on clay soils, a concrete floor needs a 4-inch base of clean graded sand, gravel or crushed stone, while on sand or gravel the base may be omitted unless drain tile is installed (SPS 321.20).

Granular Base and Compaction

The base is a drainage layer and a level bed, not a substitute for firm ground. For attached garage floors the UDC sets at least 4 inches of granular fill under at least 4 inches of concrete (SPS 321.203). Morton Buildings plans a minimum of 6 inches of uniform compacted gravel under a 6-inch shop floor, and Hansen Pole Buildings' construction manual calls for fill mechanically compacted to at least 90 percent of Modified Proctor density, topped with 2 to 6 inches of clean, drained sand or sandy gravel. If your contractor can't tell you how the base gets compacted and checked, that's the question to keep asking.

Vapor Retarder Under a Concrete Slab

For a slab under living space, Wisconsin's code requires a vapor retarder directly under the slab or under its base course, at least 6 mils thick or a reinforced material, with joints overlapped at least 6 inches and taped or sealed. It isn't required under an unconditioned attached garage (SPS 322.38). The National Ready Mixed Concrete Association goes further for interior floors: an ASTM E1745 material, a minimum of 10 mils recommended, directly under and in contact with the slab, and it notes that vapor retarders are generally not necessary for exterior slabs (CIP 29). Rib Mountain's detached garage detail draws a 6-mil retarder "if heated." For a shop you'll heat or finish later, put it in now; it's the one layer you can't add after the pour.

Concrete Slab Thickness and Reinforcement: Mesh, Rebar or Fiber

HomeGuide puts most slabs at 4 to 6 inches thick. Four inches is what Wisconsin requires on an attached garage floor and what the Rib Mountain and Franklin garage details draw. Six inches is where pole building suppliers land for heavy equipment: Hansen calls six inches "more than adequate" where heavy equipment is driven and parked, and Morton's farm shop design uses a 6-inch reinforced floor in the shop and wash bay and 12 inches under the legs of a vehicle lift. Thickness is cheap compared to tearing out a floor that was poured for cars and then asked to carry a loader.

Two details are worth checking on any reinforced slab. NRMCA recommends cutting alternate wires, or better, stopping the mesh across contraction joints so the joint can do its job. And nobody should be "hooking" mesh up into the concrete after it's placed; the steel goes on chairs or blocks before the truck arrives.

When a Thickened Edge Applies

A thickened edge turns the perimeter of a floating slab into a shallow grade beam that walls can sit on. It's the standard detail for a detached garage on a slab instead of frost footings. Franklin, Wisconsin's garage handout is a clear example: a continuous floating slab of reinforced concrete at least 4 inches thick with 6x6 wire mesh, a thickened edge all around 8 inches wide and 8 inches below the top of the slab, and two #4 bars in it. Weston's accessory structure permit asks for "grade beam details, concrete slab specifications or footings" with the application. A shed pad or hot tub pad with nothing bearing on its edge usually doesn't need one. A slab carrying walls and a roof usually does, and the local inspector has the final word.

Control Joints: Telling a Concrete Slab Where to Crack

Concrete shrinks as it cures, and a slab will crack somewhere. Joints pick the spot. The National Ready Mixed Concrete Association's rule (CIP 6) is joint spacing of 24 to 36 times the slab thickness, capped at 15 feet, with panels square or nearly so and no panel longer than 1.5 times its width. Joints are cut at least a quarter of the slab depth and never less than 1 inch. Saw cuts go in 4 to 12 hours after finishing, or 1 to 4 hours with early-entry saws; too early and the edges ravel, too late and the slab has already cracked on its own.

Isolation joints are the other kind. NRMCA says to put them between slabs and walls, columns and footings, and where slabs meet walks, stairs and other points of restraint, using premolded filler such as asphalt-impregnated fiber or compressible foam. On a pole barn floor, Morton uses expansion material or rigid foam along the splash board and an expansion joint between the floor and the outside approach.

Floating Slab or Frost Footing? Wisconsin Code and Wausau Frost

Wausau winters are the reason this question matters. NOAA's 1991 to 2020 normals for the Wausau Downtown Airport show about 163 nights a year at or below 32°F, roughly 25 days at or below zero, and 64 inches of snow (NOAA normals). Frost depth varies by year and by snow cover; in early February 2026 the state's winter outlook put frost at 1 to 2 feet for most of Wisconsin. That's one season's reading, not a design number.

For a house and anything attached to it, Wisconsin's code sets the design number. Footings and foundations go below the frost penetration level or at least 48 inches below grade, whichever is deeper, and may not be placed on frozen material (SPS 321.16). The exceptions are narrow: an engineered frost-protected shallow foundation designed to ASCE 32, and footings on cleaned bedrock. A slab that floats, meaning it rides the frost instead of reaching below it, is allowed next to a house, but structures on floating slabs "may not be physically attached" to structures on frost footings without a full-height isolation joint between them (SPS 321.15). That's the rule behind the joint at the wall in the drawing.

