How thick should a concrete slab on grade be in NC?

For most North Carolina residential uses, the correct slab on grade thickness is 4 inches for basements, interior finished floors, patios, sidewalks, and single car garages with passenger cars; 5 inches for two car garages, workshops with tool storage, and light utility buildings; and 6 inches for garages with hydraulic lifts, shops with pickup trucks or trailers, and residential outbuildings that store heavy equipment. The American Concrete Institute (ACI 302.1R) and the North Carolina Residential Code (2018 NCRC section R506) both use 3.5 inches as the absolute minimum residential slab thickness.

Do I need a vapor barrier under a concrete slab in North Carolina?

Yes, for any conditioned interior space. The 2018 North Carolina Residential Code section R506.2.3 requires a minimum 6 mil polyethylene vapor retarder between the base course and the slab in living areas, basements, and finished garages that will be heated or cooled. Best practice on Triangle metro new construction is 10 mil or 15 mil reinforced polyethylene (Class A vapor retarder per ASTM E1745), lapped 6 inches at seams and taped, extended up the perimeter form 6 inches. Skipping the vapor barrier or using the thin 4 mil sheet lets ground moisture rise through the slab and drives flooring failures, efflorescence, and mold at the base of walls.

When do I need insulation under a slab in NC?

Any conditioned interior space in North Carolina climate zones 3A and 4A (which is nearly all of the state) needs a minimum R 10 of continuous rigid foam insulation under the perimeter of the slab per 2018 NC Energy Conservation Code (NCECC) section R402. Full slab insulation (R 5 to R 10 continuous under the entire slab) is not code required but adds meaningful comfort and reduces heat loss on basement floors and heated garages. Extruded polystyrene (XPS) or high density expanded polystyrene (EPS) rated for below grade use is the standard material. Unfinished detached garages, sheds, and utility slabs do not require slab insulation.

What sub base does a concrete slab on grade need in NC piedmont soil?

The correct sub base for a slab on grade over North Carolina piedmont clay is 4 inches of compacted ABC stone (aggregate base course, NCDOT type A or B) for a residential slab, or 6 inches for a garage, workshop, or light commercial slab. Compact in 2 inch lifts to 95 percent standard Proctor density with a plate compactor. Woven geotextile fabric between the clay sub grade and the stone (NCDOT Type 4 or equivalent, 200 psi grab strength) prevents fines from migrating up into the stone and is strongly recommended in Cecil, Appling, and Georgeville clay series that cover most of the Triangle.

How much rebar or wire mesh do I need in a slab on grade in NC?

A 4 inch residential slab is typically built with either 6x6 W1.4 x W1.4 welded wire fabric (WWF), synthetic macro fibers dosed at 4 to 7.5 pounds per cubic yard, or a light rebar grid of number 3 bars on 24 inch centers. A 5 inch garage or workshop slab uses number 4 rebar on 18 inch centers or 6x6 W2.9 x W2.9 WWF. A 6 inch load rated slab uses number 4 or number 5 rebar on 12 to 16 inch centers, sometimes doubled in a top and bottom mat. Fiber only slabs are acceptable for residential floors but not for slabs that will carry vehicles, tools, or point loads.

How far apart should control joints be on a slab on grade in NC?

The industry rule from the American Concrete Institute (ACI 302.1R) is that control joint spacing (in feet) should be no more than 2.5 to 3 times the slab thickness (in inches). For a 4 inch slab that means joints at 10 to 12 feet on center in both directions. For a 5 inch slab, 12.5 to 15 feet. For a 6 inch slab, 15 to 18 feet. Control joints should be saw cut within 4 to 12 hours of placement, cut to 1/4 the slab depth (1 inch on a 4 inch slab). The layout should form roughly square panels; long thin panels crack across the middle regardless of joint spacing.

Can I pour a slab on grade directly on dirt in NC?

