http://www.wembleystadium.com/brilliantfuture/thenewdesign/keyfeatures.htm
The Sliding Roof and the Pitch
"One of the key challenges of the design team was to keep the famously high standard of the Wembley pitch while, at the same time, designing a stadium with stands that are higher and closer to the pitch than the original stadium and give better uninterrupted views.
Many new stadia have suffered from poor pitches as the stands in the stadia can leave large sections of the pitch in almost permanent shadow. Grass demands direct sunlight to grow effectively.
For this reason, the sliding roof remains an integral part of the design for the new Wembley. Options such as a palletised pitch (moving a patchwork pitch in and out of the Stadium between events) or regularly re-laying the pitch were rejected as inappropriate for Wembley.
Instead, computer models have been made of air movement and sunlight on the existing pitch and the unique moving roof designed for the new Stadium."
Yeah, right furkin eedjets.