HAVE BOUGHT AN OLD BARN, AND WAS WONDERING WILL I NEED TO TAKE ALL THE TILES OFF THE ROOF, TO PUT ON AN UNDERLAY AS THERE IS NOT ONE THERE AT PRESENT. OR IS THERE AN EASIER WAY OF WATERPROOFING AND SEALING WITHOUT HAVING TO TAKE OFF ALL THE TILES. IS THERE ANYTHING ELSE I COULD USE?. tHANK YOU FOR YOUR HELP......MARTI