For an unheated detached garage, shed, or utility outbuilding on well drained sandy loam, a 4 inch slab poured directly on compacted sub grade with no stone base can perform acceptably. On piedmont clay (the majority of the Triangle and Piedmont Triad), skipping the stone base is a false economy: the slab pumps mud through joints within 3 to 5 winters, cracks appear at the panel corners, and the surface starts to spall wherever water sits. For any conditioned interior slab, ANY slab that will see vehicle traffic, or ANY commercial use, stone base is required.

What PSI concrete should a slab on grade in NC be?

The 2018 North Carolina Residential Code (R402.2) requires a minimum 2,500 PSI concrete for residential slabs on grade. Best practice for Triangle metro new construction is 3,500 to 4,000 PSI air entrained (5 to 7 percent air entrainment) for exterior slabs (patios, driveways, sidewalks) and 3,500 PSI non air entrained for interior conditioned slabs. Garage and workshop slabs should be 4,000 PSI. Slabs that will see forklift or lift truck traffic should be 4,500 to 5,000 PSI with a minimum 4 inch slump. Skip fly ash content above 20 percent for slabs that will be surface hardened or finished immediately.

How long before I can walk, park, or store on a new slab in NC?

Foot traffic at 24 to 48 hours in typical Triangle metro weather, wheeled tool boxes and light storage at 7 days, passenger vehicles at 7 days, full weight trucks or trailers at 28 days, and forklift or lift truck traffic at 28 days minimum. According to ACI, standard 4,000 PSI concrete reaches roughly 70 percent of its 28 day design strength at 7 days. Wet cure with plastic sheeting or a spray on curing compound for the first 7 days; skipping curing on a summer Triangle pour drops final strength by 20 to 30 percent and drives surface crazing within a few months.

Concrete Slab-on-Grade Thickness Guide for NC: 4-Inch vs 5-Inch vs 6-Inch, Vapor Barrier, and Insulation for Conditioned Space

Slab thickness is the single decision that decides whether a concrete slab on grade in North Carolina lasts 50 years or cracks in 5. Homeowners and small commercial builders across Raleigh, Durham, Cary, Chapel Hill, Apex, and the Triangle metro get this wrong in the same three ways every year: pouring 3 inches where 4 was code minimum, pouring 4 inches under a garage that needed 5, and pouring 5 inches under a shop that needed 6. This guide walks through the four decisions that make a slab on grade last in North Carolina piedmont soil: correct thickness for the use, proper sub base for our clay, vapor barrier and insulation for conditioned space, and rebar or fiber reinforcement matched to the load. Real numbers, real NC code, real crew.

Quick Answer: Thickness by Use in North Carolina

Before the concrete truck rolls, match slab thickness to the actual use. The North Carolina Residential Code allows a 3.5 inch absolute minimum, but the American Concrete Institute (ACI 302.1R) and 15 years of NC piedmont experience point to these numbers as what actually lasts:

4 inch slab — residential basements, interior finished floors, patios, sidewalks, single car garages with passenger cars, sheds and outbuildings under 400 square feet.
5 inch slab — two car garages, workshops with tool storage, light utility buildings, detached shops for homeowners with a single pickup or SUV.
6 inch slab — garages with hydraulic lifts, shops with pickup trucks or trailers stored inside, residential outbuildings that store tractors or heavy equipment, light commercial floors under 3,000 pounds per square foot design load.
8 inch slab or thicker — commercial warehouse floors, forklift rated floors, and any slab with forklift or lift truck point loads (covered in our companion post on forklift rated warehouse floor thickness and reinforcement).

Get the thickness right and the reinforcement, sub base, and jointing become manageable. Get the thickness wrong and no amount of rebar will save the slab from cracking under load it was never designed to carry.

