How the Mercedes-Benz Stadium roof works: The engineering behind England's World Cup venue

Perhaps the most interesting part of England’s narrow win over Congo DR in the World Cup, was the $1.5 billion structure, that architect HOK describes as a “first-of-its-kind retractable roof”, the game was played under in Atlanta, Georgia.

The Mercedes-Benz Stadium roof works by moving eight triangular steel-and-ETFE “petals” along 16 straight tracks so they open and close like a camera aperture, exposing an oval oculus above the pitch in roughly eight minutes. We reveal how the engineering behind it actually holds together, and what it took to make an oval iris move without failure.

The roof opens like a camera aperture, not a garage door

The roof works on a radial principle: eight triangular petals slide inward and outward along straight tracks, and the offset timing makes them appear to spiral even though each moves in a straight line. Most retractable roofs use one or two large sliding panels that park in unused space beside the stadium. Mercedes-Benz Stadium instead breaks the roof into eight cantilevered petals that retract toward the perimeter, which is why the opening reads as a rotating iris rather than a sliding lid, a space-saving solution documented in detail by Facades+.

The design was inspired by the oculus of the Roman Pantheon. Lead designer Bill Johnson at HOK wanted a single beam of light to fall on the centre of the pitch and widen as the roof opened, as he told Yahoo Sports. The elliptical shape that achieved this is also what made the engineering hard: because the oculus is an oval rather than a circle, the eight petals are four different sizes and travel different distances at different speeds.

The numbers behind the structure

The scale of the structure is the reason the roof needed custom engineering rather than off-the-shelf mechanisation. 

How the roof mechanism actually moves

The petals move on a coordinated system of tracks, drive bogies and synchronised motors, all controlled from a single touchpad. Here is the sequence when an operator opens the roof.

1. A technician presses and holds the “Oculus” touchpad. Yahoo Sports’ walk-through of the control room explains that the operator must keep a finger on the pad for the full opening cycle; breaking contact halts the roof. This is a deliberate safety interlock, not a gimmick.

2. Ninety-six motors engage across the eight petals. Each petal rides on six drive bogies, and each bogie carries two 7.5-horsepower motors, for 96 motors in total, a breakdown captured in SportsField Management’s build report.

3. Each petal accelerates to its own speed. Because the petals are four different sizes covering different track lengths, the control system runs them at different speeds so they stay in sync, the top speed of a petal is roughly 30 feet per minute.

4. The supporting trusses flex under load. As the petals travel, the supporting trusses deflect by around five inches, a movement the structural model had to predict and the connections had to tolerate.

5. The petals lock into a watertight seal. In the closed position the eight petals meet to form a sealed roof; fully open, they leave an oval oculus roughly 120 metres across, according to a technical breakdown by TFC Stadiums.

The mechanisation was designed by Uni-Systems, the structural engineering by BuroHappold working with HOK, and the MEP and fire engineering by WSP, as HOK sets out in its project record. Three separate disciplines had to model the same moving object and agree on how it behaved.

Why the ETFE skin does more than keep the rain out

The roof and facade use ethylene tetrafluoroethylene (ETFE) because glass or solid panels would have turned the enclosed bowl into a greenhouse. ETFE is a lightweight, durable plastic film inflated into multi-layer “pillows,” and the stadium uses more than 143,500 square feet of it in the petals alone, the largest single-membrane ETFE application in the world, HOK notes. Choosing a lightweight membrane reduced the load the moving trusses had to carry, which in turn made the mechanisation feasible.

The facade uses the same material as a “window to the city,” a 16-storey transparent wall on the north and east sides, with a printed frit pattern covering 20% to 70% of the film depending on solar orientation to manage heat and glare, as the Facades+ enclosure analysis details. The roof and skin also help the venue hold LEED Platinum certification, which The Architect’s Newspaper reports was a first for an NFL and MLS stadium.

The people behind the petals

The part of the story that rarely makes the broadcast is how many trades it takes to make an oval iris move without failure. A television camera lingers on the roof spiralling open; it never shows the 32 factories across the US and Canada that were fabricating its steel at the same time, or the more than 18,000 sheets of shop drawings behind them. A roof like this is a coordination problem as much as an engineering one.

What makes it work is a chain of quite different specialists agreeing on the same moving object. The building and infrastructure engineers who resolve how a 720-foot truss carries its load hand off to the fabricators and mechanisation trades who turn drawings into 96 synchronised motors, and then to the control-systems and digital engineers who make eight petals of four different sizes travel at different speeds and still meet in a watertight seal. The same discipline that keeps a stadium roof honest is what keeps safety-critical structures like power and nuclear plants standing.

So when the petals opened over Atlanta on 1 July, that was the visible end of a lot of invisible work. The illusion of a roof turning like a camera lens is, underneath, thousands of people getting the tolerances right.

Work on projects like this

Morson recruits the engineers and skilled trades who deliver landmark structures across the UK and beyond. If the work behind Atlanta’s roof is the kind of thing you want your name on, these are the sectors to start with:

• Building, construction and infrastructure jobs: structural, civil and site delivery roles on major builds.

• Manufacturing and engineering jobs: fabrication, welding and mechanisation.

• IT and digital engineering jobs: control systems, BIM and coordination.

• Power, nuclear and utilities jobs: high-integrity engineering on safety-critical infrastructure.

Or search every live job to find the right role.

This article is part of our ‘How It’s Engineered’ series, written to make the engineering behind the everyday, and the extraordinary, a little clearer.

FAQ

How long does the Mercedes-Benz Stadium roof take to open?

The roof takes about eight minutes to open and around seven minutes to close, controlled from a single touchpad that an operator must hold for the full cycle, as Yahoo Sports documented from inside the venue. For comparison, retractable roofs at AT&T Stadium, Lucas Oil Stadium and State Farm Stadium each take around 12 minutes.

Does the Mercedes-Benz Stadium roof actually spin?

No. The spiralling motion is an optical illusion. The roof is made of eight separate triangular petals that each move in a straight line, offset at 45-degree angles to one another, which together read as a rotating camera aperture, an effect Yahoo Sports explains in detail.

What is the halo board at Mercedes-Benz Stadium?

The halo board is a 360-degree LED video screen that rings the oculus opening. It measures 58 feet high and 1,075 feet in circumference, roughly three football fields long if unrolled, with more than 37 million LEDs, making it the largest video board in professional sport when built, as the Atlanta Journal-Constitution reported. It was manufactured by Daktronics.

Why was ETFE used instead of glass for the roof?

ETFE was used because it is far lighter than glass and lets in daylight without turning the enclosed bowl into a greenhouse. The reduced weight lowered the load on the moving trusses, which made the retractable mechanism practical, and the stadium uses more than 143,500 square feet of ETFE, which HOK records as the largest single-membrane application in the world.

Which engineering firms built the Mercedes-Benz Stadium roof?

HOK led the architecture, BuroHappold provided structural engineering, WSP handled MEP and fire protection, and Uni-Systems designed the roof mechanisation, as HOK’s project record confirms, with the drive system detailed in SportsField Management’s build report. Steel fabrication drew on 32 factories across the US and Canada during peak production.