Edge thickening | Chalkline

Edge thickening is the increased depth of a residential slab at its perimeter and under load-bearing internal walls, forming integral edge beams (and internal stiffening beams) that resist differential ground movement. It is the defining feature of a stiffened raft slab, the dominant residential foundation system in Australia.

The edge thickening replaces a separate strip footing: instead of pouring a footing first and then placing a slab on top, the perimeter is cast monolithically with the panel. The resulting “L” or “T” cross-section at the perimeter carries the load-bearing wall above and ties the slab into the ground.

AS 2870:2011 sets the minimum depth and reinforcement schedule for the edge beam by site classification (the geotech classes from A through P). Common depths run roughly:

Class A and S: 300 to 400 mm edge beam, standard reinforcement
Class M (most metro sites): 400 to 500 mm
Class H1 and H2 (reactive clay): often 600 mm or more, with engineer-specified reinforcement

Panel thickness above the edge beam is generally 85 to 110 mm. Concrete is N20 minimum per ABCB Housing Provisions 4.2; N25 or N32 is common on reactive sites.

Three common defects show up at the pre-pour slab inspection:

Skewed reinforcement: the bottom reo cage drops below the engineer’s specified cover, especially at the corner where the edge beam meets the panel. Plastic chairs or stand-offs must hold cover before the pour.
Missed beams at junctions: an internal load-bearing wall lands without an internal stiffening beam below, or the edge beam is interrupted at an opening. The engineer’s drawings dictate; the concretor sets out.
Wrong depth: edge beam excavated to a Class A depth on a Class M site, or the bottom of the beam scraped clean of compacted bedding by the excavator.

Also known as: edge beam, perimeter beam, slab edge beam.

Category: Footings / slab construction.

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