Cantilevered Balcony Connection System

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 the element passes through is not a single thing but a set of layers each doing a different job.

At that crossing, several requirements arrive together. Load has to be transferred back into the frame. Heat flow along the projecting element has to be interrupted, or the internal surface near the junction becomes a cold spot with everything that follows from that. 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 nearest sibling is the bracket-supported balcony system, and the two can be told apart from the pavement. Look at what holds the balcony up: if the platform is an extension of the floor slab or of the floor joists, with nothing supporting it from outside, the load path crosses the envelope as a continuous element and this entry applies. If brackets, hangers or columns carry it, the load never crosses the insulated line as one member.

This is also the most familiar instance of the structural thermal break system, which holds the general case of a load-bearing element crossing an insulated line. The balcony is treated separately because the surface build-up, the drainage and the guarding above belong with the connection and are easier to read alongside it than apart from it.

Thermal separation and structural connection are the same detail seen from opposite sides. Reducing heat flow means putting less conductive material into the load path, which changes what the connection can carry and how it is reinforced. Choosing the connection first and asking for a thermal break afterwards usually produces a junction that satisfies neither, because by then there is no longer room to do both.

Continuity is the property that fails here, and it fails quietly. Insulation stopped short at the crossing leaves a path around the break; an air barrier taped up to the element but not joined back to itself leaves a gap that no inspection of the finished elevation will find. Neither shows itself until internal conditions meet a cold surface, and by that stage the layers are buried behind finishes.

The direction the balcony surface falls decides whether the junction is being loaded with water or relieved of it. A surface falling toward the building drives water at the threshold and the crossing continuously. A surface falling away moves the problem to the outer edge, where a drip and a clear soffit have to finish the job, because water shed at the edge that tracks back along the underside still arrives at the wall, only from below.

The threshold competes with everything else at this junction. A low transition from inside to the platform reduces the room available for the water-shedding layer to turn up, at exactly the point where the structure is crossing and the connection hardware sits. Resolving it is a joint decision between the structural, envelope and access requirements, and it is normally among the first things drawn rather than the last.

Guarding closes the loop. Its loads return into the projecting element, so post positions have to suit the structure; its fixings pass through the water-shedding layer, so the same positions have to suit that layer; and where the surface is a bonded finish, those fixings cannot simply be moved later. A change to the guard is therefore a change to the structure and to the water-shedding layer together.