Re-entrant corner
A re-entrant corner is an inward (L-shaped) corner where stress concentrates and shrinkage cracks start; engineers call up diagonal trimmer bars across them.
Ask Chalkline about this →A re-entrant corner is an inward-pointing corner in a slab, wall, or opening: the kind you get on an L-shaped, T-shaped, or U-shaped floor plan, or at the corner of a large opening. It is a corner where the material wraps around a notch rather than around a convex point. Stress concentrates at that inside corner, which is where shrinkage and structural cracks tend to start and then run out across the slab.
This is why engineers reinforce them. On a residential slab, NCC 2022 Housing Provisions clause 4.2.11(4) requires additional reinforcement at every re-entrant corner, and the standard detail is a trimmer bar (or a pair of bars) laid diagonally across the inside corner to carry the concentrated stress and stop a crack opening from the point. The same idea applies wherever a re-entrant corner occurs: framed openings, roof intersections, and masonry all get extra reinforcement or articulation at inside corners.
On a waffle-pod slab the pod layout has to leave a clear zone of concrete for the diagonal trimmer bars at each re-entrant corner, so the bars sit in concrete and not over a pod. The most common defect is simply missing the trimmer: the corner then cracks across the slab within months of the pour. Check the engineer’s plan for a diagonal bar at every inside corner before concrete goes in. See raft footings.
Also known as: Inside corner, internal corner, notch corner.
Category: Structural / Concrete and framing.
Related
See also
References
- ABCB NCC 2022 Housing Provisions clause 4.2.11 (verified 2026-05-10)
- AS 2870:2011 Residential slabs and footings, Standards Australia (verified 2026-05-10)
Last updated: 2026-05-30. Verified: 2026-05-10. Quarterly review for currency.