Slab-on-grade thickness: 4 inches nominal for residential floors / driveways / garages
HUD RSDG §4.6Description
Standard residential floor slabs, driveways, garage floors, and sidewalks are built at a nominal 4 inches thick per ACI 302. Where interior columns or load-bearing walls bear on the slab, thicken locally; monolithic slabs can have thickened edges as integral footings. Slabs > 10 ft in any dimension experience temperature/shrinkage cracking — specify WWF or fiber reinforcement and place control joints at 8-12 ft spacing.
Why this exists
4" is the residential workhorse — adequate for typical IRC loading and soil. Thicker locally where loads concentrate. Crack control is mandatory above 10 ft: cracks WILL happen, the design choice is where they appear.
Measurements
| Property | Operator | Value | Unit | Note |
|---|---|---|---|---|
slabThickness | min | 4 | in | Nominal residential slab thickness |
crackingThreshold | max | 10 | ft | Slab dimension above which crack control is required |
controlJointSpacing | max | 12 | ft | Maximum control-joint spacing |
Categories
Source
Solver enforcement
Browsable only — the solver does not currently enforce this directive (no spec-level data to check against). This entry exists so the architect personas can cite it in conversation and the user can read what the rule says.
Related directives
- Continuous load path from roof to foundation · HUD RSDG §2.4
- Residential structural reliability targets 1-in-100 to 1-in-1000 annual probability of failure · HUD RSDG §2.5
- Residential floor live load: 40 psf minimum (30 psf sleeping rooms) · HUD RSDG §3.4
- Wind load design uses ASCE 7 basic wind speed for the locality · HUD RSDG §3.6
- Ground snow load for Virginia: 25 psf eastern, up to 40 psf western mountains · HUD RSDG §3.7
Last reviewed 2026-05-14.