The Other Rolling Stones by Don R. Carter PE
Granted, this report is only relevant to a small percentage of Kansas City Homes – those who have pre-1960’s mortared limestone basement walls. That said, we are seeing enough problems with these walls that knowledge of a potential risk should benefit those who do have them.
I like the old limestone walls because they are more robust than concrete and by any measure a better wall than masonry block. Cost is the obvious reason they fell out of favor, clearly not performance. Typically 18” to 22” thick, these walls have so much mass that trees, water and earth will not overwhelm them, at least not to the extent that they do the lesser walls. But while the limestone itself seems to be immune to deleterious effects from ground water, this is not true for all mortars. We see about half dozen houses a year where mortar that bonds the stones together has been reduced to sand and a screwdriver easily penetrates right into the joint (photo). This means the stones are no longer connected and stability is reduced to that of a gravity wall, that is, stones dry stacked one upon another. In most cases, the wall still functions from its own dead weight, but it is no longer the wall it was intended to be. And, sand is symptomatic of a greater problem.
So what happened to the mortar? Chemistry is what happened. Things in nature try to achieve something known as pH balance. Acids have pH values under 7, alkalis above 7 and balance near 7. Rain is essentially distilled water that forms from evaporation and condensation, and being soft water previous generations often collected it for laundry and hair washing. But with no appreciable minerals in it, rain water is out of pH balance so it sucks lime out of the mortar to correct its imbalance. This takes a long time and happens primarily in locations with faulty water control such as improperly tapered grade or poorly functioning gutters and downspouts.
If you have limestone walls, use a screwdriver or ice pick to randomly stab a few mortar joints in each face. If the probe penetrates a joint by more than a fraction of inch, you are likely going to find sand. Repairs are fairly straight forward, reasonably priced and often optional. I won’t try to define the solution here, but if you see sand send us an email and I will share repair information along with some names of people who can do the work.
Don Carter is a licensed structural engineer and managing general partner of Foundation Engineering Specialists LLC, a company specializing in residential
design and assessments: email@example.com.