Why NC Piedmont Soil Makes Slab Thickness Matter More

The Triangle metro and greater Piedmont sit on Cecil, Appling, and Georgeville series piedmont clay soil, cataloged in the USDA Natural Resources Conservation Service Web Soil Survey. Three properties of these soils drive slab design decisions in North Carolina:

Low permeability. Cecil clay drains at 0.06 to 0.20 inches per hour saturated, versus 2 to 6 inches per hour for sandy loam. Water does not soak in below the slab; it stays as saturated sub grade until it can evaporate or move laterally.
Moderate to high shrink swell potential. These clays expand when wet and shrink when dry, moving the sub base under the slab if it is not stone stabilized. Movement of 1/8 to 1/4 inch across a seasonal cycle is common.
Poor bearing capacity when saturated. Dry Cecil clay carries 3,000 pounds per square foot; saturated Cecil clay drops to 500 to 800 pounds per square foot. A garage slab designed for a 4,000 pound car sitting on 800 psf saturated clay will settle unevenly if the sub base is not doing its job.

The fix is not thicker concrete alone. The fix is separating the slab from the wet clay with compacted stone base and geotextile fabric, so that the concrete slab always sits on a well drained, dimensionally stable platform regardless of how wet the surrounding soil gets. That is why a properly built 4 inch residential slab on 4 inches of compacted ABC stone routinely outlasts a poorly built 6 inch slab poured directly on graded clay.

4 Inch Slabs: Where They Work and Where They Fail

The 4 inch slab is the workhorse of residential concrete in North Carolina. Used correctly, it is the right choice for a basement floor, an interior conditioned room, a patio, a sidewalk, a single car garage with passenger cars only, and any outbuilding under about 400 square feet.

What a 4 inch slab needs to work:

Minimum 3,500 PSI concrete (2,500 PSI is code minimum but leaves no margin for the surface).
4 inches of compacted ABC stone base over woven geotextile fabric on the clay sub grade.
6x6 W1.4 x W1.4 welded wire fabric OR synthetic macro fibers at 4 to 7.5 lbs/cy OR #3 rebar on 24 inch centers as light reinforcement.
6 mil polyethylene vapor retarder in any conditioned space (10 mil is better).
Control joints saw cut at 10 to 12 feet on center in both directions within 4 to 12 hours of placement.
7 days of moist curing under plastic sheeting or a curing compound.

Where a 4 inch slab fails: under vehicles heavier than a passenger car, under any point load above about 1,500 pounds per square foot, under tool storage racks with concentrated legs, on unstabilized clay sub grade, or without control joints (which is when the slab picks its own joints and the joint pattern is called a crack).

For patio and outdoor pad specifics on a 4 inch slab, see our companion posts on residential concrete driveway thickness and rebar in NC and driveway widening and side parking pads in NC.

5 Inch Slabs: Two Car Garages, Workshops, and Light Utility

The 5 inch slab is the right choice for a residential two car garage, a homeowner workshop, a detached shop with a single pickup or SUV parked inside, and light utility buildings. It is the smallest jump above the residential 4 inch, but it changes the reinforcement, joint spacing, and cost meaningfully.

What a 5 inch slab needs to work:

3,500 to 4,000 PSI concrete, 4 to 5 inch slump.
4 to 6 inches of compacted ABC stone over geotextile fabric.
#4 rebar on 18 inch centers in a single mat set 2 inches from the bottom, OR 6x6 W2.9 x W2.9 welded wire fabric.
Thickened perimeter (12 inch by 12 inch turn down beam) where the slab abuts a masonry stem wall or where a garage door threshold requires additional bearing.
6 mil vapor retarder minimum (10 mil recommended) in any conditioned space.
Control joints saw cut at 12.5 to 15 feet on center within 4 to 12 hours.
Curing compound or wet cure for 7 days minimum.

Cost for a 5 inch reinforced garage or workshop slab in the Triangle metro currently runs 8.50 to 13.00 dollars per square foot installed, depending on rebar spec, sub base depth, and whether the perimeter includes a thickened turn down beam.

The most common 5 inch slab failure in the Triangle is a two car garage slab poured at 4 inches to save money, then cracked diagonally across the middle within 3 to 5 years once both cars started parking on it daily. The extra inch is 25 percent more concrete cost but eliminates the crack pattern entirely.

6 Inch Slabs: Lifts, Trucks, Trailers, and Heavy Equipment

The 6 inch slab is the correct choice any time the intended use includes hydraulic lifts, pickup trucks or trailers stored inside, tractor or skid steer storage, or point loads above about 3,000 pounds per square foot. It is also the right thickness for any residential outbuilding that will see a delivery truck park inside.

