I had a house in Phila. with a tile roof, built in 1920. I "replaced" the roof in 1995, and by that I literally mean re-placed: the roofers carefully took all the tile off the roof, put down two layers of 80lb roofing felt, and reinstalled all the tile. They believed that the original was done with only 1 layer of 40lb roofing felt, and that this was the only thing that had actually failed other than a couple of cracked tiles. There was some expectation that the new one might last 100 years before it would need the same process again.