Frequently Asked Questions about Historic Masonry

U.S. Heritage gets many inquiries about traditional masonry techniques and materials. Here are some of the most frequently asked questions. This list is being expanded all the time, so check back frequently.

Why should I use lime mortar when I repoint my historic building?


Spalled brick in an1860s lighthouse that was repointed with portland cement mortar about twenty years ago.

The historic stone masonry units have deteriorated dramatically, while the portland cement mortar joints remain intact.

The portland cement joints have trapped an enormous amount of water in this wall.

It' s important to understand that historic buildings are very different than modern buildings in several key ways:

First, historic brick--in particular--is softer than modern brick because modern brick is fired at a much higher temperature than was possible in the past.
Second, portland cement is used in most modern buildings--but it wasn't even available in the United States until 1871.
Third, historic masonry buildings were designed to absorb water and then release it, as opposed to modern building technology, which emphasizes waterproofing.

Because of these differences, it's very important to use compatible materials during restoration. An incompatible mortar can destroy a historic masonry structure. The most common mistake we deal with is the old masonry building that has been repointed with portland cement mortar instead of the historic lime mortar. Modern mortar mixes are made with portland cement, which is quick-setting, inexpensive, and strong. However, it can do a lot of damage to a historic building that was originally pointed with lime mortar. Portland cement is generally more rigid and less permeable than the historic masonry units, so cement mortars will cause damage to the brick and/or stone during expansion and contraction. Lime mortar, on the other hand, is more accomodating, and its lower compressive strength allows the historic masonry units to expand and contract without being damaged. You always want your mortar to be weaker than your masonry units for this reason.

Lime mortar also allows water to pass in and out of the joints, which means that water does not get trapped in the brick or the stone. Portland cement mortars are unable to wick water out of the walls, so water gets trapped and instead escapes through the historic stone or brick. This damages the building units as the water pressure builds up in the masonry unit until the face of the brick or the stone pops off, exposing the inside of the unit and making it more vulnerable to continued deterioration.

Why should I match the new repointing mortar to the historic mortar?

Obviously, you want to avoid the kind of damage described above, and in order to accurately recreate the original mortar you typically have to have the mortar analyzed. Although a building constructed before 1871 was undoubtedly pointed with lime mortar (because portland cement wasn't even available until that year) the years after 1871 are a grey area. Like any new technology, portland cement took a while to catch on--some masons picked up on it earlier, while traditionalists held fast to their lime mortars. So if you're working on a building constructed between 1871 and 1930, you're looking at a wide range of possible formulas, and mortar analysis is really the only way to confirm the composition of the original mortar. U.S. Heritage can analyze your mortar sample to determine the formulation of the binder, (i.e., the lime and/or portland cement) and we also identify the aggregates so that your restoration mortar looks and performs just like the original. For this reason, mortar analysis is also valuable for building constructed before 1871.

There is one important caveat: we've worked on projects where the original mortar was too hard for the building. This is generally a phenomenon of the 1920s and 30s, when builders were a bit too enthusiastic about the strength and performance of portland cement. This period produced some of the hardest mortars we've ever seen, and we do sometimes recommend a weaker formulation than the original. Our goal is to provide the most appropriate restoration mortar, and while this generally means recreating the original there are the occasional exceptions.

The old lime mortar in my building is crumbling and falling out. What should I do?

Which one is in better shape?

...the one on the left, or the one on the right?
Answer: the one on the right-the masonry units are in perfect condition because the lime mortar has failed. The one on the left has been pointed with portland cement, and not only are the bricks deteriorated, but there's enough trapped water to sustain plant life.

You should thank your lucky stars. This is the best possible scenario, and we're thrilled whenever we see it--not just because we like historic lime mortar more than most people, but also because it's a clear indicator that your building is in perfect condition.

Here's why: many people assume that old, crumbly mortar is a bad thing. On the contrary! If the historic mortar is failing but the brick and/or stone of the building is intact, then the mortar has worked perfectly. The original mortar was soft enough that it permitted the masonry units to expand and contract in extreme conditions without sustaining damage, and when the mortar began to fail it lost its adhesive qualities and started to fall out. That’s not a problem--that's an ideal situation. Now all you have to do is scrape out the loose mortar, put any loose stone or brick back in place, and repoint with a restoration mortar.

Mortar is supposed to be a sacrificial element, which is to say that it's supposed to fail after about fifty years. It's much easier to repoint your building every half-century or so than it is to repair or replace historic brick, stone, or terra cotta. Trust us. (If you've already got damaged masonry units, though, see the next question.)

I'm restoring my historic stone and brick building, which was tuckpointed years ago with portland cement. The brick and stone is spalling now--can I patch the brick and the stone? How do I replace the bricks that are damaged beyond repair?

We encourage customers to retain as much historic building fabric as possible. We offer a patching product line, the Heritage Repair Mortar series, which is designed for stone, terra cotta, and brick repair. If you are looking to replace brick, we also offer a brick search service.

What is hydraulic lime mortar?

Hydraulic lime mortar is a lime mortar that hardens in damp or even wet conditions through a chemical reaction rather than exposure to air. (This is in contrast to standard, or non-hydraulic lime mortars, which harden by absorbing carbon dioxide, a process known as carbonation.) Hydraulic qualities are created when clay is included in the lime mixture.

We recommend hydraulic lime mortars for buildings or building components (like chimneys) that are located in damp conditions or suffer a lot of weathering.

Is there a reliable source of unbiased information on masonry restoration and preservation?

Yes. The Historic Preservation Service division of the National Park Service (NPS) has established the official standards for the preservation of historic structures; these standards are The Secretary of the Interior's Standards for the Treatment of Historic Buildings. The NPS also offers extensive web resources to help you interpret and apply these standards, and their online resource center includes their Preservation Briefs, which are the professional standard for preservation and restoration in the United States.