What a 6 inch slab needs to work:

4,000 PSI concrete minimum, 4 inch slump.
6 inches of compacted ABC stone over geotextile fabric.
#4 or #5 rebar on 12 to 16 inch centers in a single mat, sometimes doubled top and bottom mat for hydraulic lift bays and truck point load zones.
Thickened perimeter grade beam (12 inch by 24 inch) at any point where a wall bears on the slab edge.
10 mil or 15 mil reinforced vapor retarder in any conditioned space, taped and lapped.
Control joints saw cut at 15 to 18 feet on center in both directions.
28 day cure before any full weight truck, trailer, or lift traffic.

Cost for a 6 inch load rated slab in the Triangle metro currently runs 11.00 to 16.50 dollars per square foot installed. That is a real number that catches homeowners off guard, especially when they are pricing a home shop against a 4 inch residential quote and wondering why the shop is 60 percent more per square foot. The answer is 50 percent more concrete, double the rebar, extra sub base depth, and a longer cure cycle.

Vapor Barrier: Why It Matters in NC and What Grade to Use

Any conditioned interior slab in North Carolina needs a vapor retarder between the sub base and the concrete. The 2018 North Carolina Residential Code section R506.2.3 sets the minimum at 6 mil polyethylene sheeting; the American Concrete Institute (ACI 302.2R) and modern flooring manufacturers push toward 10 mil or 15 mil reinforced polyethylene rated Class A per ASTM E1745.

What the vapor barrier does: ground moisture in the sub base rises through concrete as vapor. Water vapor moving up through the slab drives efflorescence (white salt deposits on the surface), delaminates most flooring adhesives, causes cupping in engineered wood floors, and grows mold at the base of wall assemblies. A 4 mil sheet degrades under construction traffic and does not stop the vapor drive; a properly installed 10 mil sheet does.

Install spec that actually holds up:

Roll the sheet directly on top of the compacted stone base, not between two lifts of stone.
Lap all seams 6 inches minimum and tape with butyl or foil backed tape (Poly Guard or Stego Tape).
Extend the sheet up the perimeter form or foundation wall 6 inches minimum, tape to the wall.
Do not puncture the sheet with rebar chairs; use plastic bricks or dobies instead.
Any hole or tear larger than a nickel gets patched with a 12 inch square of the same material and taped four sides before the pour.

Cost adder for upgrading from a code minimum 6 mil sheet to a 15 mil ASTM E1745 Class A sheet is roughly 0.15 to 0.30 dollars per square foot in materials. On a 1,200 square foot conditioned garage slab that is 180 to 360 dollars — a rounding error against the total slab cost and the single easiest upgrade to justify.

Insulation Under the Slab: When NC Code Requires It

The 2018 North Carolina Energy Conservation Code (NCECC section R402) requires a minimum R 10 continuous rigid foam insulation at the perimeter of any heated slab in climate zones 3A and 4A (which covers essentially all of NC). Full slab insulation under the entire floor is not code required for residential construction but is standard on basements, heated garages, and any slab that will bear finish flooring in a conditioned room.

Two typical install patterns:

Perimeter only: 2 inch XPS or high density EPS in a vertical strip along the inside face of the foundation wall, extending down 24 inches or to the footing. Blocks the primary edge heat loss path. Minimum code compliant for most Triangle metro new construction.
Full slab: 2 inch XPS or high density EPS in a horizontal sheet under the entire slab, on top of the compacted stone and under the vapor barrier. Adds R 10 to R 12 across the whole floor, meaningfully improves comfort on basement and heated garage floors, and reduces heat loss to the ground by 20 to 40 percent depending on soil temperature.

Which XPS or EPS grade to use: for below slab applications, specify a foam rated for below grade use with a minimum compressive strength of 25 psi (or 40 psi under a heavier commercial slab). Owens Corning FOAMULAR 250 XPS and Insulfoam GeoFoam EPS39 are two common Triangle metro brands that meet spec. Do not use ordinary construction foam boards from a big box store — they crush under slab loads and lose R value permanently.

