What's the code minimum for a slab on grade inside a house: thickness, base, and what goes under it?

IRC R506 (2021/2024) governs residential slabs-on-grade.

Thickness: 3½ inches minimum (R506.1). Exception: 3 inches allowed for garage floors only if no vehicle loads (rarely practical).

Base Course (Capillary Break): 4 inches minimum of clean, graded crushed stone, gravel, or crushed blast-furnace slag (R506.2.1). Material must pass a 2-inch sieve; ≤5% passing a No. 4 sieve. Compacted.

Vapor Retarder: 6-mil (0.006 in.) polyethylene or equivalent Class A vapor retarder (ASTM E1745) required under slab in heated/cooled spaces (R506.2.3).

• Lap joints 6 inches minimum, taped.
• Seal all penetrations (pipes, columns).
• Place directly under slab (on top of base course)—not under the base. ACI 302.1R recommends this to prevent blotching/curling; IRC allows it above or below base, but above is standard practice.

Reinforcement: Not required by code for non-structural slabs, but #3 rebar at 18" o.c. each way or 6x6-W2.9xW2.9 WWF is standard for crack control. Support on chairs—not pulled up during pour.

Prep: Subgrade uniform, compacted, free of organics/debris (R506.2). Dampen base before pour to prevent rapid moisture loss.

Garage note: Slab must slope ⅛"/ft minimum toward door or drain (R309.1).

Commercial/structural slabs follow ACI 318/360—different rules.

For a slab-on-grade inside a house (interior residential floor), the code minimum is governed primarily by the International Residential Code (IRC), specifically Section R506 (Concrete Floor Slabs on Ground).

Code Minimum Requirements (IRC R506):

1. Slab Thickness

• Minimum thickness: 3.5 inches (IRC R506.2.1) Exception: For garages, patios, walks, and driveways: 3.5 inches minimum. Note: For interior living spaces, 4 inches is common practice and often required by local amendments or structural design (especially if supporting load-bearing walls or heavy finishes). Always check local jurisdiction — many require 4" minimum for habitable areas.

2. Base (Subbase)

• Minimum 4 inches of granular fill (gravel, crushed stone, or sand) compacted (IRC R506.2.2) Purpose: Provides drainage, prevents capillary rise of moisture, and offers uniform support. Material: Must be clean, well-graded aggregate (e.g., #57 stone) or sand. Avoid expansive soils or organic material. Compaction: Must be compacted to at least 95% standard Proctor density (per IRC R403.1.1.1, referenced in foundation prep).

3. What Goes Under It (Vapor Retarder)

• 6-mil polyethylene vapor retarder (or equivalent) placed directly under the slab (IRC R506.2.3) Placement: Over the compacted base, under the slab. Seams: Must be lapped at least 6 inches and sealed or taped (IRC R506.2.3.1). Penetrations: Around pipes, columns, etc., must be sealed to maintain continuity. Exception: Not required in detached garages, accessory structures, or unheated storage areas (IRC R506.2.3 Exception).

Additional Notes:

• Insulation: Not required by IRC for interior slabs unless in Climate Zones 4–8 and slab is heated (then R-10 insulation at slab edge per R402.2.11). Unheated interior slabs typically don’t require insulation unless local energy code mandates it (e.g., IECC).
• Reinforcement: Not code-minimum for thickness ≤5", but welded wire mesh (6x6 W1.4xW1.4) or rebar grid is strongly recommended to control cracking — especially over 100 sq ft or where cracks would be objectionable.
• Joints: Control joints required at intervals not exceeding 24–36 times slab thickness (e.g., 8–10 ft for 4" slab) per ACI 302.1R, though IRC doesn’t specify — follow concrete contractor best practices.

Practical Recommendation:

For interior residential slabs: 4" thick concrete over **4" compact