I wonder what the criteria are for deciding whether to simply add a layer or to strip everything off and start over.
For a shingled roof, it's 2 layers max. If you've already got two layers on there when you decide it's time to repair, they strip both off before laying down the new shingles. Don't know if a tarpaper and roofing cement roof has different criteria.
When my father and I redid the roof of his vacation home, the manufacturer's instructions seemed to imply something like up to five layers, depending on the condition of the lower ones. It was not terribly clear, though.
(no subject)
For a shingled roof, it's 2 layers max. If you've already got two layers on there when you decide it's time to repair, they strip both off before laying down the new shingles. Don't know if a tarpaper and roofing cement roof has different criteria.
(no subject)
Regulated by Building Codes Here in USA