Why a pre-slab inspection matters: one Glen Waverley site lesson

What our inspector saw before the pour

The client had engaged us for an independent construction-stage inspection of a new French Provincial-style house in Glen Waverley. Houses like this are common in the area: big façades, detailed mouldings and high ceilings.

At slab stage none of that tells you whether the hidden work is right. The reinforcement, membrane and under-slab services were still exposed. At one point, the reinforcing steel was too close to a PVC sanitary drainage pipe where it mattered structurally. There was not enough room for our inspector to be satisfied that the pipe had the separation, protection and allowance for movement that the detail needed.

Our inspector raised it before the pour.

The builder said he had been building luxury homes for 30 years, had never heard of this problem and had followed the drawings. The drawings did not say the steel had to be moved, so he asked to be shown the requirement.

That was a reasonable request. An inspector should be able to explain the basis for any significant finding. Our inspector showed him the relevant slab and footing requirements and how they work together with the drainage requirements. The answer was not on one line of the structural plan. It came from reading the structural detail, the relevant Australian Standards and the plumbing installation together.

The concern was movement, not an immediate leak

We were not saying the steel would cut the pipe as soon as the concrete set. We were saying the pipe and steel were touching, or nearly touching, with very little room for the movement every house goes through.

Concrete shrinks. Slabs and footings move a little. Changes in soil moisture and settlement after building can also move foundations. The Building and Plumbing Commission says some footing movement is normal, and that foundation movement can be caused by soil settling after a house is built.1

Where a pipe runs near or through structural concrete, its location, support, protection and room to move all matter. The Australian standard for residential slabs and footings deals with services passing through beams and footings, and the sanitary drainage standard deals with pipes near and through structural elements.23 A South Australian government plumbing advisory makes the same general point: pipework must be protected from mechanical damage, properly supported and kept out of structural concrete unless the structural design allows for it.4

If steel, concrete and PVC are locked together in the wrong place, later movement can put concentrated load on the pipe. That does not mean it will fail. It is a risk that costs little to check before the pour and a lot to reach afterwards.

If a drain under a finished house cracks or distorts, the people living there may not see where the problem is coming from. Finding and fixing it can mean lifting floors and saw-cutting the slab. Before the pour, the builder, plumber and engineer can sort out the same detail while everything is still exposed.

The drawings are not the only rules that apply

“It is not on the drawing” sounds convincing, because builders are expected to follow the approved plans. They are. But Victorian domestic building work is not governed by drawings alone. Consumer Affairs Victoria explains that the implied warranties require work to follow the plans and specifications in the contract and also to comply with all laws and legal requirements.5

For a house, the full set of requirements can include:

• the building permit and endorsed drawings;
• the engineer’s design, notes and approved revisions;
• the edition of the NCC that applies to the project;
• Australian Standards called up by the NCC or the project documents;
• plumbing and drainage requirements;
• manufacturer installation instructions;
• the building contract and specifications;
• written directions, approved variations and site-specific engineering advice.

No drawing set can repeat every rule from every trade standard. A structural drawing may show where the steel goes and a plumbing layout may show where the drain goes. Problems tend to appear where the two meet, which is why someone needs to look at the work itself.

Thirty years is a long time, but it may not be many houses

The builder told us he had been building for 30 years. He also said he usually finished about one of these houses a year. That is roughly 30 houses.

Thirty houses is real experience. But “30 years” can sound like much more work than it is. Years in the industry, number of projects, variety of sites and how often someone has been corrected are different things.

Experience only improves the work when the builder hears about problems. If owners assume an expensive builder does not need an independent inspection, a detail like this may never be questioned. If a hidden pipe starts leaking years later, the builder may never find out.

An error does not become correct because it has been repeated for 30 years.

Codes and standards also change. A better question than “How long have you been building?” is “Which requirement applies to this detail on this project, and what shows the work meets it?”

Premium homes still need inspecting

“Luxury” describes the price and the finishes. It says nothing about whether the slab is right. Buyers can see stone, joinery and high ceilings, but nobody can see the under-slab drainage once the concrete is poured.

A large custom home can actually have more to coordinate: more service penetrations, more wet areas, more structural changes and more details that are not in a standard display-home design. An independent inspection does not assume the builder is bad. It checks the work on this site before it is covered.

What the mandatory inspection does and does not tell you

A member of the Home Audit team previously worked as an inspector within a Victorian building surveying practice, and was routinely expected to complete more than ten new-home inspections in a single day. Travel, access, photos, document checks and reports all had to fit into that day. The employee attended the sites, and the principal of the practice signed the inspection documents.

Not every building surveying practice works that way, and mandatory inspections have an important role. But a signature does not tell an owner how long was spent on site, what was checked, what could not be seen, or whether the person who signed ever saw the slab.

Consumer Affairs Victoria makes a similar distinction: building surveyors inspect for compliance with the Building Act and building regulations, but do not supervise the work or check it against the building contract. It also says owners can engage an independent building consultant before making stage payments.67

The Building and Plumbing Commission’s practice note on mandatory inspections says such an inspection should check the work against the Act, regulations and permit up to that stage, not just one element, and that the relevant building surveyor stays responsible when someone else inspects for them.8

The two inspections do different jobs, and one does not replace the other. For the arguments owners often hear against an independent inspection, see what builders may tell owners about independent stage inspections.

What to arrange before a slab pour

A pre-slab inspection needs time to happen. It cannot be done once the concrete truck has arrived or the steel and services are covered. Before the pour, check that:

1. The stage is ready. Excavation, formwork or pods, reinforcement, membrane and the relevant services should be far enough along to inspect.
2. The documents are available. Give the inspector the structural drawings, slab design, soil report, architectural plans, any revisions and whatever service drawings you have.
3. Access is arranged. Agree on notice, induction, PPE and attendance with the builder beforehand.
4. The work stays uncovered. Make sure the builder knows an inspection is booked and will not pour or cover the work first.
5. There is a way to respond to findings. Ask who will review the report, how disputed items will be answered and whether the engineer or plumber needs to confirm anything in writing.
6. Fixes are checked. A promise to fix something is not the same as seeing it fixed. For important items, ask for photos, documents or a reinspection before the pour.

An independent inspector does not redesign the slab, certify the plumbing or replace the building surveyor, engineer, plumber or builder. The inspector records what can be seen, explains any concern and says when the right practitioner needs to confirm or correct a detail.

Check the slab before it disappears

What made this inspection useful was the timing. The pipe and steel were still visible, the builder could ask for the requirement, our inspector could show it, and the detail could be sorted out before the concrete went in.

Years in business, an expensive contract and approved drawings all count for something. They are not a substitute for looking at what has been built.