Abstract: To study the seismic behavior and stress mechanism of roof joints with mortar layers of different thicknesses under horizontal load, a parametric finite element analysis of roof joints was conducted using ABAQUS. Considering the effects of mortar thickness and loading type on the failure modes and bearing capacity curve of the roof joint, eight specimens with mortar thicknesses between 0 mm to 50 mm under either monotonic or cyclic loading were simulated. Compared with the test results, the finite element model is proved to be accurate enough for further analysis. It is noted that as the thickness of mortar layer increased, the failure mode of the anchor rods of the roof joint is changed from shear failure mode to bending failure mode. Therefore, the lateral resistance of the roof joint is reduced as the thickness of the mortar layer increases. The four anchor rods of a roof joint transfer similar amount of lateral force. The lateral resistance of the roof joint under cyclic loading is reduced by 30% compared with that under monotonic loading. Based on the analysis results, evaluation equations of the lateral resistance of roof joints were proposed which take into account the stress mechanism and the deformation of anchor rods. The lateral resistance is consisted of the shear resistance of anchor rods, the friction force between mortar layer and concrete, and the horizontal component of axial force of anchor rods. Moreover, a reduction factor of 0.7 is adopted to consider the effect of cyclic loading.