that's one of the worst thing about having roommates. When they don't pull their weight. For all my wrongs as a human being I have NEVER EVER EVER EVER not paid rent on time in my life. Ever. Rent is the number 1 priority in life money wise. The roof over your head, it's the number 1 thing and it PISSED me the fuck off when it wasn't the case for some people. "Eh a roof over my head, who gives a fuck, i'd rather go out and party..."
When I had roommates and was in charge of the lease (which was the case for most of those 5 years, at least 3 of them anyway), I used to split the bills evenly down to the cent and I still got accused of overcharging by a roommate who never ended up paying their whole share. It's not my fucking fault you don't know the price of living, and that's despite the fact I would put the bills on the fridge door.
I had calculated that over the 5 years I did the roommates thing, there was around 2000$ total that was owed to me by roommates that I never got. yeah sure it would have been way more expensive to not have them but that still you know..2000$ out of my pockets to make other people live. About 400$ a year, it might not seem like much but when you're as poor as I am, it really is. It's like three months of food to me.