10 min read · Updated 2026-02-18
Concrete Slab Guide — Foundation Basics for Steel Buildings
How to spec, pour, and cure the concrete slab under your steel building. Thickness, rebar, anchor bolts, and common mistakes.
Key takeaways
- ▸Most steel buildings sit on a 4–6" reinforced slab with turn-down footings.
- ▸Anchor bolt placement is critical — a 1/2" mislocation delays erection by days.
- ▸Cure the slab at least 7 days before erection; 14 is safer.
- ▸Vapor barrier is non-negotiable for any conditioned interior space.
Standard slab thicknesses
Residential garages typically use a 4" slab with #4 rebar on 24" grid. Workshops and light commercial buildings use 5" with #4 on 18" grid. Heavy commercial and industrial use 6"–8" with #5 rebar and often post-tensioning.
Turn-down footings
The perimeter of the slab is thickened to 12"–24" below grade to carry the primary frame loads. Iron Forge foundation drawings show exact turn-down dimensions and rebar layout for your specific building.
Anchor bolts
Anchor bolts are set in wet concrete using a template. A 1/2" mislocation can delay erection by 2–3 days while the crew grinds and re-drills — spend the time to get placement right.
Vapor barrier
A 10-mil vapor barrier under the slab is required for any interior space that will be conditioned or used for storage. Without it, moisture wicks up through the slab and destroys stored items, flooring, and interior air quality.