Yes I don't know of a way of doing it in one simple step.
You could start drawing a perpendicular line as Stuart suggests, then when it's perpendicular right click and convert it to a guide line. Then you could dimension to the intersection of the guide lines and the roof.
Alternatively, if you could dimension to the mid or end points, then you could insert the dimension string at any angle and rotate it afterwards.
Or you could temporarily rotate the grid to be parallel with the roof then dimension anywhere and snap to one of the dimension line options that will become available, as long as you have 'Relative to Rotated Grid' enabled in your Work Environment.
It's always been kinda strange how linear dimensions don't rubber band from the first temporary dimension point.
Cheers,
Link.