How a Monolithic Slab with Thickened Edge Works

A monolithic slab (also called a turn-down edge slab) combines the structural footing and the floor slab into a single poured element. This eliminates the need for separate trench footing pours, stem walls, and subsequent slab pours:

• Slab Deck: The interior floor area, typically 4 inches thick for residential workshops or 5 to 6 inches for heavy vehicle garages.
• Perimeter Turn-Down Footing: The outer edge is dug deeper (typically 12" to 24" total depth) and wider (typically 12" wide) to bear exterior wall and roof loads directly into load-bearing soil below the local frost depth.

Calculation Methodology

To prevent double-counting the deck thickness at the perimeter, this calculator computes:

1. Deck Volume = Slab Length × Slab Width × Slab Thickness
2. Centerline Perimeter = 2 × (Length + Width) - 4 × (Edge Width)
3. Net Turn-Down Depth = Total Edge Depth - Slab Thickness
4. Turn-Down Footing Volume = Centerline Perimeter × Edge Width × Net Turn-Down Depth
5. Total Volume = (Deck Volume + Turn-Down Volume) × (1 + Waste %)