Primitive roots were the basic building blocks of Primitive Adûnaic words. In Lowdham’s Report, Tolkien used the term “bases” instead of roots. Words that are etymologically related can be attributed to a common root, but it is unlikely such a root served a genuine grammatical function in Primitive Adûnaic. True Primitive Adûnaic roots had either two consonants (biconsonantal-root) or three consonants (triconsonantal-root) combined with a characteristic-vowel (a, i or u), for example: ✶Ad. √ZIR “love, desire” or ✶Ad. √KALAB “fall”. There were also uniconsonantal-forms used for affixes, inflections and pronominal stems, but these were not considered true roots (SD/415-6).

Words could be formed from roots using various rules describing the possible word patterns. There were probably grammatical rules underlying these patterns, but they can no longer be discerned. A common pattern was to have the characteristic vowel after the first and second consonants; such words were called the full-form of the root (SD/422), for example ✶ziri or ✶kalab. Other patterns were variations on the full form.