This seems to be a common install in this area. Roofers are installing 15-20 year asphalt over 100+ year old slate.
I have spoke to slate roof specialists, Gerry Beaumont (Thanks mate) and now I am trying to call some roofers in the area to get there input.
If I see this I call it out as an improper install and recommend a specialist to evaluate. Today was an interlocking T asphalt shingle (Are these nailed or is there a different installation as they are interlocked)
As I looked down the street there where 1/3 with the slate in general servicable condition. 1/3 had asphalt over slate and 1/3 had asphalt tear offs.
Be carefull that you idenify the slate underneath or a very large lawsuit could be in the horizon.