I just pulled the ridge transition between my tile and flat roof so I could lay down a strip of ice and water shield. I had the flat roof replaced but they didnt want to touch the tile roof so they just tucked the bitumen under and left me a roll of ice and water shield.

The tiles were installed with galvanized nails which were mostly failing and laid some pretty scrappy looking strips of wood. I need to reinstall these tiles with copper or stainless fasteners. What would be the recommended fastener to source?

Also, quite a few are flaking on the underside. What is the cause of this? Is it just expected as they age?

Is there a recommendation for sealant or mortar to fill the joints?

Thank you!