If you are fortunate enough to have an historic home with a wealth of stone or old brick, chances are at some point you will have to have repointing work done. Repointing brick or stone is actually replacing the mortar to help ensure the wall lasts for a long time. In order to do this, masons use a hammer and chisel to remove about one inch of mortar from between each brick or stone, then they refill the area with new mortar.
But what kind of mortar do they use? That is what you should be most concerned about, because the wrong mortar can spell disaster for a historic home.
Why the proper mortar matters
Let's start by talking about the most common types of mortar. In new home construction, almost all mortar is based on Portland cement. But for much older homes, the mortar might have been lime-based. The difference is dramatic. Portland cement dries quickly and is a very hard mortar, making it difficult for water to seep through. Lime-based mortars take much longer to set up but are much more permeable than the 'soft' rock or brick that was often used in old houses.
This is important because water is the biggest threat to any home, new or old. As brick or stone becomes wet from water vapor, the water has to go somewhere. In a home with lime-based mortar, that water seeps away through the mortar and avoids damaging the stone or brick. But if Portland cement is used, it is actually less porous than the brick and stone, which means the water takes the path of least resistance. It comes out through the stone or brick. The result is often spalling of the surface, which can lead to significant damage.
How to ensure the right mortar is used
When it's time to repoint the brick or stone, ensure the contractor knows what he or she is doing -- they are working on a very old home that probably does not use Portland cement-based mortar. Though many masons take the historic nature into consideration, it is your responsibility to ensure they do.
You can start by making certain of your home's age. If it was built before the 1860s, you likely have a very soft brick, which necessitates lime-based mortar. On the other hand, homes built after the 1890s took advantage of new brick-making techniques, and thus probably have harder brick. These bricks are likely mortared with a mixture that includes Portland cement and lime. A home with stone should always use a softer mortar, as it can often be impossible to ascertain just how hard or soft the stones are.
To be absolutely certain of the type of mortar to use, obtain a sample of the original mortar. An older home has likely been repointed many times, so look in hard-to-reach spots, such as under the eaves or behind the porch. These areas might have been avoided in earlier repointing efforts.
If you're still not sure, ask the mason to repoint only a small section -- perhaps only one square foot -- and then determine if the new mortar matches. Never be afraid to insist on a softer mortar! Your old house is depending upon you to make the right decision.