You are a landlord to your roomate, therefore, you need to follow the Washington State residential landlord tenant laws. Here is a good summary of what you, as a landlord, needs to know: http://www.wsba.org/media/publications/pamphlets/landlord-tenant.htm
If there is no sublease written agreement, your roommate is a month to month "at will" tenant, but is still protected by the law that provides rights to tenants. Make sure you follow the law closely. Here is a link to the Washington law:
Oftentimes the simple 20 day advance notice that is required to terminate a tenancy will be all that it takes as the tenant will get the message. In your situation it will lead to a bit of discomfort, to say the least.
You might also find a good self-help book of forms in the Tacoma Public Library. Good luck.