I’m starting work on a kitchen addition that needs a new roof.  The addition was done about 25 years ago and the flat tar and gravel roof they put on does’nt really go with the nice heritage house that it’s attached to. Besides that,  the roof is  in pretty bad shape.  The old flat roof will get framed over with a new gable roof.  It has to intersect the existing gable roof on the main house.  The new ridge will be about 6″ under the house ridge.   I plan on running sleepers down the valleys to lay the valley rafters on.  So first off , is it a good idea to remove the old asphalt shingles where my sleepers are going to sit,  or is it okay to put the sleepers right over top of the shingles.  Second, when I get to sheathing it, can I run my sheathing right up to the shingles and then cut out a strip to slide in the valley flashing.  After removing or cutting the nails of course.  Or is it better to remove all the shingles  where my new roof is going to sit befor I even start any framing.

  Thanks in advance for any help.