There are a few different ways that you can waterproof parapet walls. One option is to use a waterproofing membrane, which is a sheet of material that is applied to the surface of the wall. This will create a barrier that will prevent water from penetrating into the wall. Another option is to use sealant, which is applied to the joints between the bricks or stones. This will fill in any gaps and prevent water from seeping through. Finally, you can also use waterproofing paint, which is applied to the surface of the wall. This will create a waterproof barrier that will protect the wall from water damage.