Detached Garages and Accessory Buildings

Here's the part out-of-state advice gets wrong for Wisconsin. Except for the fire separation rule in SPS 321.08(1), the state dwelling code says its provisions "do not apply to detached garages or to any accessory buildings detached from the dwelling" (SPS 320.05(4)). That doesn't mean anything goes. Municipalities write their own rules, and some pull the UDC back in: Rib Mountain's accessory building supplement says permanent detached garages, carports and sheds must follow applicable UDC provisions. Whether a detached garage or shop around Wausau sits on a thickened-edge floating slab or on frost footings is a question for the local building inspector, asked before the slab is formed. Full foundations under a home are a separate subject and outside this guide.

Concrete Slab Permits Around Wausau and Marathon County

Permits around here follow the building on the slab more than the slab itself. Every municipality draws its lines differently, and in Marathon County that can mean a city, a village or a town office. What we could verify on official pages:

• Village of Weston. Its accessory structure permit (updated November 2025) treats yard sheds as 200 sq ft or less and detached garages as 201 sq ft or more, takes applications through its online portal, and asks for concrete slab specifications or footing details. It also lists maximum accessory floor area by zoning district and setbacks for hard surfaces.
• Village of Kronenwetter. Garages and yard sheds require a building permit, and plans must show footing detail and foundation plans.
• Rib Mountain. Its 2022 accessory building supplement draws a 4-inch slab over at least 4 inches of granular fill, a 6-mil vapor retarder if heated, and anchor bolts at 72 inches on center, and caps combined garage, carport and shed area at 1,000 sq ft. It also notes that property in a natural resource protection overlay may need Marathon County or Wisconsin DNR approvals before the town issues a permit.
• City of Wausau, Schofield, Rothschild, Marathon City and the towns. Call the local building inspection office before you order anything. Ask about size thresholds, setbacks, whether the slab alone needs a permit, and what slab or footing detail they want to see.

A plain slab with nothing on it is often treated differently from a garage, but don't assume. Lot-coverage and setback rules can still apply to a pad even where no building permit is needed.

Pole Barn Concrete Floors: Pouring a Pole Building Floor

A pole barn floor is poured backwards compared to a garage. The posts carry the building, so the floor usually goes in after the shell is up. Hansen Pole Buildings calls that "the best time to pour it," with concrete normally placed through the big door, and its manual says at the very least the roof should be on, because pouring with only columns standing risks knocking one out of plumb. Here's how the sequence usually runs:

1. Shell up, roof on. Posts set and the roof installed. Some wall steel can be left off for truck or pump access (Hansen).
2. Skirt board as the form. The pressure-treated skirt board, or splash plank, becomes the slab form. Morton pours the floor top about 4 in. above grade; thicker floors gain depth below.
3. Fill and compact. Native soil stripped and fill compacted. Then the granular base, 6 in. under a 6 in. floor in Morton's farm shop design.
4. Vapor retarder. Run up the sides and onto the splash plank, seams overlapped at least 6 in. and taped. Hansen prefers 15 mil over 6 mil because it survives the pour.
5. Drains, heat, insulation. Floor drains, trench drains, wash-bay slopes, PEX tubing and under-slab foam all go in now. None of it can be added later.
6. Door openings. Temporary 2x4 forms across doors. Under overhead doors, Morton sets the concrete 3 in. above grade, with an expansion joint to the outside approach.
7. Pour and protect. Concrete in through the door. If liners are already up, cover them with plastic. Never pour on frozen ground (Morton).
8. Cut and cure. Saw joints on schedule, cure 3 to 7 days, and keep heavy equipment off until the concrete has strength.

Pole Building Floor Thickness for Equipment and Door Aprons

Hansen's manual describes the usual floor as 4 inches nominal, with about 3-3/4 inches of skirt board left showing above it. That's a car-and-mower floor. For a tractor, a skid steer, or a loaded trailer parked for the winter, the suppliers above go to 6 inches and reinforce it. The door apron and outside approach take the hardest use, because that's where wheels turn and where meltwater freezes. Morton says to design exterior approaches for heavy loads and exterior conditions.

Floor Drains and Wash Bay Slopes

Decide on drains before anything else. Hansen notes that the International Building Code wants vehicle parking floors sloped to move liquids but gives no number, and says roughly 1/8 inch per foot is the usual consensus. For attached garages, Wisconsin's code is explicit: slope to the overhead door, to exterior grade, or to an interior floor drain that complies with the state plumbing code. A floor drain in a shop may come with its own plumbing permit and, in some places, an oil separator; ask the local office.

