Choosing How a Balcony Is Held Up

A cantilevered balcony is a projection of the floor structure itself. Whatever the floor is made of continues outward past the line of the wall, and the platform outside is the same element as the floor inside. That continuity is the point of the arrangement structurally, and it is also the difficulty, because the wall it passes through is not a single thing but a set of layers each doing a different job.

In a bracket-supported balcony the platform does not come out of the building. It is hung on the wall, propped against it, or stood beside it on its own columns, which removes the continuous element passing through the insulated line and replaces it with a set of discrete connections.

At a cantilever crossing, several requirements arrive together. Load has to be transferred back into the frame, heat flow along the projecting element has to be interrupted, and the insulation, the air barrier and the water-shedding layers all have to continue past the crossing without a break. Solving any of these in isolation tends to compromise the others.

The entry names the usual failure of process rather than of product: choosing the connection first and asking for a thermal break afterwards generally produces a junction that satisfies neither, because by then there is no longer room to do both. Reducing heat flow means putting less conductive material into the load path, which changes what the connection can carry.

The substitution is the whole design. Each connection has to find something structural behind the finish to fix into, has to pass through whatever controls water and air at that point, and has to stay protected against corrosion in a position nobody will look at again.

The number and position of penetrations then becomes the variable everything else follows from. Each bracket has to land on structure, and structure sits where the frame is rather than where the elevation would prefer the balcony to be. Moving a bracket to suit the elevation takes the fixing off structure; keeping it on structure moves the platform.

Sealing and drainage at each fixing are the same problem rather than separate ones. A closed penetration relies on water not standing against it, so the platform has to shed water away from the wall and the wall has to shed water clear of the fixings.

Where the frame sits close against the elevation, water running down the wall reaches the fixings first and stays there, loading the closure continuously instead of occasionally. The entry names the corresponding misunderstanding: closing a penetration is treated as the answer, when keeping water away from it is what allows that closure to last.

A projecting element is exposed on every face while the floor inside is not, so it does not move uniformly along its length, and where the surface build-up, the finish and the sealed joints accommodate that difference is a design question.

An externally supported balcony and the building move independently, and the entry makes the decision explicit: whether the connection is meant to hold them rigidly together or to allow relative movement has to be decided rather than left to the detail. Holding the two rigidly at several points removes that freedom, and the movement then appears wherever the assembly is weakest.

On a cantilever, connection hardware sits inside the build-up and cannot be inspected once the junction is closed. Its protection against corrosion, and the detailing that keeps water away from it, are settled at design stage and rarely revisited.

On a bracket arrangement, the fixings sit inside the wall build-up where they cannot be seen, at a point wetted by whatever gets past the closure, and often in contact with a different metal in the bracket. Breaks in a protective coating at cut edges and drilled holes are the usual starting point, and the only later evidence that the decision was wrong is movement or staining appearing at the platform.

The exchange is visible again here. Being outside the wall, bracket supports can be inspected, but only if someone can reach them, and access at height and who arranges it belongs in the handover rather than being discovered long afterwards. The arrangement also permits an open platform, leaving the frame and the fixings visible from below.

A cantilever leaves less to see and more to record. What was actually installed at the crossing is seldom visible afterwards, so recording the connection type, the break, the layer arrangement and the fixing positions is the only way a later alteration can proceed without opening the junction up.