The picture above shows the roof in its fully closed position. As you can see, it does not cover the pitch and if rain is falling at an angle, some guests in Level 1 seating may get wet. During concerts, if you have standing tickets on the pitch, you will be open to the elements.
This is how Wembley Stadium has been designed as it allows for the playing surface to receive maximum sunlight. Wembley Stadium does not move the sliding roof whilst visitors are in the Stadium.