Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
arrectārĭus (adr-), a, um, adj. [arrectus], in an erect position, erect, perpendicular; hence, arrectaria, the upright posts of a wall (opp. transversarii, cross-beams), only Vitr. 2, 8, and 7, 3.