Any feedback on this one? Other than being sloppy work, does leaving the shingles on a roof deck that gets an addition framed on top (30 years ago) present a problem?
see it often
never had a particular concern.
I would be concerned about the opening where the light is shining through .:D:D:D
Probably not.
rarely done correctly
overframe of any roof best practice:
requires the removal of existing shingles and installation of new frame in direct contact with preexisting roof deck
also note the doubled plate for the rafter ends on the existing roof
Hi Barry, how log has this been considered “Best Practice”.
I too often see 30 odd year old rood additions like the one depicted in the first photo.
as you know when anything other than comp is involved roof covering material is usually removed
why not comp?
Every contractor knows that comp is just really thick tar paper with grit for extra friction! :p;-)
Barry is correct, that is best practice of overframing.
And debris from the stripping of the existing composition roof is usually taken down and left clean.
That was an unaceptable execution of the work by the contractor, whether it was 30 years ago or now.
Hell, I would have been fired 30 years ago to leave a mess like that.
And Barry, when did it start snowing in your area for that picture? :mrgreen:
And what is that light at the end of the tunnel?
Yes the light at the end of the tunnel.
I blew up the photo. That jackstud ( and God knows what it is resting on looks like it is wicking water at the bottom.
Looks wet, saturated and stained. My PC?.
Suspect: jackstud. roof opening in eaf,wooden shakes in new attic or addon and old roof covering.
Recommend: licensed contractor remove all old shingles and install valor barrier, investage jackstud, repair opening in the eafe.
Installing the rafters ontop of shingles will aloow for eventual movement and reseteling. The valley nails will posibly lift and problems can occure.
I have seen many different buildings with part of part an intact old roof underneath is all.
It does not make sense to me that every shingle need to be removed. 
Being in a part of the country where there are very many 100-150+ year old houses with additions, I have seen hundreds of cases where the new addition roof framing has been installed directly over original wood and/or composite shingles…no major problems were ever seen even though it does not “look” professional.
The minor sags/uneveness I have seen is from the original house framing not even being close to today’s minimum codes…for example- hand hewn rafters in a ~200 year old post & beam that were 4’ on center (the first house I owned and renovated…the family I bought it from bought it back from me when I moved to Halifax area)… and another i inspected a few miles up the road of the same vintage had roof rafters 6’ on center but surpisingly stood quite straight and level.