The OP doesn't not suggest that the relationship started well after they started living together. The OP said they were in a relationship that started in 2017.What about people who live in same roof with another 5 people in same address and later become committed to each other as gf bf are they considered common law ? Does living on same address consider common law?
Your question regarding roommates, while being silly, is different. If people lived as roommates but did not have a relationship that included a significant degree of attachment, intimacy, and mutual support, then it's not common law. If they lived together for a year and then became committed, the common law status starts twelve months after they became committed.
The problem, though, is if you lived together with roommates for five years before you became committed to each other, you lived together for five years at the same address. You'd need to prove to the visa officer, somehow, that the relationship started after five years of living together. And that may be hard.