Repair Masonry Joints with Sealants
Your masonry expansion joints expand and contract depending on the weather and the temperature. (It's their job.) Eventually however, the original sealant in these joints degrades, cracks, and needs to be replaced. Water seeps in and then freezes and thaws, widening cracks and then making its way into your building through the gaps.
The fix: Get the help of an expert with the diligence to remove old sealant, install a backer rod to fill the gap, and then install the right sealant for your building.