This work used free vibration tests based on impulse excitation technique and experimental modal analysis using an impact hammer and an accelerometer to assess the local and global effects of the hygrothermal ageing of GFRP specimens exposed to salt-spray environment at 35°C and 60°C over a period of 136 days. Through the change in elastic properties, fifteen natural frequencies and damping ratios, the effects of degradation are associated with reductions in elastic properties and natural frequencies and an increase in damping ratios. In turn, the increase in elastic properties and natural frequencies – associated with a reduction in damping ratios – is mainly attributed to the post-curing effect. •Hygrothermal ageing of FRP angle members and coupons is reported.•Impulse excitation technique is adopted to monitor reduction of elastic properties.•Experimental modal analysis is used to study variations in natural frequencies and damping ratios.•Combined influence of material degradation and ageing is discussed.