Jesse,
Just forget all the numbers I threw at you, if the Roubaix has a Tiagra and uses the Tiagra, the Ultegra should work provided they are close in year of manufacture. 10 speed Ultegra is relatively new and if the Ultegra is close it should work. If it has the bottom bracket with it you are golden. Sorry for all the numbers and complexity I used. I am an engineer and plead guilty to having Engineer's Disease, bad. I over thought this one, best of luck with the swap and please let us know how it goes if it gets done. Happy New Year.

Bill