Construction glossary · Concrete and masonry
What is slab on grade in construction?
A slab on grade (SOG) is a concrete slab placed directly on prepared ground, with the soil providing continuous support, so it serves as the building's ground floor rather than spanning between supports. A typical commercial assembly, from the bottom up, is compacted subgrade, a granular base course, a vapor barrier, reinforcement (welded wire mesh, rebar, or fibers), and the concrete slab itself, commonly 4 in to 6 in thick with a troweled finish and a curing application.
Updated June 2026 · Reviewed by the Ruh construction team
Slab-on-grade cross section
Estimating concrete and masonry scope this week? Watch Ruh take off a real plan in 30 minutes.
Book a walkthroughSlab on grade describes any concrete slab that bears continuously on soil or compacted fill instead of spanning between beams or walls. You will see it called out on the foundation plan and in the structural general notes, with the assembly drawn in the typical slab details on the structural sheets and the concrete specified in Division 03 (03 30 00, cast-in-place concrete). The geotechnical report governs what happens below it: subgrade preparation, base course thickness, and compaction requirements, which often hide in Division 31. Concrete subs, GCs, and foundation contractors all price it, and on most commercial jobs it is one of the largest single concrete line items. New estimators make three common mistakes. They price the concrete and forget the layers below it (base, vapor barrier, reinforcement). They miss thickened edges and interior thickenings at column lines and bearing walls, which add real cubic yards. And they skip the small items that add up: joint sawing, curing compound, and slab depressions at tile or terrazzo areas shown only on the architectural finish plans.
The slab is taken off in square feet of plan area, then converted to cubic yards for the concrete buy: area in SF times thickness in feet, divided by 27 (there are 27 CF in 1 CY). A 5 in slab is 0.4167 ft thick, so every 1,000 SF is about 15.4 CY before waste; most estimators carry 5 to 10 percent waste for subgrade irregularity and pump or chute loss. Base course and vapor barrier are measured in SF (add laps for the barrier), mesh in SF with a 10 to 15 percent lap allowance, rebar in pounds or tons, and finishing and curing in SF. Thickened edges and interior thickenings are taken off separately in LF and converted to added CY. Quantities come from the foundation plan; thickness and reinforcing come from the slab schedule or the structural general notes.
Worked example
Take a 10,000 SF warehouse slab on grade, 5 in thick, with illustrative US unit costs. Concrete volume first: 10,000 SF x 0.4167 ft (5 in divided by 12) = 4,167 CF; 4,167 CF / 27 = 154 CY; add 8 percent waste and carry 167 CY. Now price each layer. Compacted stone base, 4 in: 10,000 SF x $0.80 per SF = $8,000. Vapor barrier, 15 mil taped: 10,000 SF x $0.45 per SF = $4,500. Welded wire mesh: 10,000 SF x $0.55 per SF = $5,500. Concrete: 167 CY x $165 per CY = $27,555. Place and finish (laser screed, hard trowel): 10,000 SF x $1.10 per SF = $11,000. Cure and seal: 10,000 SF x $0.25 per SF = $2,500. Total: $59,055, or about $5.91 per SF. The ready mix is under half the total; the layers around it are where new estimators leave money out.
Try Ruh on a real bid. 100% money-back guarantee if you are not satisfied.*
*Scoped delivery, terms apply. Read the guarantee terms
How Ruh handles slab on grade
Ruh reads the foundation plan and typical slab details, measures the slab on grade area, picks up the thickness and each assembly layer (base, vapor barrier, reinforcement, finish, cure), and converts the quantities to SF, CY, and LF. It then prices those quantities against the contractor's own price book, their real unit costs, and hands the estimator a line-item draft of the full slab assembly. The estimator reviews, adjusts waste factors and thickened edge quantities against the subgrade conditions, and signs off.
See concrete estimating softwareSlab on grade: frequently asked questions
Is a slab on grade the same as a foundation?+
Not usually in commercial work. The slab carries floor loads and bears on the soil, while column and wall loads run to separate spread footings or grade beams. The exception is monolithic (thickened edge) construction, common in light commercial, where the slab and footing pour together. Keep slab and foundation concrete as separate line items either way.
How thick is a typical commercial slab on grade?+
Light commercial and office slabs are commonly 4 in, retail and warehouse floors run 5 in to 6 in, and heavy industrial slabs with rack storage or forklift traffic can be 6 in to 8 in or more. The actual thickness is on the structural slab plan or in the general notes, and it can vary by area within one building, so never price from a single assumed thickness.
Does a slab on grade need a vapor barrier?+
Under enclosed, conditioned buildings, yes; specs typically call for a 10 mil or 15 mil polyethylene sheet (often referencing ASTM E1745) directly below the slab, with taped laps and seals at penetrations. Exterior slabs and some unconditioned spaces may omit it. It is a cheap line that gets expensive when missed, since flooring failures over damp slabs end up in disputes.
Still measuring by hand? Your next takeoff can run while you review this one.
See it on your plansRelated terms
Keep going: read the full guide or explore concrete estimating software.
See Ruh price a bid from your own drawings.
AI takeoff and estimating on your price book, your estimator signs off.
Figures on this page are illustrative. Construction estimates depend on project-specific conditions, source documents, market pricing, and professional judgment. Ruh's AI assists the estimator and does not replace professional review: your team reviews, validates, and approves every estimate, bid, and pricing decision.