A shop slab is 4 inches where cars, a workbench and storage live, 5 inches for a half-ton truck and a lift, and 6 inches where equipment, a loaded trailer or a vehicle lift will sit, with the perimeter thickened or turned down to the building supplier's drawing.
Updated November 10, 2026 · DFW Stone & Concrete
The base under it matters more than the inch on top, and the vapor retarder, steel placement and joint grid are what keep it flat and dry.
Guidance from the flatwork trade puts 4 inches as sufficient for passenger vehicles in the 3,000-to-6,000-pound range and 5 or 6 inches for trucks and large RVs, with edges thickened 1 to 2 inches because edges carry more load with less support. Going from 4 to 5 inches can raise load capacity by nearly half for roughly 20 percent more concrete. A pole-building supplier's advice for heavy equipment is the same: 6 inches is more than adequate where equipment is driven and parked, 4 inches for lighter use, and the money goes to site preparation first.
Source: Hansen Pole Buildings, concrete floor thickness for heavy equipment
A metal building's columns and walls bear on the perimeter, so the slab is thickened or turned down there, or set on a separate grade beam, with anchor bolts to the manufacturer's bolt plan. ACI 332, the residential concrete code, separates slab thickness, joints and reinforcement (its slabs-on-ground chapter) from footings and foundation walls, which is the structure a quote should follow: field thickness, edge depth and width, steel in each. Verify the edge detail against the building supplier's foundation drawing and ask whether your city or the building size requires an engineer's sealed design.
Source: ACI 332-20, Code Requirements for Residential Concrete (preview)
The slab lives on its base. Remove topsoil, scarify and condition the clay, place select fill or crushed stone in compacted lifts, and bring the finished floor above surrounding grade so water runs off every side. Fort Worth's parkway standard gives usable language, subgrade compacted to 95 percent of standard Proctor at optimum moisture, that you can ask for on a pad anywhere. On Blackland clay the pad and its drainage are the difference between a flat floor and a floor that rocks.
Source: City of Fort Worth, Parkway Standards
If the shop will be conditioned, floored, or used to store anything that cannot get damp, a vapor retarder goes directly under the slab. The residential code baseline is at least 6 mil between the base and the slab, with exceptions for unenclosed flatwork and small accessory buildings. Trade guidance goes further: place the membrane directly under the concrete rather than under fill (fill traps water and worsens curling), overlap seams at least 6 inches and tape them, turn it up at walls, seal around pipes, and use a 15-mil or heavier ASTM E1745 Class A sheet where ready-mix trucks, pumps, laser screeds or buggies will travel on it. Pick by puncture resistance, not by mil alone.
Source: For Construction Pros, selecting and installing vapor retarders under concrete floors
The code requires reinforcement to be supported so it stays in the top half to top third of the slab throughout the pour, which means bars on chairs tied at crossings, not mesh pulled up with a hook. Fort Worth's #3 at 18 inches on center both ways on 3-to-3½-inch chairs is a representative residential spec; a 6-inch equipment floor or a thickened edge takes larger bars per the engineer. Reinforcement holds a cracked slab together; it does not prevent cracking, which is the joint grid's job.
Source: EVstudio, IRC code requirements for concrete slabs on grade
The NRMCA's Concrete in Practice sheet on slab-on-grade joints sets the rules: contraction joints at 24 to 36 times the slab thickness (about 10 feet on a 4-inch slab), a recommended maximum of 15 feet, panels square or nearly so with length no more than 1½ times width and no L-shapes; joint depth at least one-quarter of the thickness and never less than 1 inch; conventional saw cuts within 4 to 12 hours of finishing, early-entry cuts 1 to 4 hours after. Isolation joints with premolded filler separate the slab from columns and walls. Dowels, carefully aligned, go in construction joints that carry heavy loads.
Source: NRMCA Concrete in Practice CIP 6, joints in concrete slabs on grade
The Portland Cement Association's shorter version: contraction joints at two times the slab thickness in feet (up to 2½ with larger aggregate), and no more than 15 feet apart on thick, heavily loaded slabs. If the quote shows a joint grid that matches either rule, the crew has planned the cracks. If it shows none, ask.
Source: Portland Cement Association, FAQ: joints, what type are used and where
Most strength gain happens in the first month, and the longer concrete is kept moist the stronger it gets. Ask for the curing method (compound or wet cure) and the date the building crew can set columns and the date equipment can roll in. A slab that will be finished inside also needs to dry, which is separate from curing.
Source: Portland Cement Association, curing in construction
For cars, a workbench and storage on a good base, yes. For a lift, a loaded trailer or equipment, go to 5 or 6 and thicken the edges.
Code exempts unenclosed flatwork and small accessory structures; an enclosed shop you might finish later is cheap to protect now with a 10-to-15-mil sheet.
Ask the quote to state it. The building supplier's drawing or an engineer sets it; 3,000 to 4,000 psi is the common residential range, and one fireplace-kit manual on this site specifies 4,000 for its pad.
Pattern, color method, sealer, stone and joint, or field thickness, edge, steel, base and membrane: one independent, insured North Texas contractor looks at the lot and writes it all down with dates.
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