The Roost RSA 400 takes things further and, besides delivering every function of the Roost Smart Battery, it detects the usual trio: smoke, CO and fire and, additionally, it can also sense the presence of natural gas a unique addition for this type of detector. Even if the Roost RSA 400 could fit among the smart detectors, when compared with Halo Smoke Alarm, Onelink or Nest Protect, its design is pretty unremarkable. So, similarly to the traditional smoke detectors, the case of the Roost RSA 400 is circular, while towards the bottom, the body gets narrower and finishes into a larger, thin plate where the device gets in contact with the ceiling or wall. Since the plastic case is covered by a white matte finish, it will easily blend in on a ceiling it actually looks like a flush mount lighting and, since it is a relatively small device it measures 5. 63 x 5. 63 x 1.