ABSTRACT: This paper presents experimental and numerical studies on pre-damaged and post-damaged collar-jointed masonry walls subjected to external loading. The collar-jointed masonry walls were created by constructing a wall parallel to an existing single-leaf wall and "tied" to it using the collar joint technique. For the predamaged case, both walls were constructed and bonded together at the same time. For the post-damaged case, the single leaf masonry wall was tested initially until partial damage occurred and then a second leaf was constructed and bonded to it, making it essentially a means of retrofitting. From the results, it was found that the pre-damaged wall can carry 50% more load than the post-damaged one when the collar joint was not keyed into the existing damaged wall and the when no mechanical tie was used between the existing and the second 'retrofit' wall. Also, a three dimensional numerical model has been developed using the Finite Element (FE) software MIDAS FEA. Bricks were modeled as rigid bodies connected together with zero thickness interfaces representing the mortar joints. The numerical results were compared against the experimental findings and good agreement obtained.