Why does water get in where the balcony meets the wall?

Water gets in where the balcony meets the wall because that line is a cold joint, and cold joints leak. The wall bears on the balcony slab, so the two were poured at different times and never bonded into one piece. A coved bead of the right sealant, inspected and replaced as it ages, keeps water out of that seam for a fraction of what the repair costs once it has been left.

What makes the balcony-to-wall junction a cold joint?

A cold joint is the seam that forms where fresh concrete is poured against concrete that has already begun to set, and it is a common water path because the two pours never fully bond. At a balcony, that seam runs along the base of the wall for the full width of the slab.

The reason is structural. A balcony slab does not begin at the building line. It starts inside the suite and continues outward, with most of the slab inside and a smaller portion projecting beyond the wall. That is what allows the balcony to cantilever. The exterior wall then sits on top of that slab.

So the wall is quite literally standing on the balcony floor. Where it lands, there is a joint, and that joint holds moisture.

What does the inexpensive version of this repair look like?

The preventive detail is a coved bead of specialty sealant where the balcony floor meets the wall, applied well and replaced as it ages. Coving means the sealant is tooled into a rounded fillet across the corner rather than pressed flat into a square angle, because a curved transition sheds water and does not tear under movement.

Neither the material nor the labour is expensive. This belongs on the maintenance schedule alongside every other exterior sealant on the building: inspect it, and reapply or replace it as needed.

That is the whole argument here. A maintained bead costs a fraction of the repairs below, and it is the one that gets skipped.

What happens when the joint is left alone?

Once the sealant has failed and water has been in the seam for years, the joint stops being a caulking item and becomes a waterproofing repair. At that point the balcony is treated as a cold joint properly, and the build-up runs in three stages: fibreglass applied up the wall as an upturn, sealer injected into the joint itself to fill the seam rather than cover it, then fibreglass carried down from the wall and tied into the floor so wall, corner and slab are one continuous sealed surface.

That is real work at real cost, normally done as part of a larger balcony restoration once there is already significant damage.

The blunt version: apply and maintain the caulking and you never need the fibreglass. Those two paragraphs describe the same joint at two very different prices.

On a residential balcony project in Hamilton the defects were exactly this pair, cracks in the slab plus cold joints where the balcony floor met the wall, repaired with an industrial-strength adhesive, crack filler, fibreglass fleece, a waterproofing coating and a polyaspartic base and top coat with non-slip aggregate.

Why does a cold joint hold water rather than simply leaking?

A cold joint at a balcony holds water because the seam is enclosed on both sides, slab below and wall above, with nowhere for trapped moisture to evaporate to. It behaves the way a badly coated balcony slab does: water gets in, cannot get out, freezes, and works the seam wider every winter.

Water in that joint also travels. The seam runs under the wall and into the building line, which is why junction leaks are frequently reported as interior complaints rather than balcony complaints, and why the source is often missed on a first inspection.

What should a property manager do about the perimeter joint?

Put balcony perimeter caulking on the maintenance schedule rather than the capital schedule, and inspect it on the same cycle as the rest of the building’s sealants. You are looking for sealant that has hardened, cracked, pulled away from either face, or gone missing in sections.

Where the bead is gone and the joint is open, have it assessed before scoping a full balcony programme, because whether the building is buying sealant or buying fibreglass and injection changes the number substantially, and it changes per balcony rather than per building.

Scope the coating on the slab and the outer edge at the same time. Our balcony questions page, the balcony services page and the cold joint repair page set out how the phases fit together.

Start by checking the perimeter bead on a handful of units across different elevations, running a fingernail along it to see whether it has gone hard or lost adhesion. Fifteen minutes tells you whether this is still a maintenance item. We will walk a sample with you and mark which junctions have moved past that point.