The Nitya pitru tarpaNam formimg part of Brahma Yajnam has to be done every day using plain water without any tila. This is meant for invoking the pitru devatas. There is no AvAhanam for one's pitru, pitAmaha and prapitAmaha. This is to be done daily by even those whose father is alive. The proper method of doing this has to be learnt from a Guru.
The shaNNavati pitru tarpaNam invoking one's pitru, pitAmaha and prapitAmaha has to be done only by those whose father is not alive. This tarpaNam using tila is done during amAvAsya, sankramaNam ( birth of month) , ashtaka, etc