What documents do I need to add my name to Dad's bank account?

If he is competent and you agree that it is a good idea to have your name on his accounts, go to the bank together with your valid IDs and have your name added. A better option may be to get a POA if his bank honors them. This protects your father's assets while allowing you to write checks if needed. Having your name on the accounts is handy, but circumstances could arise that puts the money at risk. For example, if you had an accident and were sued, any accounts with your name on it would be considered as your money. Or if you were to go into debt, the same would hold true.

The good side of having your name on the account is that you can avoid probate if you are his only heir. You can also get around the problems of banks that are not comfortable with POAs. You and your dad should decide which is the best way to go. Personally, we have my name on my mother's bank accounts so I can do her banking. She assigned the POA to my brother -- no good reason except that she thinks legal things are in the male domain.

Is he competent? Has he been declared legally incompetent?