Waffle Slab Calculator

How it works

module = (pan + rib)² · voids per m² = pan² × pan depth ÷ module · concrete = ribbed area × (pan depth + topping − voids per m²) + solid area × total depth · pans = ⌈ribbed area ÷ module⌉

A waffle slab (losa reticular, two-way joist slab) replaces most of a thick solid slab with a grid of ribs, using reusable pans or lost polystyrene or block void formers (casetones). Each module is the pan plus one rib, so the concrete per square metre is the full depth minus the void volume. Solid zones around columns and along edge beams are added at full depth. ACI 318 (§9.8) treats it as joist construction when ribs are at least 4″ (10 cm) wide, no deeper than 3.5 times their width, spaced no more than 30″ (76 cm) clear, with a topping at least 2″ (5 cm) and 1/12 of the clear spacing. Reinforcement must come from a structural design.

Worked example

An 8 × 6 m (48.0 m²) slab with 60 × 60 × 25 cm pans, 15 cm ribs and a 5 cm topping: each 0.75 × 0.75 m module leaves 0.16 m³ of void per m², so the ribbed 90% (43.2 m²) takes 43.2 × (0.30 − 0.16) = 6.05 m³ and the solid 10% takes 4.8 × 0.30 = 1.44 m³ — 7.49 m³ of concrete (7.86 m³ with 5% waste). That is an equivalent 15.6 cm solid thickness, 48% less than a solid 30 cm slab, and it needs 77 pans.

Sources: ACI 318-19 — Building Code Requirements for Structural Concrete, §9.8 joist systems · CRSI — Design of two-way joist (waffle) slabs

Frequently asked questions