A number of historical and residential buildings were constructed with traditional brick-flat-arch roofs. The seismic behavior of this type of diaphragms has shown that they have a poor seismic performance. Several methods were suggested to retrofit this type of diaphragms. In this research, the in-plane seismic behavior of retrofitted brick flat arch diaphragms using transverse beams is investigated with experimental models. For this purpose, four full-scale experiments of roof diaphragm were conducted under cyclic loading. The results of the two first experiments showed that non-retrofitted traditional flat arch roofs have insufficient stiffness, shear capacity and integrity. In the retrofitted models however, the transverse beams within all the spans of roof can improve the in-plane behavior of this type of diaphragm to the extent that acceptable improvement in integrity and ductility of the diaphragm was observed in retrofitted roof, but the transverse beams could not properly improve the other seismic parameters of the diaphragm such as its shear capacity and stiffness. Therefore, this retrofitting method might not be an adequate method to secure the appropriate in-plane behavior of flat-arch roofs.