Joined: 06/23/2014 04:28 AM EDT
tiles today are for looks rather than function. it's the underlayment (double felt or membrane) that prevents leakage. air gap allows for runoff and drying. any old style tile job would never have straight lines as the tile itself was the protection against leaks; most installs did not even have sheathing; tiles were laid on battens secured to the rafters.