A steel floor platform, made up of perforated metal decking over lightweight structural steel, provides a base for the pavilion at the same level as the Serpentine Gallery floor. The 30m-diameter balloon sits on a 5m-high polycarbonate drum, made up of two circular walls of polycarbonate panels, 1.6m apart. The café and all the infrastructure and services sit within the void of the wall, which is made up of 55 segments. A bespoke polycarbonate mullion assembly, located at approximately 1.2m centres, provides the support for the open-cell polycarbonate panels that form the wall.
Hidden within the wall is the pavilion's primary structure, which consists of four 305mm steel columns on screw piles, designed to restrain the balloon's three-tonne uplift. The four columns support a ring-beam, also located within the translucent wall cavity, which acts as a bumper for the balloon when it is in the down position.
The ring-beam has 55 short arms, or brackets, which support cables that run through each polycarbonate mullion and maintain the wall's vertical position. Arup Associate Carolina Bartram explains that cables were chosen because they would be the least visible through the polycarbonate wall, which is illuminated from within at night.
Other details were adapted from the worlds of ballooning, airships and sailing to enable the roof canopy to be raised and lowered. Rotating circular pulleys found on boats, called sheaves, are used to guide the ropes up and down. A 'flower pot' detail, a four-sided steel bucket lined with nylon, was developed for the tethering point at the top of the four columns to guide the cable into place during windy conditions. Four synchronised winches on electric switches are used to move the canopy up and down, which takes about one minute. Seven stays, tied to Platipus anchors, provide additional restraint for the canopy, spreading the load so that reinforcing patches on the canopy could be eliminated.
The fabric, selected to maximise translucency, is a fine polyester mesh, laminated between two layers of clear PVC.
The location of seams in the fabric and the cutting pattern were established using the 3D model. Inside the canopy, a clear horizontal PVC membrane, fixed to the canopy's interior surface using highfrequency welding, separates the helium in the top from the air below. Because the sun makes the helium expand, the bottom compartment is vented to maintain a constant internal pressure that keeps everything taut. The canopy does not leak, but the balloon exterior slowly transmits helium, and it is anticipated that the canopy will need to be topped up about once a month. All the pumps, as well as lighting, are in the void between the walls.
A 10mless than or equal to aluminium frame is recessed into the underside of the roof canopy to create an enclosure for the space below. The ceiling of this recess is made of ETFE to provide a clear window into the underside of the canopy. As the internal pressure of the balloon will cause the ETFE to bulge down, a system of 25mm polyester belts is located on approximately 900mm centres to hold the ETFE in place.
The vertical faces of the ceiling recess are printed with graphics by artist Thomas Demand.