Concrete Spalling On Balconies And Facades

A brown stain running down the underside of a balcony is not a paint problem. It is rust, and the rust is coming from inside the concrete.

What is actually happening

Concrete is reinforced with steel bars. The concrete around them is alkaline, and that alkalinity is what stops the steel rusting.

Over decades, water and salt work their way in. The concrete around the steel loses its protection, and the steel begins to corrode.

Rusting steel expands. It can grow to several times its original volume, and it does that inside a slab that has nowhere to give. So the concrete cracks, then lifts, then falls off in pieces.

That is spalling. In strata it usually gets called concrete cancer, which is a fair description of how it behaves: it starts small, it spreads along the bar, and it does not stop on its own.

The warning signs in order

Rust staining. A brown streak on a soffit or a wall. This is the earliest visible sign and the cheapest point to act.

Hairline cracking in a straight line. Cracks that run parallel to each other at even spacing are following the reinforcement underneath.

Drummy concrete. Tap the surface and it sounds hollow. The concrete has separated from the steel but has not fallen yet.

Loose or fallen pieces. Now it is a safety issue as well as a repair.

Balcony edges, soffits and the undersides of slabs go first, because they take the most water and have the least cover over the steel.

Why it accelerates near the coast

Salt in the air does two things. It reaches the steel faster than water alone, and it keeps working once it arrives.

Buildings within a few kilometres of the water see this earlier and more often than buildings inland. Exposed balconies on the weather side of a building are almost always the first to show it.

The repair is not patching

Filling the hole and painting over it fails, usually within a year or two.

A proper repair removes the damaged concrete back to sound material, exposes the steel fully, cleans it back to bare metal, treats it, then rebuilds the section with a repair mortar designed for the job.

If any corroded steel is left buried behind the patch, it keeps expanding and the patch pops off. That is why the cut-out always goes further than the visible damage.

Where a large area is affected, or the bar has lost section, an engineer decides whether the element still has capacity. That is a structural question, not a maintenance one.

Stopping the next one

Waterproofing and drainage are what decide whether it comes back.

A balcony that drains properly and has an intact membrane keeps water away from the slab. A balcony that ponds is feeding water into the concrete every time it rains.

So the repair usually runs with a [balcony tiling and drainage](/blog/balcony-tiling-and-drainage) review and a coating on the exposed faces, because fixing the concrete and leaving the water is doing half the job.

Who pays

Balcony slabs and building facades are almost always common property.

That makes spalling a strata matter rather than a lot owner one, and it usually means a scheduled programme across multiple balconies rather than a single repair. The question of [common property or lot owner](/blog/common-property-or-lot-owner-who-pays) comes up on nearly every one of these jobs.

The honest timing advice

Rust staining is cheap to deal with. Fallen concrete is not.

The gap between those two stages is usually years, which is exactly why it gets deferred. Every year of deferral means more steel affected and a bigger cut-out when it is finally done.

If you can see brown streaks on a soffit, get it looked at while it is still streaks.