If you're a warrior you can intervene to the roof from the flag room by targeting someone right on the corner of the roof(near the drop down to 2nd level) and fly up at intervene speed down the tunnel and up to the roof to stand next to them.
You can also intervene to the roof of the entrance to the tunnel by targeting a friendly standing on the top in los and intervening to them as you run past.
These tend to get messed up if you're hit by cc right as you click it and can waste your intervene cd.