You need to inform your insurance company of the roommate and that he or she has their own insurance policy (which would appear to be a non-owner's policy if they do not own a car to insure) to find out if that person would need to be placed on your insurance policy as a driver to be covered by your car insurance policy.

State laws and insurance companies guidelines vary though most all insurance policies will require you to inform your car insurance provider about all of the individuals living in your residence (under one roof as you put it) that are licensed. If you do not want certain licensed household members listed on your policy then some insurers allow you to exclude that person.

If you choose to exclude your roommate you would normally do so by signing a waiver stating that this person will not drive the vehicle and if they did (even in an emergency) they would not be covered by your coverages. In exchange for not having this person on your policy or driving your car than your premiums should not increase.

If a person wants their roommate to be able to covered to drive their car than the car owner would likely need to add that person to their policy as a driver and make sure to not sign any exclusion waiver for that person. To make sure a roommate is properly covered by insurance to drive a vehicle the car owner should inform his/her insurance company of the licensed household member (roommate) that may occasionally drive the car. The insurance company will then determine if the roommate needs to be added to the policy to be covered and if so what the rate would be for that.

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