The design intent was to look for a maximum of visual transparency and lightness, in order to preserve the interior arena "cavea" space intact, and to reduce as much as possiible the impact of the visible structure from the outside. The adopted solution was to cover the whole arena area with a “Geiger” system (a «Tensegrity» type structure). A radial truss compression ring provides a neutral forces balance and the required bracing. A single membrane of ETFE (Ethylene Tetra Fluorine Ethylene) up to 95% transparent, provides the weather waterproofing, without compromising the acoustic venue performances. The choice of the Geiger system, radial (rather than Fullerian, spatial) allows to follow the arena geometry and to underline the concentric oval rings by means of more visual structural elements (the struts « floating »). Some of the south side roof panels are openable, to allow the entry of large-format scenes using a crane. A constant natural ventilation, as well the smoke exaust in case of fire, will be provided at all time by the opening of the perimeter truss ring, by the arches of the Arena, and by the overlap of the roof ETFE rings. The rain water are quickly directed to the radial channels (avoiding also the runoff) and collected on a main channel ring. The ETFE membrane used in single film has no thermal properties, and avoids the possible greenhouse effect, allowing UV rays to pass. The ETFE doesn’t change colour over the years. Its surface is practically self-cleaning,