Unfinished detached garages, sheds, and utility slabs do not require slab insulation per NC code. If the space is unconditioned and will never be heated or cooled, insulation is a discretionary upgrade for a homeowner planning a future conversion.

Reinforcement: Rebar, Wire Mesh, and Fibers by Slab Thickness

Reinforcement in a slab on grade does two things: it holds shrinkage cracks tight (so they stay hairline instead of opening up) and it distributes point loads across a wider area of the slab. It does not prevent all cracking — concrete cracks; the reinforcement decides whether those cracks stay hairline or become failures.

By slab thickness, current NC best practice:

Slab thickness	Primary reinforcement	Alternate
4 inch residential	6x6 W1.4xW1.4 WWF or #3 rebar @ 24" o.c.	Synthetic macro fibers @ 4-7.5 lbs/cy
5 inch garage / workshop	#4 rebar @ 18" o.c. single mat	6x6 W2.9xW2.9 WWF
6 inch load rated	#4 or #5 rebar @ 12-16" o.c. single mat	Doubled top and bottom mat under lift bays
8 inch commercial	#5 rebar @ 12" o.c. top and bottom mat	Structural fibers plus rebar for hybrid designs

Fiber only slabs — synthetic or steel macro fibers as the only reinforcement — are acceptable for residential 4 inch slabs and light utility floors, but not for slabs that will carry vehicles, tools, or point loads. The tensile capacity of a fiber only slab is a fraction of a rebar reinforced slab of the same thickness. If a builder or crew quotes a fiber only 5 inch garage slab, that is a shortcut; the slab will perform for a few years, then crack diagonally across the panels under repeat vehicle loading.

Rebar placement matters as much as size. The rebar or WWF should sit 2 inches above the sub base for a 4 inch slab (so the steel is in the lower third of the section, resisting bottom tension under load). Wire chairs, plastic bricks, or dobies keep the steel elevated during the pour. Placing rebar directly on the vapor barrier and then walking on it during the pour is one of the most common preventable failures in NC residential concrete work.

Load Table: Real Vehicle and Equipment Weights vs Slab Thickness

The right way to size a slab on grade is to work backward from the intended point loads. The table below covers common residential and light commercial loads seen across the Triangle metro:

Use / equipment	Typical point load	Minimum slab thickness
Passenger car (Honda Civic, Toyota Camry)	800 - 1,000 lbs per tire	4 inch
SUV (Highlander, Explorer, Grand Cherokee)	1,100 - 1,400 lbs per tire	4 inch (5 inch preferred for 2 daily)
Full size pickup (F-150, Silverado, Ram 1500)	1,400 - 1,800 lbs per tire	5 inch
Heavy duty pickup (F-250/F-350, Silverado 2500)	1,800 - 2,600 lbs per tire	5 to 6 inch
Boat + trailer (tandem axle)	2,000 - 3,500 lbs per tire	5 inch, 6 inch if stored loaded
Two post hydraulic lift (10,000 lb rated)	5,000 lbs per column base plate	6 inch reinforced doubled mat
Four post hydraulic lift (12,000 lb rated)	3,000 lbs per column base plate	6 inch reinforced single mat
Compact tractor (25-40 HP)	1,500 - 2,500 lbs per tire	5 to 6 inch
Skid steer / compact loader	3,000 - 5,000 lbs per tire	6 inch minimum
Forklift (3,000-5,000 lb capacity)	3,500 - 6,000 lbs per drive wheel	6 to 8 inch reinforced

The load table is a starting point, not a substitute for a structural engineer on any commercial or industrial slab. For residential homeowners planning a shop or garage upgrade, matching the point load to slab thickness before the pour is what separates a slab that lasts 40 years from one that develops a corner crack under the lift column in year 3.

