We're thinking of redoing the shingles on our roof, do we tear the old ones out or put on another layer?

The house is about 15 years old, we're in Toronto so the winters get pretty harsh. Any roofers out there with any suggestions of what to do or not do?We're thinking of redoing the shingles on our roof, do we tear the old ones out or put on another layer?
in order for the roof to last for the Length of time on the packaging you need to tear of the old and lay down a new vapor barrier.if you g over the existing shingles you will end up rotting the wood from moistureWe're thinking of redoing the shingles on our roof, do we tear the old ones out or put on another layer?
I've always seen and actually have assisted the ';tear off';/disposal and replace w/new paper and shingles, for rough winters (lots of snow) we use the metal roofing, heavy snowfall will fall/slide off and not accumulate where you suffer damage w/conventional roofing, i.e. leaks, collapse etc...may cost a lil' more, but really really lasts, prob lifetime :-) also they come in various colours.....Happy roofin' hope I was a smidge helpful...
Take off the old shingles. When I lived in New Jersey in 2001, we were told that there could be up to three layers, but no more. Removing the old and the paper underneath you can see any defects, rotten spots, mildew, etc. Good Luck ! ! !
My husband was a roofer for ten years, and they always tore off the old layer of shingles. Apparently, there are many problems that can arise from leaving old layers on. It's more work, but better in the end.
some people place old over new;but,to do it correctly,take off the old shingles.


this allows you to see if there is any wood damage on the roof.check to see if any wood needs to be replaced.if so,replace them.
you can place new over old , depending on so many things..


it is best to remove old ones ,if any damage is visible at all.-
take off the old ones,