Should I be doing something with reserve cash I get from rent?

[David Czech]

This might also be a bit of a personal finance question as well but wanted advice. Is there a set amount I should have in reserves to feel comfortable in case something catastrophic happens? I'm a bit paranoid so right now, I'm putting away everything I receive from rent I receive and have enough for lets say, a total roof replacement, if necessary. 

Should I just keep putting this money away or are there smarter things to do with it? I guess I'm also putting it away in case a tenant moves out but that's my mind also going crazy. Any advice would be appreciated.