It is possible to install metal roofing over old sheets without tearing them off if the local construction regulation allows it. Although removal of the old roof is the preferred course of action, removal is untidy and raises the expense of the activity. Metal roofing is a lifetime investment that can last for 50 years or more if it’s a decent one. The average life span of Asphalt roofs is around 20 to 30 years, depending on the installation and quality of the shingle. A potential issue with this sort of installation is trapped water vapor. Whenever water vapor is trapped between the metal roofing and old roofing, moisture can accumulate and create mold and rot. Be it as it may, roofers can install a vented metal roof that mitigates this potential issue. Or on the other hand, installing the new metal roofing over furring strips might raise the metal and cause a ventilating air pocket between layers.