What is a necessary repair?

In a word: everything

Q: When you own the property, who suffers if an item is not repaired? (no matter how small)
A: You (the owner)

Ex. If there is a broken door knob, say the cost is $20, are you going to insist the tenant fix it/pay for it? or are you just going to do the right thing and fix it? The average house in the Bay Area is about $600,000. The average door knob set is about $20.00. You see where I'm going with this. If you don't fix the door knob your tenants are going to kick/slam the door. Then the door will become damaged and you have to replace that for $2-300 dollars (materials + labor).

A house is like a car and it needs constant maintenance. In theory, if everything is used properly it will never break. But do you think your tenants will take the best care of everything? It's human nature to not take care of things that you don't own.

Q: Are you saying I need to pay for everything?
A: No, but what I am saying there are alot of little costs that as the owner, you have to accept. If a tenant breaks a window, you need to fix it right away. You cannot wait for the tenant to give you money and then you go repair it. If you don't fix it, you are allowing a safety hazard to exist and that might be more trouble down the road if the house is burglarized because there was no window. In the case of the window, yes you can bill the tenant and you should. Also, you can collect the repair bill from the security deposit after the tenant moves out if they refuse to pay you at the time of the damage.