tectum

Tectum, tecti is a second declension neuter that began life as the perfect participle of tego, "to cover": the covered thing. From roof it goes, by the usual shorthand of a part for the whole, to the house itself, and the plural tecta is ordinary Latin for "buildings, dwellings" with no roof in view at all.

The trap is that the participle never went away, so tectum in a sentence may be the noun or may be an ordinary passive participle agreeing with something neuter, and tecta may be the plural noun or a feminine singular participle. Read the syntax, not the ending. Same family: toga (the covering you wear), tegumentum, protego, detego, and through them English protect, detect, integument and tile.