Cost Table: Slab on Grade Installed Pricing in the Triangle Metro (2026)

The table below is a realistic 2026 installed cost range for slabs on grade in the greater Raleigh Durham metro, assuming ABC stone base over compacted piedmont clay sub grade, standard rebar or WWF reinforcement per thickness, and 3,500 to 4,000 PSI concrete. Vapor barrier and insulation are additive on conditioned interior slabs.

Slab type	Thickness + reinforcement	Installed cost (Triangle range)
Residential patio or sidewalk	4 inch, fiber or light WWF	$6.50 - $9.00 per sq ft
Basement or interior conditioned slab	4 inch, WWF, 10 mil vapor barrier	$7.50 - $10.50 per sq ft
Single car garage or shed slab	4 inch, WWF, thickened perimeter	$7.00 - $10.00 per sq ft
Two car garage or workshop slab	5 inch, #4 rebar @ 18", thickened perimeter	$8.50 - $13.00 per sq ft
Home shop with lift or trailer storage	6 inch, #4 rebar @ 16", grade beam perimeter	$11.00 - $16.50 per sq ft
Light commercial or metal building slab	6 inch, doubled #4 mat, engineered grade beam	$13.00 - $18.50 per sq ft
Slab insulation upgrade (perimeter R 10)	2 inch XPS strip, 24 inch deep	+$1.20 - $2.00 per linear ft
Slab insulation upgrade (full R 10)	2 inch XPS under entire slab	+$2.50 - $4.50 per sq ft
Vapor barrier upgrade (6 mil to 15 mil Class A)	15 mil reinforced poly, taped	+$0.15 - $0.30 per sq ft

The numbers above assume a level or nearly level lot, standard access for concrete trucks, and no unusual excavation or drainage work. Rolling lots, remote pumped pours, and lots requiring significant fill or cut all shift the price upward.

Common Failure Modes We See on NC Slabs on Grade

After 15 years of pours, repairs, and tear outs across the Triangle metro, the same six slab failures appear on nearly every rebuild we do:

Under thickness for the use. A 4 inch slab poured under a two car garage, or a 5 inch slab poured under a shop with a lift. Diagonal cracks appear within 3 to 5 years, right where the vehicle tire path meets the panel edge.
No stone base or under compacted stone base. Slab poured directly on clay, or on 2 inches of loose stone instead of 4 inches compacted. Slab pumps mud through joints, corners settle, and cracks appear at the panel edges within 2 to 4 winters.
No vapor barrier or a 4 mil sheet with holes. Efflorescence bloom on the top of the slab within a year, flooring adhesive failure within 3 years, mold at the base of wall assemblies within 5.
Missing or improperly cut control joints. Joints saw cut too shallow (less than 1/4 of slab depth), or spaced beyond 3 times the slab thickness in feet. Slab picks its own joints, and the joint pattern reads as random cracks radiating from panel corners.
Wrong PSI or wrong slump. 2,500 PSI concrete used where 3,500 was needed, or 6 inch slump concrete used where 4 inch was needed. Surface scaling, low compressive strength, and rebar corrosion within 5 to 10 years.
No cure or cure aborted at 3 days. Curing compound skipped, or plastic sheeting pulled at day 3 instead of day 7. Final strength drops 20 to 30 percent, surface crazes, and the slab develops a permanent chalky top that never seals well.

Every one of those is preventable at install for a fraction of what the repair costs later. This is why we pour slabs on grade on our pay on completion model: Local Concrete funds every load of stone, every yard of concrete, every roll of vapor barrier, and every sheet of insulation up front, and the property owner pays nothing until the slab is placed, cured, and walked. If the slab is wrong, we do it over before we invoice.

Sequencing: The Order a Slab Gets Built Before the Pour

A correctly built slab on grade in North Carolina follows a strict sequence. Skip a step, do them out of order, and the reinforcement or vapor barrier gets buried under stone that then has to be excavated to fix.

