Well, if the stadium is multi-use venue, a roof can be very useful and this is the case with the Millennium Stadium.
It means the arena can host everything from showcase events like the indoor special Welsh rally stage, boxing matches, cricket and pop concerts without anyone (particularly the spectators) getting wet.
And whilst it cost a cool £121 million to build, it costs just £2.54 (in electricity) to open and takes 20 minutes.
Well, the only other famous stadium with a roof in the UK has got to be Centre Court at Wimbledon.
The cost is so high as a full roof sitting above the huge expanse of a stadium that can move fully in or out is a serious piece of engineering.
The Millennium Stadium roofs has 4,500 components, 3.2km of tensioning cable, 200,000 nuts and bolts, and weighs in at 400 tonnes.
Yes, Cardiff’s Millennium stadium has one, but it’s certainly the exception.
In fact, it’s actually the second largest capacity stadium in the world with a fully-retractable roof.
The rules state that both teams have to agree for the roof to be closed and once it’s closed it can’t be re-opened during the match.But this in itself can cause problems with shadows being cast over half the pitch and so on, but that’s a whole different ball game, so let’s stick to retractable roofs for now.In theory a retractable roof is the best option, as you can host ‘indoor’ events, nurture your hallowed turf for rugby and football and close it to prevent fixtures being cancelled. As we mentioned, Cardiff is one of only a handful in the whole of Europe and it’s certainly the biggest, with its 74,500 capacity.So without that unanimous agreement t was an open, not shut case this time.As for why Wales wanted the roof closed, well, it was apparently a question of ambiance and atmosphere for the match, allegedly so the Welsh rugby fans could intimidate the away fans with their singing!