This blog's topic and title sound pretty complicated. However, if you are a historic building enthusiast and spend a bit of time enjoying walks around the historic parts of our city, you see this issue often. Many brick walls have concentrated brick joint deterioration near the bottom of the wall. It is common. Have you ever seen this and wondered why that happens? A few reference pictures follow.
Take the wall in the picture below, for example. You can see, in the photo below we have drawn a red line around the lower portion of the wall. At that area of the brick masonry, the brick joints have deteriorated and mostly fallen out at the face of the wall.
The next photo shows the same area of the wall, but up closer. You can see that the mortar joints are recessed and have deteriorated at the surface of the wall.
This is very common, but why it happens may be surprising. One of the two main reasons may be commonsense and intuitive, but the other reason is probably not obvious. The two main reasons for deterioration at the lower portion of a wall, adjacent to hardscape, follow:
A simple phenomenon called Rain Splash-back happens almost every time there is significant rain.
Another phenomenon called Rising Damp often happens in old brick and masonry foundations.
We'll discuss both of these issues and a third concept called a Damp Proof Course, abbreviated as DPC.
Rain splash-back is simply the act of rainwater landing on a hard surface next to the wall and then bouncing back onto the wall. This is a very common occurrence, as we said above it happens almost every time it rains outside, but rainwater bouncing back onto the lower portion of a wall is not as benign as it seems. Over time, as many common rainstorms naturally occur, rain splash-back causes a concentration of water to land on the vertical lower wall areas. While that happens again and again, over a time period of several years, the lower area of brick is exposed to much more moisture than the upper areas.
Brick seems solid and secure and impermeable, but it really isn't impermeable, brick has a degree of permeability. This means that the lower area of brick, while being exposed to so much more water, also has some water getting into the outer face of the mortar joints. Over several years that occurrence causes the lower mortar joints to deteriorate at a faster rate.
Everything explained above makes sense and basically explains the issue, but rain splash-back is not the only root cause. Something insidious is also often happening below the surface, Rising Damp. Masonry foundations, like masonry and brick walls, are also somewhat permeable. However, even more than the wall above ground, the foundation below ground is often exposed to even more moisture. From area to area, sub-grade earth has different levels of moisture, but soils usually contain a high degree of moisture. This means the somewhat permeable foundation is exposed to that sub-grade moisture, and that foundation does something surprising. The foundation, through the capillary action of water will actually wick water upwards. That is counterintuitive because we think of water as naturally flowing downwards, but in some cases water will actually wick upwards. When this happens in masonry walls, we call this phenomenon Rising Damp.
To stop Rising Damp, modern building Code requires the installation of a Damp Proof Course (DPC). In modern masonry construction, a DPC is built with a low permeability or impermeable material such as a bitumen roll membrane or synthetic roll membranes. In the times of historic construction they did not have technologies such as polyvinyl or modified bitumen. Nonetheless, even 100+ years ago, builders were aware of the phenomenon of Rising Damp, and to deter Rising Damp they often used slate tiles embedded directly into the mortar joints at a specific location, usually above the splash-back zone.
This means that the area below the DPC is often exposed to some degree of rising damp. Rising Damp will cause accelerated deterioration of the lowest mortar joints in a brick wall. For these reasons, we often see mortar missing at joints at the lower part of the wall.
Our company can help with upkeep and maintenance of buildings and masonry building assemblies to deter and repair issues like this. If you have questions and think we might be able to help, reach out and say hello. You can email here at out contact page.