Tensile Roof in ETFE Cushions
An ETFE cushion does not hold up because it is stretched: it holds up because it is inflated. Its stiffness is air at slight overpressure, which means the roof has a mechanical plant that never switches off — the first roof with an electricity bill.
Overview
TAV. 00ETFE changed the way large spans are covered because it weighs one to two kilos per square metre instead of the twenty-five of glazing: the structure no longer has to carry the envelope's weight, only to resist wind and snow, and that allows grids and sections glass would never permit. The price of that lightness is that the film on its own has no stiffness: it gains it by layering two or more sheets and inflating the cavity between them. Everything else follows — a permanent, redundant inflation plant, dehumidified air so no condensation forms inside the cushion, a control logic that raises the pressure when snow arrives, and drainage in the frame for the water that forms on the inner film. In exchange you get two things glass does not do: it transmits ultraviolet light, so plants grow beneath it, and in a fire it shrinks away, opening a vent that lets the smoke out.
The site score
TAV. HThe detail of the craftthe frame that clamps the edge
The structure and the frame
Erecting the frame structure and the perimeter aluminium extrusions, checking the geometry and the continuity of the profile.
Here the governing load pulls up, it doesn't push down. Look at the anchors, not the beams.site notebook — editorial synthesis
click a beat · arrow keys ← → walk the site · dashed pauses are the chemistry at work
The structural inversion
When the envelope weighs nothing, wind takes over
On a conventional roof the governing load case is weight: structure, covering, snow, everything pushes down, and the anchors work in compression. With an envelope of a kilo and a half per square metre that weight practically disappears, and what governs is wind, which over a curved surface generates suction and pulls upwards. It is a complete inversion: the anchors work in withdrawal, the connections must be checked in tension, and the most delicate stage becomes erection, when the structure does not yet have the loads that will stabilise it.
Load cases and pressure coefficients on light, curved surfaces must come from the wind action standards and, for non-standard geometry, from wind tunnel testing: what is explained here is why the sign of the load reverses.
What glass does not do
ETFE transmits ultraviolet — which is why biospheres exist
Ordinary glass blocks almost all ultraviolet radiation, which is why in a conventional greenhouse certain plants do not complete their cycle. ETFE lets it through, and that opens a use no other transparent envelope allows: covering botanical gardens and biospheres with a controlled internal climate while keeping the natural radiation. The same property, though, has to be read the other way round where there are people and materials below: ultraviolet degrades fabrics, fades finishes and reaches skin as it would outdoors.
Spectral transmission depends on the specific film, its thickness and its print: it must be read from the system's data sheet. Where people spend long periods under the roof, the choice must be assessed together with whoever designs the lighting and comfort.
How it ages (and what betrays it)
TAV. PQuestions from the site
TAV. QHow loud is the rain?
Very loud, and it is the most frequent complaint about these roofs. A film a few hundredths of a millimetre thick under tension is an acoustic membrane: the drop excites it and the cushion radiates the sound downwards. The effect can be reduced — more layers, prints that add surface mass, non-planar geometry — but it cannot be removed, and it is right to say so in advance. In an atrium or a stadium it is not a problem; in a room where people speak or listen to music, ETFE is not the right roof and no measure will make it one.
How long does it last, and what gets replaced?
The film is extremely stable under ultraviolet light — that is why it is used outdoors — and the oldest installations comfortably pass twenty years with no significant loss of clarity. What gets replaced sooner is everything else: the inflation units, their filters, the frame gaskets, the sensors. As with dynamic glass, the real risk is not the material ageing, it is the plant nobody maintains because at handover nobody explained that it existed.
Can it be used at reachable height?
Technically yes, in practice it is a choice to weigh carefully. The film is thin: it resists hail and diffuse impact well, but a deliberate cut opens it, and a cut cushion deflates. The repair is easy and quick, but it has to be done. That is why in projects exposed to the public ETFE is kept at height or behind a guard, and reachable areas are resolved with other materials — multiwall polycarbonate, for instance, which has far greater mechanical resilience at a similar cost.
What happens with snow?
It is the load case that has to be managed with the plant, not only with the geometry. Under snow the internal pressure is raised to increase the cushion's stiffness and prevent a hollow forming in which snow accumulates: once the hollow forms, the load concentrates and feeds itself. In snowy climates two more things are added: geometry curved enough for the snow to slide, and in some cases heating of the inflation air, which warms the film from inside and helps melting. These are design decisions, because they require installed power and cannot be added later.