When it comes to layers, we have one rule. We never install over an existing layer. Having more than one layer of shingles on your roof can cause damage in the long run and can even be against local building code in some areas. You see, each layer has its own warranty from the manufacturer. So when you go over an existing layer with a new one, you automatically forfeit both warranties from the manufacturers. And that means if something goes wrong with the new installation, there won’t be any manufacturer warranty.