I have done a number of new roofs on new houses.  For the first time I have to add a second layer of 3-tab shingles over an existing 3-tab roof.

Here’s my question: how do you work the new layer of shingles with the step flashing along the side of a chimney, for example.  In a first-time roof, obviously the rows of shingle are laid between the rows of step-flashing.  When you add the next layer of shingles, do you have to remove the existing layer of shingles in the area of the step flashing so that the new shingles can go between the step-flashings?  if so, doesn’t that leave an unsightly “bump” in the new roofing at that place?  If you don;t remove the old shingles next to the step-flashing, how do you keep the water from running in  between the old and new layers of shingles?

Thanks for any help

Bob Chapman