What to Check Before Your Concrete Slab Is Poured: A Homeowner's Pre-Slab Guide

Your concrete slab is quite literally the foundation of your entire home. Once it's poured, there's no going back — any problems underneath are locked in forever. That's why the pre-slab inspection is arguably the most important inspection of your entire build.

Here's what every homeowner should check before that concrete truck arrives.

When to Do This Inspection

The pre-slab inspection happens after:

• Excavation and site preparation is complete
• Footings are dug to the correct depth
• Plumbing rough-in is installed (pipes in the slab)
• Steel reinforcement (rebar and mesh) is placed
• Vapour barrier (plastic membrane) is laid

Your certifier (private or council) must sign off BEFORE any concrete is poured. But don't rely solely on the certifier — they have limited time on site and may miss things.

Steel Reinforcement

This is critical. The steel gives your slab its tensile strength.

• Bar size and spacing: Check against the engineering drawings. Typical residential is N12 bars at 200mm centres, but this varies by soil class and slab design.
• Bar chairs: Steel must be lifted off the ground on bar chairs (plastic supports). The bottom cover should be minimum 40mm for ground contact. If the steel is sitting on the ground, it will rust and fail.
• Lapping: Where bars overlap, the minimum lap length is typically 500mm (40 x bar diameter for N12). They should be tied with wire at the overlap.
• Edge bars and trench mesh: Check that footing trenches have the correct trench mesh or individual bars as per the engineer's design.

Plumbing Rough-In

• Check all pipe positions against the floor plan — once concrete is poured, moving a toilet or shower waste is extremely expensive
• Pipes should be properly supported and not resting on steel reinforcement
• All pipe ends should be capped to prevent concrete entering
• Water test should be completed (plumber fills pipes and checks for leaks under pressure)

Vapour Barrier

• The plastic membrane (typically 200μm polyethylene) should cover the entire slab area
• Joints should overlap by minimum 200mm and be taped
• No tears, holes, or damage — if you can see through the membrane, it's too thin or damaged
• Turned up at edges to prevent moisture wicking up through the slab edge

Formwork & Levels

• Edge formwork (boards around the perimeter) should be straight, level, and at the correct height
• Check the finished floor level (FFL) against the site plan — getting this wrong affects drainage, steps, and compliance
• Step-downs for wet areas (bathrooms, laundries) should be formed correctly (typically 25mm)
• Garage slab should step down from the house slab (check your plans)

Termite Management

Your slab design should include a termite management system. Common options:

• Chemical barrier: Termiticide applied under and around the slab (look for a certificate from the pest controller)
• Physical barrier: Stainless steel mesh or crusite sheeting installed under the slab and turned up at edges
• Combination: Both chemical and physical barriers

Whichever system is specified, verify it's actually installed — this is a common corner-cutting area.

Take Photos

This is crucial: take extensive photos of everything before concrete is poured. Once the concrete goes in, you'll never see any of this again. Photograph:

• All steel reinforcement from multiple angles
• Plumbing pipe locations
• Vapour barrier installation
• Termite barrier installation
• Any areas where you have concerns

HomeOwner Guardian makes it easy to organise your pre-slab photos by stage and tag them for future reference. If a dispute arises later, these photos are your evidence.

Red Flags

Walk away (or call your certifier immediately) if you see:

• Steel sitting directly on the ground with no bar chairs
• Damaged or torn vapour barrier with no repairs
• Pipes without caps or end protection
• The builder pressuring you to approve before you've finished inspecting
• Concrete trucks already on site before the certifier has signed off

Your slab sets the tone for the entire build. A builder who cuts corners here will cut corners everywhere else. Document everything with HomeOwner Guardian and hold your builder accountable from the start.