Repairing Masonry Mortar in a Historic Home

This article is focused on masonry in home built before 1850.

Chances are, unless your historic house has been recently renovated, your masonry is looking a little wimpy. Every time I sweep my kitchen floor, I collect sand that sheds from my stone wall. It’s amazing that the walls are still straight and the floors remain even. Even though I don’t obsess about the state of my walls, having the mortar touched up is something I’m researching.

Many homeowners make the mistake of treating their old walls, like new walls. Apparently fixing your pre-1840s masonry is a little more complicated than just hitting up the Home Depot over the weekend and slapping some quick-dry cement into the cracks. In fact, as I’m learning, using the wrong materials to patch your masonry can do irrevocable damage. Instead, doing any sort of work to your historic walls needs careful consideration. Don’t think hiring a professional will get you off the hook either since most modern masons aren’t savvy about what goes into a quality restoration.

The National Park Service has released a series of preservation briefs. Repointing Mortar Joints in Historic Masonry Buildings covers the more basic points of masonry restoration and will point you in the right direction. They explain a little about the history of masonry and the different types of mortar used. The article also stresses that having your walls re-pointed is not something you can rush. Before starting the project make sure you can devote the time and resources to having it done right.

The first thing you need to do is take a good look at your wall. Is it damaged due to some other source rather than just age? Is it really damaged or just looks a little worse for wear? Is there an compromise in the structural integrity? Having your mortar pointed isn’t going to solve an underlying problem. You’ll need to fix the other problems first. This might be a good time to enlist the assistance of a professional. A historic architect might be worth consulting with to see the extent of the work needed. The National Park Service recommends the following agencies; the American Institute for Conservation of Historic and Artistic Works (AIC), the Association for Preservation Technology (APT), and the American Institute of Architects (AIA).

The next step is to choose a type of mortar. Chances are you don’t have a laboratory in your basement so you’ll have to send out a sample to find a match to the existing mortar. Getting the best match possible is key to preserving your house. You can send a chunk of the existing mortar to a specialist who can perform either a wet chemical or instrumental test. Details of these tests are outlined in Repointing Mortar Joints in Historic Masonry Buildings. One such place that offers this service is The U.S. Heritage Group. The U.S. Heritage Group can also work with you and your mason to create a suitable mortar for your project.

The most stressed rule is that the mortar you use for repairing your brick wall should be less hard than the bricks. If your house was built before 1840, the bricks used were not fired at as high a temperature as more modern bricks. Therefore it is imperative that the mortar be softer than modern-day cement, which is very hard. Don’t be nervous, the mortar from the 1840s and earlier is sacrificial and is supposed to wear away with time. Its permeable quality helps whisk moisture away from the brick itself. Not having adequate moisture management means the moisture may collect in your bricks, which
are porous, leading to damaged bricks. If you suspect that previous owners were not so thoughtful in their renovations, it might be prudent to re-point and use the correct mortar rather than wait for a more serious problem to arise.

The components of mortar vary depending on the age and original materials. All mortar has a base of sand which for an older building should be natural, not synthesized. Then lime is added to bind the sand together. There are all sorts of limes available that are processed in various ways. Analysis can tell you what sort of other ingredients are in your original mortar, including clay, shells and bits of partially burned lime. And finally, water, which should be free of impurities. Traditional lime-based mortar is soft and porous. It allows for moisture to be channeled away from the brick. Lime mortar also keeps it’s general volume during temperature changes which lessens the chance for shifting or settling.

Scheduling your repointing project is very important. Temperatures should be warm, between 40 and 90 degrees Fahrenheit. The area being repaired should be a relatively shady area and be given ample time to fully dry, slowly. You absolutely can not rush repointing so be prepared to do without that area for a while. Repointing also makes a huge mess so indoor areas will have to be sealed off thoroughly.

Repointing isn’t something a novice home-repairer should try to undertake. It’s well worth it to get an experienced professional. Leave time to do proper due diligence when picking out a contractor or mason and make sure to choose someone with a through background in historic masonry using appropriate materials.

For experienced masons who want to become familiar with the techniques of historic mortar joinery, The U.S. Heritage Group offers a two-day, hand-on workshop for masons, architects, and contractors wishing to learn how to work with historically accurate mortars and masonry. I am tempted to take this workshop myself so I can be more knowledgeable about the process.

The above outline is just a very preliminary introduction to repointing your historic brick/masonry work. There is good deal more involved with the process. Both sites referenced in this article have extensive information covering all the aspects of repointing.

Anyone who owns an old house knows that maintenance is ever ongoing. Even the best quality historic mortar will have to be replaced every fifty years. You have to have a love of the process of renewal as much as the finished product. Repointing is a very satisfying project resulting in beautiful walls and using the correct mortar on your house insures the architectural and structural integrity of your house for years to come.