When we buy property with a roommate/partner, everything needs to be spelled out in writing.  

@Andrey GorokhovskiyI hate spell check.

Good to see you got the spelling right!

On your lease change the "monthly" payment to "bi-weekly" and spell out that means every other week.  

It is also used in white magic spells of protection.

For a relatively small amount of money (your premium) the insurance company is agreeing to take on a specified amount of risk (policy coverage) that is spelled out ad nauseum in your policy.As the property owner YOU have to decide the level of risk you are willing to accept.

Spells out material terms and assumptions.

If you go that route you need several documents to protect you and effectuate taking over mortgage payments (POA, notice to release, etc. )If you only have a strict 90 days to do this then in your agreement that should be spelled out but several things depend on your relative and how secure he wants to be in the fact that you will basically hand the house back over to him at the end of 90 days if you haven't sold it.

This Operating Agreement will also spell out important things like how profits and losses are divided.

I don't see the details spelled out.I am self employed.