This paper reports and discusses the results of experiments performed on masonry barrel vaults strengthened externally with a composite material. The vaults characterized by 125-mm thickness, 2000-mm internal span and 730-mm rise, were built of solid clay bricks and lime mortar. As a strengthening glass fiber grids or carbon fiber grids were used. They were embedded in a polymer-cement mortar at the vaults extrados. The main aim of presented research was to determine load-carrying capacity and examine failure modes of tested specimens. The results of performed tests show that observed failure modes depended on reinforcement ratio of strengthening layer. The specimen strengthened with one layer of glass fiber grid failed due to fibers rupture, whereas the vault strengthened with carbon fiber grid failed due to sliding along a mortar joint just above the abutment.