Adding a Dormer
 

I'm adding a large dormer to an existing finished roof. When I get to roofing, is it possible cut existing roofing along the valley line and work the valley flashing up under the existing comp shingles or do I need to remove all the shingles above the new valley?
Thanks in advance

Can you provide any more detail?

Unless there are unforseen details, that is what I have generally done. I cut the existing shingles so that the cut line ends up 3" from the centre of the valley. Then very carefully get any nails out that will be in the way of the flashing, pull any staples in the roofing felt and slide the flashing up after the dormer is framed and sheathed. Certainly re-roofing the whole side will give you the absolutely best job, but I have had no problems with a retro-fit.

Thanks Bonus, How wide of valley flashing have you had success with in sliding under existing shingles after making your cut? I'm working with a 4-12 pitch.

Are you adding on in the valley?

I am actually making a new valley by attaching a new roof perpendicular to an existing one. The attaching roof meets at the peak and eave of existing roof.

I generally use 24" valley flashing with a 'w' profile, (so there's an inverted 'v' running down the center of the valley). This gives you 12" each side.