The vapour barrier and steel cover under a new slab
A Maddingley pre-pour inspection found the vapour barrier stopping short of edge beams and steel without enough cover. Why it matters on cold, wet ground.
In short: Once concrete is placed, the membrane underneath and the steel position are hidden for good. The pre-pour visit is the only check.
What the barrier does
A vapour barrier under a slab limits ground moisture rising into the concrete and the floor coverings above. It has to run under the whole slab and down into the edge and internal beams.
What we found
On a slab inspection in Maddingley in December 2025 the barrier did not extend under the entire slab and all the beams as required. Elsewhere on the same job the steel sat too close to the form face and the mesh needed more bar chairs.
Cover and chairs
Concrete cover protects reinforcement from moisture and corrosion. Bar chairs hold the mesh at the right height while concrete is placed. Too few chairs and the mesh is walked down into the bottom of the slab.
Why it matters on the Ballarat side of the ranges
New estates in Lucas, Alfredton and Delacombe are built through long wet winters. Ground that stays damp for months puts more demand on the barrier and the concrete cover than a dry site does.
Timing
The slab inspection happens after the steel, pods and barrier are in and before the pour is booked. Ask your builder for the pour date as early as possible.
What happens next
The builder fixes the items before concrete. We can return to check, or the builder can send photos for review where practical.
Written by our local inspection team from work carried out in Ballarat. Licensed building practitioners, Building & Plumbing Commission registration CDB-U 53425.