Excavate to full depth (slab plus stone base plus insulation if applicable), match the intended cross slope on the sub grade, remove root balls and organic material.
Install perimeter footing or turn down beam if the slab is monolithic with the foundation.
Lay woven geotextile fabric across the entire sub grade, lap seams 12 inches minimum, extend up the perimeter form.
Place and compact ABC stone in 2 to 4 inch lifts to full depth (4 inches residential, 6 inches garage or commercial), plate compactor or vibratory roller to 95 percent standard Proctor.
Install rigid foam insulation if the slab is a conditioned space and the design calls for full slab R value, edges butted tight, taped at seams.
Roll out vapor barrier across the entire slab area, lap and tape seams, extend 6 inches up the perimeter and tape to the form or wall.
Set reinforcement on chairs or dobies at the correct elevation (2 inches above the sub base for a 4 inch slab, roughly middle third of the slab section).
Form the slab edges, set control joint locations by chalk line, install any embeds (anchor bolts, floor drains, plumbing stub ups).
Pour concrete at the design PSI and slump, screed off the forms, bull float, and edge.
Saw cut control joints within 4 to 12 hours of placement, depth of 1/4 the slab thickness, on the pre marked layout.
Cure with plastic sheeting or curing compound for 7 days minimum, protect from foot traffic 24 to 48 hours, protect from vehicle traffic 7 days minimum (28 days for full weight trucks).

Steps 3, 4, 5, and 6 are the ones most often compressed. They are also the four that decide whether the slab lasts 50 years or 5.

Key Takeaways

Match slab thickness to the actual use: 4 inch for residential interior and single car garages, 5 inch for two car garages and workshops, 6 inch for lifts, trucks, and heavy equipment.
North Carolina piedmont clay soil requires 4 to 6 inches of compacted ABC stone over woven geotextile fabric. Skipping the stone base is the single most common preventable slab failure in the Triangle metro.
Any conditioned interior slab needs a vapor barrier (6 mil minimum code, 10 to 15 mil ASTM E1745 Class A is best practice) between the stone base and the concrete.
The 2018 NC Energy Conservation Code requires R 10 continuous rigid foam insulation at the perimeter of any heated slab in climate zones 3A and 4A.
Reinforcement scales with thickness: light WWF or fibers for 4 inch, #4 rebar at 18 inches for 5 inch, #4 or #5 rebar at 12 to 16 inches for 6 inch. Fiber only slabs are not acceptable under vehicles or point loads.
Control joint spacing (in feet) should be no more than 2.5 to 3 times the slab thickness (in inches) per ACI 302.1R. Joints must be saw cut within 4 to 12 hours of the pour.
Total installed cost in the Triangle metro currently runs $6.50 to $10.50 per square foot for a 4 inch residential slab, $8.50 to $13.00 for a 5 inch garage or workshop slab, and $11.00 to $16.50 for a 6 inch load rated slab.
Local Concrete Contractor funds every yard of stone, every yard of concrete, every roll of vapor barrier, and every sheet of insulation up front. Property owners pay nothing until the slab is placed, cured, and walked.

Ready to Spec a Slab for Your Raleigh, Durham, or Triangle Property?

If you are building a garage, a workshop, a home shop with a lift, a metal building slab, or a basement floor anywhere in the greater Raleigh Durham corridor, we can walk the site with you, confirm the intended use, run the load numbers, and price the slab honestly. Pay nothing until the work is complete — Local Concrete Contractor funds every load of material and every hour of labor up front. Call (704) 318-2440 or request a free on site estimate at localconcretecontractor.com.

Authority references: American Concrete Institute ACI 302.1R Guide to Concrete Floor and Slab Construction, concrete.org; American Concrete Institute ACI 302.2R Guide for Concrete Slabs That Receive Moisture Sensitive Flooring Materials; 2018 North Carolina Residential Code section R506 (Concrete Floors on Ground), NC Department of Insurance Office of the State Fire Marshal, ncdoi.gov; 2018 North Carolina Energy Conservation Code section R402 (Building Thermal Envelope); ASTM E1745 Standard Specification for Plastic Water Vapor Retarders Used in Contact with Soil or Granular Fill Under Concrete Slabs, astm.org; USDA Natural Resources Conservation Service Web Soil Survey (Cecil, Appling, Georgeville piedmont clay series), nrcs.usda.gov.