(Hope I am not too late - it is already morning here
)
In the 3D window with the roof selected, go to Design > Align 3D texture > Set Origin, then pick the point at which your texture should start its repeat pattern.
(Edit - Reading again I see you are talking about a cover fill image, not a material texture, so you must be in either AC12 or 13. This information would have been helpful, add to your signature for future reference. And ignore the advice above, sorry)