Whether someone WANTS to have their existing shingles removed can often be a matter of personal preference after weighing the pros and cons. As far as what MUST be done with regards to a tear-off prior to installing a metal roof, we look to building code. According to building code, there are two situations in which you MUST have the existing shingles removed. The first would be if there are two or more layers of existing shingles. The second would be if there is only one layer, but there is extensive rotted decking which would need to be replaced.