Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
sub-strŭo, xi, ctum, 3, v. a., to build beneath, to underbuild, lay; lit.: fundamentum, Plaut. Most. 1, 2, 40: intervalla montium, Vitr. 8, 7 med.; cf.: intervalla substructis canalibus junguntur, Plin. 33, 4, 21, § 74: locus substructus, Varr. L. L. 5, § 155 Müll.: Capitolium saxo quadrato, Liv. 6, 4: substruendo iter facere, Dig. 8, 1, 10: vias glareā, i. e. to lay, to pave, Liv. 41, 27.
Absol.: non alte substruitur, the foundation is not laid deeply, Vitr. 8, 6, 5.
P. a. as subst.: substructum, i, n., = substructio, Vitr. 8, 7 med.