The colegio del Cardenal was built in Monforte de Lemos at the end of the 16C, under the influence of the monastery of El Escorial. The Lisbon earthquake in 1755 had an effect on it and most of its vaults suffer from movements and have visible cracks.
This paper studies the vaults of the presbytery, one of the most damaged. It is a granite barrel vault of only one bay, with lunettes. The study comprises (1) the search for meaningful data from the historical documents, (2) the precise metric survey of the geometry and the quartering, (3) the relation with the coeval construction treatises, and (4) the elaboration of a hypothesis about the geometric and constructive original definition of the vault.