Chances are good that if you own or work on old houses, you’re going to come across masonry in some form or another. Depending on the home’s region, style, and how long ago it was built, you might be dealing with foundations, walls, chimneys, window and door sills, and decorative architectural details constructed from stone, brick, block, or tile. The common thread in all of these is mortar, the glue that holds everything together.

Like any part of an old building, mortar joints are a maintenance item. Over time, mortar is pushed and pulled by freeze/thaw cycles and eroded by weather, and mortar joints need to be scraped back and refilled in a process known as pointing. But mortar is a general word that is used for a wide variety of different mixtures made from a variety of ingredients in different ratios.

In the end, the ingredients and ratios used to make the mortar determine how it performs—or in many unfortunate cases, how it leads to a lot of irreversible damage to original masonry work. But by understanding the basic ingredients of mortar and how they have changed over time, you can choose the right mix for your project, ensuring that the masonry units don’t spall or crack.

Here’s an overly simplified timeline of mortar: Up until the mid-1800s, masons used a mixture of lime putty and sand. Lime putty was made on the job site by combining lime or quicklime with water, a practice known as slaking. By 1900, masons slowly began incorporating portland cement—a fast-curing, dense, hard-setting hydraulic cement—into the mortar mixture as an additive to speed set time and increase strength. By 1950, portland cement was dominant, and knowledge of using lime mortars was largely lost.

A Sacrificial Solution

If all we needed mortar to do was hold a wall together, then we would have long ago abandoned those sand-and-cement mixtures and switched to some space-age adhesive. Aside from aesthetics, the reason we continue to use mortar is that, in addition to holding the masonry units together, it also acts as a sacrificial component.

Expansion and contraction, freezing and thawing, and structural shifts all put strain on masonry assemblies. If the mortar is denser and stronger than the masonry units it’s holding together, then it’s the masonry units that will eventually fail. But using mortar that is softer and more vapor-permeable than the masonry units means it’s the mortar that absorbs the damage.

How Old Are My Bricks?

Although there are experts in the field of dating historic houses and components, the most common practice for figuring out the age of a masonry assembly is to make an educated guess based on property documents and other historical records. Bricks that are at least 100 years old may look inconsistent in size or shape rather than uniform and rectilinear, they may be smooth surfaced rather than rough, and they often have dark specks or streaks.

But be aware that in poorer communities with less access to modern technology, people may have made bricks differently from how they were made in prosperous areas, so there could be two very different-looking bricks from different regions but the same time in history.

Some Options to Consider

Mortar is a mixture of sand and a binder. The binder is one of three things: nonhydraulic lime, hydraulic lime, or portland cement. Nonhydraulic lime cures through exposure to carbon dioxide, which it absorbs through the air as it dries. If kept wet it will never cure, and so it can be stored indefinitely. portland cement cures solely by water and becomes very strong. Hydraulic lime is a compromise between the two. It cures similarly to cement but through different chemical processes. Over time it carbonates just like nonhydraulic lime, giving it a good mix of strength and breathability.

Nonhydraulic lime is the softest option—Purists argue that the best mortar option for working on old homes is lime, which is a powder derived from heating limestone. When mixed with water, nonhydraulic (NH) lime (which is different from NHL and is also sometimes called AS, or air-setting, lime) becomes a creamy paste with some plasticity to it, which is referred to as lime putty.

Add sand to the putty and it yields a mortar that is soft, porous, and easily worked but that requires intentional care over a period of days to weeks to properly harden. NH lime can be purchased as putty or already mixed with sand as lime mortar or lime putty mortar. Because it cures by exposure to carbon dioxide rather than water, it can be stored indefinitely if sealed in an airtight container or kept under a thin film of water.

Like portland cement–based products, you want to mix lime mortar to the driest formulation that is still workable. Too watered down and the mixture will be weak. It’s also important to ensure that the mortar dries slowly as it cures (anywhere from 48 hours to three weeks, depending on the type) so that it doesn’t “flash cure” by drying out too quickly.

This is typically done by protecting surfaces with wet burlap, misting the mortar with water, and so forth. Note that lime is extremely caustic when wet, so gloves and other PPE are a must. A good trick is to keep ordinary vinegar on-site, which neutralizes the lime. NH lime putty, which requires the addition of sand, sells for around $105 for 5 gal. NH lime mortar, which has the sand already added to the mix, is about $60 for 5 gal.

Natural hydraulic lime is stronger and more predictable—Relatively new to the U.S., natural hydraulic lime (NHL) has been in use since the 1990s. This type of lime is sold as a powder, to which sand is added (usually in a ratio of two to three parts sand to one part NHL), and then the lime and sand are mixed with water to start the setting process. But unlike portland cement mortars, this water-triggered setting process stems from using limestone that naturally contains clay and certain impurities that harden and set without any additives.

After the water-setting part of the lime hardens, the rest of the material sets with exposure to air, just like an air-setting lime. The main difference is that the water-setting part allows it to gain resilience much quicker than a pure air-setting lime. NHL is available in three standard strengths or hardnesses–NHL 2.0, 3.5, and 5.0. Once opened and exposed to moisture in the air, a bag of NHL may be good for two to three days, after which it should be discarded. A 40-lb. to 55-lb. bag sells for about $60.

Portland-cement mortars are modern options that are still safe—Named for the region where it’s mined and produced, portland cement is a hydraulic cement—typically gray in color, but white is also available—that begins to set once exposed to water. When choosing a type of portland cement–based mortar, you will find Types M, S, N, O, and rarely, K.

Ordered from highest strength to lowest strength, these can be remembered using the mnemonic “MaSoNwOrK,” where every other letter matches a type of mortar. For many old buildings built after the 1880s, Types N and O are the go-to products. Though a Type N mix is specified at a minimum of 750 psi at 28 days, the actual strength can be two or three times stronger thanks to the purity of modern cement. This is true of all modern cement mortars, including Type O.

Be sure also to understand the difference between “mortar mix” and “mason mix.” The former is preblended cement and sand, so you just need to add water; the latter usually includes additives to increase workability or reduce set time. In contrast to nonhydraulic and hydraulic lime, portland cement is somewhat predictable in terms of set time.

Although Types S and N are commonly available at home centers, the softer Types N and O may need to be purchased from a masonry supply house, ordered online (a 40-lb. bag of Type O sells for about $240), or made yourself by portioning out cement, hydrated lime, and sand.

Thick as a Brick

Bricks used in old houses often fall victim to changing trends and well-intentioned efforts to freshen up the exterior with a new coat of paint. But adding a coat of paint to a brick wall can quite literally destroy it. Modern paints are designed around the intention of keeping water out at all costs, so they are waterproof, stretchy, and tenaciously sticky.

Bricks, however, are like a sponge. They get wet, then they dry out. When you add a coat of paint (or many waterproofing sealants, for that matter) to the surface of the bricks and water finds its way behind the paint, which it always will, the porous bricks hold the moisture and can’t release it.

This typically leads to the outer surface of the bricks flaking off, which is referred to as spalling. Unlike a degraded mortar joint, which can be repointed, once the face of a brick has spalled, the entire brick must be replaced. If you insist on coating bricks, always opt for a vapor-open coating that won’t trap moisture. Lime wash is a typical safe choice that’s appropriate for a historic home.

— Justin Fink owns and operates Fink & Son Historic Restoration in Connecticut. Photos by Matthew Millham (except where noted).

From Fine Homebuilding #332

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