Slab-on-Grade Estimation — Concrete, Mesh, Vapor Barrier
A slab-on-grade looks like the simplest concrete element, but the takeoff has to account for an uneven subgrade and everything layered beneath it.
Concrete volume
Add 5–10% for an irregular subgrade and edge thickenings. Thickened edges and internal grade beams are computed separately and added.
Reinforcement
- Welded wire mesh: mesh area = slab area × (1 + lap factor). Sheets lap one full square (~150–200 mm) each way, so add ~10–15% to the net area, plus 5% waste. Convert to standard sheet count.
- Rebar mat (heavier slabs): take off both directions at the specified spacing, with laps, like a suspended slab.
- Dowels at construction and expansion joints — count by joint length / dowel spacing.
Under-slab layers
| Layer | Takeoff |
|---|---|
| Vapor barrier (0.15–0.2 mm PE) | slab area × 1.15 for laps and turn-ups |
| Granular sub-base | area × compacted thickness (typ. 100–150 mm) × bulking |
| Sand blinding | area × ~25 mm |
| Termite treatment (if specified) | by treated area |
Joints and finish
Sawcut control joints at ~24–36 × slab thickness spacing — take off by linear metre. Include joint sealant, isolation joint material at columns and walls, curing compound or wet-cure materials, and the specified surface finish (float, trowel, or hardener).
EngEst Pro’s Structural Calculator estimates slab-on-grade concrete with a subgrade tolerance, mesh or rebar with laps, and the vapor barrier and sub-base — all itemised in the BOM with joint materials.
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