Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
sug-grunda (subg-), ae, f. [cf. grando; Sanscr. hrāduni, storm; Gr. χάλαζα, hail], the lower border of a roof, the eaves, Varr. R. R. 3, 3, 5; Vitr. 10, 21; Dig. 9, 3, 5, § 6; 50, 16, 242, § 1.
Called also sug-grundium, Vitr. 2, 9, 16; Plin. 25, 13, 102, § 160; and suggrundātio, Vitr. 4, 2.