Do I have to tear off my old roof when replacing? Or can I install the new roof over the old one?

On both low and steep slope roofs, it is always preferable to remove the existing roof down to the decking.  Code states that existing residential shingle roofs must be removed before new shingles are installed as only one roof system may be in place at any time. However, low slope roofs are allowed to have two roof systems installed so some roofs may be eligible for a recover in certain situations. There are a numerous amount of criteria that an existing roofing system must meet in order to be eligible for a recover.