I have a single story flat roofed extension attached to the main building. The slit that was cut into the main building to insert the flashing into is quite large (about 1cm thickness). It also seems the gap goes all the way through the wall too. On rainy days (dependant on the direction of the rain) water can get in through the gap and I am wondering what I should do to remediate this issue. I was thinking about filling the entire strip with some mortar. Thoughts? advise?