I'm Canadian, but this is how my roommate rental income worked this past year:
-450.00/month rent for one of three bedrooms, plus shared everything else. I had assumed I could claim 20% rental space (1 out of 5 rooms) but my accountant said 33 %, given that the roommate used the other rooms as well.
-I could claim 33% of all housing-related expenses that the roommate also benefited from, such as property taxes, house insurance, utilities, cable/internet, furnace cleaning, any house maintenance, such as painting and a new window and yard maintenance when hired out. My mortgage is paid off, so couldn't claim that.
-As the rent is taxed at my marginal rate, it wasn't really worth it to me financially, especially when I factored in the irritation factor of having someone else around all the time plus the extra housework they created for me. So, personally, I won't do it again once they move on, but it depends a lot on your tolerance for others and how much you financially benefit.