Class 2 Builder vs General Builder: When the DBP Act Applies
When does NSW building work need a registered Class 2 builder? How the DBP Act works, what it covers, and how to check a builder's registration.
Read the articleConcrete cancer is one of the most common defects we are called to on Sydney strata buildings, and one of the most misunderstood. It usually starts as a rust stain or a hairline crack on a balcony edge. Left alone, it ends with pieces of concrete falling from slab edges, balconies and columns, and a much bigger repair bill. This guide explains what concrete cancer is, how it should be repaired, and what an owners corporation should expect from the process.
What concrete cancer actually is
Concrete cancer is the everyday name for corrosion of the steel reinforcement inside concrete. Healthy concrete protects the steel: it is alkaline, and the layer of concrete over the bars, called the cover, keeps water and salts away from them. When that protection breaks down, the steel starts to rust.
Rust takes up several times the volume of the steel it replaces. As it builds, it pushes against the surrounding concrete until the concrete cracks and breaks away. That breaking away is called spalling, and it exposes more steel to water and air, so the damage speeds up.
Why it is so common in Sydney
- Salt air. Coastal buildings from the Eastern Suburbs to the Northern Beaches are exposed to airborne chlorides, which attack the steel directly.
- Carbonation. Over decades, carbon dioxide in the air reduces the alkalinity of concrete, so older buildings lose their natural protection.
- Thin cover. Many buildings from the 1960s to the 1980s were built with less concrete over the reinforcement than current standards require.
- Water getting in. Failed balcony membranes, blocked drainage and leaking planter boxes keep the concrete wet, which feeds corrosion.
The warning signs
Owners and strata managers should look for:
- Rust stains bleeding from concrete, especially on balcony edges, soffits and columns.
- Cracks that follow the line of the reinforcement.
- Bulging, flaking or missing concrete.
- Concrete that sounds hollow or drummy when tapped.
- Leaks or efflorescence, the white salty deposit, under balconies and slabs.
If you see pieces of concrete coming away, treat it as a safety issue. Fence off the area below and get it inspected quickly.
How a proper repair works
A lasting concrete cancer repair follows a clear sequence. Skipping steps is why so many patch repairs fail within a few years.
- Investigation. A structural or remedial engineer inspects the building, sounds the concrete to map hollow areas, and may test cover depth, carbonation and chloride levels. The result is a report and a repair specification.
- Scope and quote. The builder prices the engineer’s scope, with access, the likely quantity of repairs and provisional sums for areas that can only be measured once the concrete is opened up.
- Access and protection. Scaffolding, a swing stage or elevated work platforms, plus protection for residents, cars and common property.
- Break out. Damaged concrete is removed back past the corroded steel to sound material.
- Treat or replace the steel. Rust is removed, badly corroded bars are replaced or supplemented, and the steel is primed.
- Reinstate. The section is rebuilt with the repair mortar the engineer specifies, then finished to match.
- Protect. Coatings, membranes or sealers are applied to slow the next round of corrosion, and the water source, such as a failed balcony membrane, is fixed.
The last step matters most. A repair that ignores why the steel corroded in the first place will simply move the problem along the slab.
Approvals and the DBP Act
In NSW, apartment buildings are Class 2 buildings, and remedial work on structure or waterproofing generally involves building elements regulated by the Design and Building Practitioners Act 2020. That means the repair design is prepared and declared by a registered design practitioner, and the work is carried out and declared by a registered building practitioner. Before you engage a builder for concrete cancer work on a Class 2 building, check that they hold that registration. Our article on Class 2 builders vs general builders explains the difference.
Inside the strata scheme, the owners corporation is responsible for maintaining and repairing common property, which usually includes the structure and balcony slabs. Major works are typically approved at a general meeting and funded from the capital works fund or a special levy, so the committee needs clear scopes, pricing and a programme to put to owners.
What drives the cost
The cost of concrete cancer repair depends on:
- How much of the concrete is affected, which is only fully known once it is opened up.
- Access: scaffolding, swing stages and traffic or pedestrian management.
- The number of balconies and units involved.
- The engineering requirements, including any steel replacement and protective coatings.
- Associated work such as new balcony membranes, balustrades and finishes.
That is why we price from a defined scope and flag provisional sums upfront, rather than give a headline number that grows on site.
Living in the building while the work happens
Concrete repair is noisy and dusty, but most of it can be done with residents at home. Expect written notices before work starts, agreed access to balconies, set noise windows, protection of common property and regular updates the strata manager can share with owners.
Getting started
If your building is showing the warning signs, start with an engineer’s inspection and report. If you already have a report or scope of works, we can price it, programme it and give your committee the documentation it needs.
Shafbuild is a licensed remedial builder and registered NSW Class 2 Building Practitioner based in Kensington. Learn more about our remedial builders Sydney services, or get in touch to discuss your building.