Why would I use a limewash instead of typical latex or oil-based paint?

Limewashes represent the best of both form and function. They're beautiful and breathable, making them the perfect choice for both historic and new buildings. Unlike typical modern paints, they don't trap water and they age beautifully--instead of getting shabby, they develop a lovely mellow patina over time.

Limewashes are also nontoxic and have no noxious fumes, and they're suitable for exterior and interior applications--in fact, limewashes were historically used in hospitals and sickrooms because of lime's hygienic qualities. U.S. Heritage offers our European-style limewashes in a range of colors, including traditional whitewash.

I'd like to use a limewash on my house, but it's already got a coat of paint on it. Will limewash adhere to a painted surface?

Limewash should only be applied to porous, water-permeable surfaces, so it will not perform well if it's applied over an existing coat of latex or oil-based paint or sealer. Limewash is ideal for unpainted stucco and plaster, and it is also suitable for porous brick and stone. Because it requires a porous substrate, it generally does not work well over granite or marble.

Unfortunately, many historic masonry walls have been painted over the years, and impermeable paints and coatings cause major problems for old stone and brick. It is usually advisable to remove these coatings, regardless of whether or not you plan to apply a limewash or other lime-based parging. A word of caution is in order, however-it is very important to hire only well-qualified masonry cleaning contractors. Inappropriate cleaning chemicals and techniques can cause irreversible damage to historic masonry. For more information, read the National Park Service Preservation Brief 1 "Assessing Cleaning and Water-Repellent Treatments for Historic Masonry Buildings" and Preservation Brief 6 "Dangers of Abrasive Cleaning to Historic Buildings."

What is the difference between slaked lime putty for frescoes and the regular slaked lime?

Slaked lime putty is quicklime (burned limestone) that has been mixed with water and allowed to age. Slaked lime putty for frescoes is finer than regular slaked lime, and it's been aged longer in order to make it smoother. This is crucial for fresco work, which depends on very smooth surfaces.

What is the difference between fresco lime plaster and the more common lime plaster?

Both of these plasters are typically used for interior applications, but fresco lime plaster is composed of aged lime putty and fine aggregates.

What is a lime render?

A lime render is a coating of lime-based material that was traditionally applied over rough, unsightly brickwork or rubblestone masonry walls. Non-hydraulic lime stuccos and hydraulic lime stuccos are common lime-based renders.

In addition to these traditional applications, lime renders can restore some measure of protection to masonry buildings that have been sandblasted. (Sandblasting a masonry building is incredibly damaging. Think of a brick like a loaf of bread-the crusty exterior protects a soft interior, and when you damage this crust you expose the vulnerable interior. Sandblasting dramatically accelerates building deterioration.)

I'm restoring a stone wall that has a number of fine cracks. Is it possible to fill these cracks?

Yes. Dispersed Hydrated Lime (DHL) fills fine cracks with breathable, flexible lime mixture. Click here to learn more about DHL.

What is the difference between tuckpointing and repointing?

Tuckpointing is a very old English technique that disguises wide mortar joints as more narrow ones. Historically, masons would point the building with a mortar that was tinted to look like the surrounding mortar units, and then they would cut a channel into this flush joint. They would fill this channel with a mortar of a contrasting color that would then appear to be the mortar joint.

Over time, the term "tuckpointing" has become disassociated from this original meaning, and it is often used synonymously with "repointing," which involves the removal of deteriorated mortar and the application of new mortar. One key difference is that in common usage the term "tuckpointing" doesn't generally involve the removal of old deteriorated mortar.

How can I learn more about traditional materials and techniques?

There are a number of great publications available, and the internet has made it easier than ever to access information from around the world. Anyone who is planning a masonry restoration project should thoroughly review the National Park Service's online preservation resources. We also offer lime mortar workshops, which include both hands-on experience and classroom learning.

I'm building a new house with salvaged historic brick. Should I use a traditional lime mortar?

Yes. As we have already mentioned on this page, historic brick was fired at a lower temperature and is typically softer than modern brick. Construction techniques have changed, but it's still important that the mortar accomodate the historic brick.

You recommended the onsite masonry contractor training for my mason. But he has thirty years of experience! Why would he need training?

Due to the prevalence of portland cement mortars (notably the standard Type N), very few masons have worked with traditional lime mortars. Portland cement versus lime is really a case of apples and oranges--traditional materials require a different approach, which is why we teach proper preparation, application, and curing techniques. Proper training prevents material failure and minimizes labor costs while protecting historic buildings from irreversible damage, so we strongly encourage an upfront investment in training in order to prevent major problems later in the project.

Another advantage of the onsite contractor training is that we install the mockup panel using the lime mortar materials. This ensures that contractors have a quality control standard. Here's an example of one situation we had where onsite contractor training would have preempted this problem:

	U.S. Heritage produced a custom matched mortar for this project. The mockup panel, which clearly doesn't match, was installed by a masonry contractor who had not been trained in traditional masonry materials and techniques. The material is very white because the contractor added too much water (in an attempt to make it feel more like portland cement mortar) and it's also smeared all over the face of the brick.		We removed the contractor's sample and reinstalled it using the proper techniques. This mockup sample is installed using the exact same materials as the one on the left, but it's now indiscernable from the rest of the wall. In such situations, the problem is often incorrectly attributed to lime mortar failure, when in fact the issue is actually a lack of familarity with the material. With this in mind, we offer consulting services specifically to ensure that the material is being used correctly.
Without training...		...and with our assistance

For more information about traditional lime mortars, joint preparation, and installation, we recommend the National Park Service Preservation Brief 2: Repointing Mortar Joints in Historic Masonry Buildings.