In the US, being *roommates* doesn't imply sharing the room. (**Note**: this question isn't about the situation where people share sleeping quarters, such as in a dormitory with separate beds but just one room - the term *roommate* is descriptive there. It's about the situation where the people *don't* share sleeping quarters but do share the rest of the house or apartment.)

> 
> In the UK, the term "roommate" means a person living in the same bedroom, whereas in the United States and Canada, "roommate" and "housemate" are used interchangeably regardless whether a bedroom is shared, although it is common in US universities that having a roommate implies sharing a room together.
>  [- wikipedia](https://en.wikipedia.org/wiki/Roommate)
> 
> 
> 

**How did the term *roommate* come to mean someone who shares a house but not the room?**

Things I've checked:

* An EL&U search threw up a question on the [etymology of the *-mate* suffix](https://english.stackexchange.com/q/140809/142322); I'm interested in the etymology of the *room-* part.
* I also found a question about using the term *roommate* in an [office setting](https://english.stackexchange.com/q/411685/142322); my question is about the standard domestic setting.
* Etymonline's entry for [roommate](https://www.etymonline.com/word/roommate) is rather plain, simply listing *room* and *mate* as the roots. The entry for [*room*](https://www.etymonline.com/word/room?ref=etymonline_crossreference#etymonline_v_16519) is somewhat more illuminating, but the noun entry seems to point to a contiguous space, whereas the US usage of *roommate* relates to sharing a divided space (a house).
* [Wikidiff](https://wikidiff.com/housemate/roommate), [ODO](https://en.oxforddictionaries.com/definition/us/roommate) and [M-W](https://www.merriam-webster.com/dictionary/roommate) don't add much beyond the basic definition of the term.

So if North American roommates don't share the room, where does the *room-* prefix come from? The closest justification I've come across talks about shared living spaces: living room, dining room, kitchen, etc, but that doesn't seem to go far enough, except in the generic and unidiomatic sense that everyone who happens to be in a 'room' of any description are 'roommates' - ballroom, restaurant, waiting room, etc.