I was looking at an architectural detail the other day and came to the stunning realization that I didn’t really know the purpose of mortar in masonry construction. I know, I know…I sell masonry materials for a living. How did I miss this? I know a lot about masonry. I could answer, “What is mortar?” I could answer, “Where is mortar?” I could even answer, “How is mortar?” But I couldn’t answer, “Why is mortar?”
I had always had some kind of vague idea that it was the glue that held a masonry wall together. But as I looked at a detail for a York Thru-wall Flashing (which ideally goes through the entire thickness of the wall), I caught myself thinking, “But wouldn’t that be a bond breaker? Wouldn’t a line of copper or stainless steel going through the wall interrupt the adhesion? Wouldn’t adding that cause the whole wall to fall down!?!”
How does this not ruin the wall? How!?!?!?!
Catastrophizing is a forte of mine.
There must be an obvious answer. Brick buildings have had thru-wall flashings on them for years and years with no issue. So how does this work?
Whenever I have questions about masonry I turn to the International Masonry Institute. The IMI provides education, technical support, research and training on all topics masonry. I spoke to Brian Trimble and Casey Weisdock, both Directors of Industry Development & Technical Services for the IMI. “The flashing does act as a bond break, but you have the weight of masonry bearing on the flashing and that flashing has some coefficient of friction that helps hold the wall in place.,” Brian said. “Also weight/gravity keeps the material in place.”
Casey reminded me that while “mortar isn't an adhesive with a capital A”, it's bonding capacity does play a role bonding the bricks together. The bricks need to stick together somewhat in order to have “panels” that are secured back via a brick tie to the structure of the building.
In short:
While mortar does serve as an adhesive, holding masonry units together, it also serves as a cushion to create full bearing between units. Walls are primarily held together by their own weight and because they are typically anchored to the structure of the building. That’s why the addition of a thru-wall flashing doesn’t compromise the structural integrity of the wall.