√RON¹ root. “arch over, roof in”
This root first appeared as ᴹ√ROD “roof, cave” in The Etymologies of the 1930s with derivatives like N. rosta- “to hollow out, excavate” and ᴹQ. rondo/N. rhonn “cave”, having instead the sense “domed roof” in Ilk. rond which served as the basis for (at the time Ilkorin) name Ilk. Elrond = “vault of heaven” (Ety/ROD; EtyAC/ROD). In the Quendi and Eldar essay from 1959-60, Tolkien instead had the second element of this name as S. rond “a vaulted or arched roof as seen from below (and usually not visible from outside); a (large) hall or chamber so roofed” as a derivative of *rono “arch over, roof in” (WJ/414).
Neo-Eldarin: For purposes of Neo-Eldarin, I think it is best to assume this root was √ROD, because some of its early forms (rosta-) can be derived from this root but not later RON(D)-.
References ✧ VT39/9; WJ/414-415
ᴹ√ROD root. “roof, cave”
References ✧ Ety/EL, OS, ROD; EtyAC/NAUK, ROD