The application of Textile Reinforced Mortars (TRMs) may cause a significant increase of the lateral capacity of unreinforced masonry elements. This paper presents relationships between the durability and the governing material properties of GFRP (Glass Fibre Reinforced Polymers) grids used to produce TRMs. Measurements of the tensile strength were made using specimens cut off from GFRP grids before and after ageing in aqueous solution and high cycle fatigue testing with a number of cycles of 60,000, 150,000 and 300,000. The tensile strength of two GFRP grids was tested after up to 210 days of storage in deionised water and NaCl solution. A degradation in tensile strength up to 30.2% and 10.8% was recorded for the specimens subjected to treatment in aqueous solution and to high cycle fatigue testing, respectively. This degradation indicated that extended storage in a wet environment may cause significant decreases of mechanical properties.