Owners of older homes with slate roofs are faced with considerable expense if the heavy slate has to be removed and disposed of. In some cases, metal roofs have been installed over old slate roofs to help avoid this expense.
Most slate roofs were originally installed over solid or nearly-solid decking rather than over spaced sheathing. This can facilitate putting a metal roof over the old slate. Also, different types and grades of slate have aged differently over the years and some are less brittle and lend themselves to being roofed over.
Metal roofs should be installed over slate only if the slate is on solid or near-solid decking. Next, to allow for the thickness of the slate, only shake and tile profile roofs that have considerable airspace built into the panels should be installed over them. In all cases, the product manufacturer should be consulted before proceeding with the installation and any special instructions should be adhered to. Fasteners should be selected in advance on the basis of what fastener will not cause the slate to crack objectionably. A certain amount of limited cracking will always occur but it should not be excessive. In some cases, a self-drilling, self-tapping stainless steel screw might work well. If the metal roof you’re installing normally uses aluminum nails, consider using stainless steel nails due to their extra hardness. Proper underlayment should always be installed over the slate and, to minimize fasteners, it can be installed as the roofing progresses rather than all in advance. Make sure that the local building inspector approves of the application before starting the job.