How old is the roof? are you sure its not asbestos? I just re roofed my workshop. It is a standard double garage that was built in 1973. Twenty years ago i extended it & simply added 6ft on the back end. New fibre cement board matched in perfectly on the new bit. By this year the old part was giving more & more trouble, leaks, split sheets due to ice damage etc. Decision was made to replace it.
I used insulated steel roofing sheet, The job went well & its nice & warm inside.
As for the asbestos i kept the fibre cement boards separate when we took them off that will go in the hardcore. The rest some 16 9ft asbestos sheets plus ridge caps had to be taken away by a licenced contractor, It has to be wrapped in polythene in packages that can be lifted by two men so two sheets at a time. That lot cost me just under 500 quid. Guys were clean & efficient & i got the proper paperwork. Interesting that the guy said since the first lockdown started they have never had so much work in their lives, flat out 6 days a week. Must be doing alright though not my choice of career!