The basis for Elvish “tree” words, this root first appeared in The Etymologies of the 1930s as an extension of ᴹ√GALA “thrive” (Ety/GALAD). This replaced the earliest derivation of “tree” from the Qenya Lexicon of the 1910s, where the Qenya word for “tree” ᴱQ. alda was derived from ᴱ√ALA “spread” (QL/29). In The Etymologies, the Quenya form of this word remained the same, but the 1910s Gnomish words G. âl “wood” and †alwen “tree” (GL/19) became the 1930s Noldorin word N. galadh “tree” (Ety/GALA). Quenya and Sindarin retained these words for “tree” thereafter, and while Tolkien did not mention the root √GALAD again, his continued use of primitive ✶galadā “tree” (Let/426; PE17/153; PE21/74; UT/266) made it clear this root remained valid.
Middle Primitive Elvish
top
root. cover, roof, roof, cover
tōp-
verb. to roof
kast
adverb. towards the top
kas
root. head
galad
root. tree
galadā
noun. tree
kas
noun. head
khar
root. helmet
kharan
root. helmet
kharap
root. helmet
A root in The Etymologies of the 1930s glossed “cover, roof” with derivatives like ᴹQ. tópa-/N. toba- “to roof”, ᴹQ. tópa “roof”, and N. tobas “roofing” (Ety/TOP). It was grouped together with and was apparently a variant of ᴹ√TUP (EtyAC/TOP), a root with a much lengthier history; see that entry for details. For purposes of Neo-Eldarin, I think this root could mean “roof” as opposed to √TUP = “cover”.