Covering a stadium is one of the more difficult problems in large-scale construction. The difficulty is not the size of the roof but the absence of anywhere to support it.
Nothing can stand in the middle
A roof over a stadium cannot use internal columns, because any support inside the bowl would block sightlines for a large number of seats.
The entire structure must therefore be carried at its edges, which means every load travels outward to the rim before reaching the ground.
Spans of that size behave quite differently from ordinary building structures, and the engineering is closer to bridge design than to conventional roofing.
Wind governs the design
A large roof surface is essentially a wing, and wind passing over it generates uplift that can exceed the weight of the structure itself.
Designs are therefore checked against suction as well as loading, and anchorage against being lifted is often the governing condition.
Wind tunnel testing on scale models is standard practice, because airflow around a bowl with open ends is difficult to predict by calculation alone.
The pitch still needs light
Grass requires sunlight, so a roof that covers the seating usually leaves the playing area open or uses translucent material above it.
Translucent panels admit light but also trap heat and restrict airflow, which creates a humid microclimate that grass tolerates poorly.
Fully closing a stadium generally means accepting a hybrid or artificial surface, or investing in the lighting rigs and ventilation needed to sustain natural grass.
Retractable roofs multiply the problems
A moving roof must carry its own weight while travelling, which means the supporting structure is designed for the load in every intermediate position.
Seals, drainage and the mechanical drive all require maintenance, and a system used infrequently is more prone to problems than one operated regularly.
Operators consequently open and close them on a schedule rather than only when the weather demands, simply to keep the mechanism exercised.
Acoustics change with the cover
A roof reflects crowd noise back into the bowl instead of letting it escape, which is why covered stadiums are noticeably louder than open ones.
That effect is often intentional, and the underside is shaped and surfaced to direct sound toward the pitch rather than to absorb it.
The same reflection complicates public address and broadcast audio, so sound systems in covered venues are designed alongside the roof rather than fitted afterwards.