Heated Pole Barn Floors: PEX and Insulation

In-floor heat is a decision for the day the base is graded. Hansen's own recommendation for a heated pole building is rigid insulation under the entire floor, vertical insulation around the whole perimeter, and PEX-AL-PEX tubing divided into zones. The Department of Energy's Building America guidance on slab edge insulation says slabs independent of the foundation wall should be insulated vertically at the edge and horizontally at the perimeter or under the whole slab. HomeGuide puts radiant floor heating in concrete at $11 to $25 per square foot, which can cost more than the slab it's buried in.

Frost Heave at the Big Door

The building's weight sits on its posts, not its slab, so a pole barn floor floats. That's fine until the ground at the door heaves and the overhead door won't close. Hansen's fix starts with drainage: grade away from the building at least 5 percent for 10 feet and put downspouts at least 10 feet out. Morton notes that frost footings through overhead door openings can keep the fit tight under the door. On silty Marathon County ground, the drainage part is the one to get right first.

Pouring and Curing a Concrete Slab in Wausau's Cold Season

NRMCA defines cold weather for concrete as more than 3 days in a row averaging under 40°F, with the air no warmer than 50°F for more than half of any day (CIP 27). Around Wausau that's a good part of spring and fall. Concrete can be poured then, but the job changes:

• No frozen ground. Snow and ice come off, and the base has to be above freezing, which may mean thawing it with blankets or heat first.
• Protect it to 500 psi. Fresh concrete freezes below about 25°F and can lose more than half its potential strength. NRMCA says to protect it until it reaches 500 psi, about two days after placement.
• Slower set. A 20°F drop in concrete temperature roughly doubles setting time, so finishing runs late into the day.
• Blankets and vented heat. Insulated blankets cover corners and edges first, since those lose heat fastest. Heaters inside an enclosure need venting; unvented exhaust causes a dusty surface.
• Accelerators aren't antifreeze. NRMCA is plain that accelerating admixtures do not prevent concrete from freezing.

Curing is the step people skip in any season. NRMCA's curing guidance (CIP 11) calls for typically 3 to 7 days of keeping the slab moist and above 50°F, using wet coverings, plastic at least 4 mils extended past the edges, or a curing compound sprayed when the surface sheen disappears. It also says concrete in a dry environment can lose as much as half its potential strength. HomeGuide's rule of thumb: foot traffic in 1 to 2 days, vehicles after 1 to 2 weeks.

Concrete Slab Cost: National Ranges and What Moves the Price

These aren't our prices and they aren't quotes. They're national figures from HomeGuide's concrete slab cost guide (February 3, 2026), which puts an installed slab at $6 to $12 per square foot, with materials at $3 to $7 and labor at $3 to $5. Central Wisconsin bids can land anywhere around them.

By project, HomeGuide lists a 10x10 to 20x20 shed foundation at $600 to $4,800 installed. For pole barns, HomeGuide's pole barn guide (January 21, 2026) prices an optional concrete slab floor at $5 to $10 per square foot installed, depending on thickness and reinforcement, and lists building permits at $500 to $2,000. Fixr's pole barn guide (updated October 13, 2022) gives a lower $2 to $4 per square foot for a concrete floor and $8 to $12 per square foot for a frost-proof slab foundation. The gap between those two sources is a good reminder that national numbers are a rough frame, not a budget.

What Moves a Concrete Slab Price

• Base work. HomeGuide lists gravel at $1 to $3 per square foot and yard leveling at $0.40 to $2.00. Stripping soft silt and bringing in fill on a rural site can be the biggest line on the bid.
• Thickness and steel. Each step from 4 to 6 to 8 inches adds about $2 per square foot in HomeGuide's table, before rebar or a thickened edge.
• Frost detail. A floating slab costs far less than frost footings, which is why the inspector's answer belongs before the bid, not after.
• What's buried in it. Drains, PEX and under-slab foam are all priced before the pour.
• Tear-out. HomeGuide puts concrete removal at $3 to $8 per square foot, including disposal.
• Access and season. A pump truck where a chute can't reach, and blankets and heat for a late-fall pour.

Questions to Ask a Concrete Slab Contractor

Whoever you hire, these questions separate a bid that's been thought through from a square-foot number:

• What's going under it, and how deep are you digging?Listen for topsoil removal, base material and thickness, and how compaction gets done.
• How thick, and what reinforcement, for what I'm putting on it?Tell them the heaviest thing that will ever sit or drive on the slab. Ask where the steel will sit during the pour.
• Floating slab or frost footings, and has the inspector agreed?For anything with walls on it, get the local office's answer before the slab is formed.
• Where will the joints go, and when do you cut them?Ask for a joint layout on paper. NRMCA's limits are in the joint table above.
• Is there a vapor retarder, and how thick?Required under heated living space; strongly worth it under any shop you might heat later.
• Which way does water go?To the door, to grade or to a drain. Ask for the slope in writing.
• What happens if it's cold on pour day?Blankets, heat, ground thawing, or a new date. Ask who decides.
• What's in the price and what isn't?Permits, excavation, fill, removal of old concrete, pump truck, curing and sealing.