A retractable roof is a covering that can be rolled or folded aside so a building's interior is exposed to the open air. In a stadium it gives a venue two modes: protected from the weather when it needs to be, and open when it does not.
Why venues build them
Stadium retractable roofs are generally used where bad weather, extreme heat or extreme cold is common during a sport's season. They let traditionally outdoor sports be played in better conditions and keep spectators comfortable.
They are also the answer to a problem the first generation of indoor stadiums created. The domes built mainly in the 1960s, 1970s and early 1980s were permanently closed. A retractable roof also allows the same sports to be played outdoors when the weather is good.
A third reason is the grass. A retractable roof can allow natural grass to grow in places where extreme heat or cold would otherwise make installing and maintaining a natural field too expensive.
How they differ
Retractable roofs vary a great deal in shape, material and movement. American Family Field's roof opens like a fan, while Toyota Stadium in Japan uses one that folds up like an accordion. Most are made of metal, but some, such as State Farm Stadium's, are made of water-resistant fabric.
T-Mobile Park is a special case. Extended, its roof acts like an umbrella over the field and seats, with no side walls, so it never seals the stadium into a climate-controlled space; no other roof of this kind in North America is the same.
The first ones
- Civic Arena, Pittsburgh (1961). The first retractable-roof sports venue. The domed roof spanned 170,000 square feet and was built from eight equal segments; six of them slid back under the remaining two. It has since been demolished.
- Olympic Stadium, Montreal. Planned as the first outdoor retractable-roof stadium for the 1976 Summer Olympics. Construction trouble delayed the roof until 1987, and it could not open until 1988. A permanent fixed roof replaced it in 1998.
- Rod Laver Arena, Melbourne (1988). It opened in January 1988 as Centre Court of the National Tennis Centre, the first Grand Slam venue with a roof that could open. It lets matches continue in rain, extreme heat and bushfire smoke.
- Rogers Centre, Toronto (1989). SkyDome opened with a working retractable roof, a first among North America's major team-sports stadiums.
BC Place in Vancouver shows the other route to a retractable roof: conversion. It was an air-supported domed stadium until the roof was deflated for the last time on 4 May 2010. It was then rebuilt with a cable-supported retractable roof, which was the largest of its type.
Rules of play
In North American major leagues, specific rules govern when a roof may move, and they differ by league and by stadium. In general, if a game starts with the roof open and conditions worsen, the home team may ask for it to be closed, with the approval of the officials and the visiting team. A roof that is closed after a game begins typically has to stay closed.