In most of our market areas, where homes typically are wood framed structures, stone houses occupy a special place. Stone masonry is a construction method of the classic antiquity of Europe, where most of our ancestors came from. Stone masonry has endurance.
This article may help you understand some of the characteristics of this material and perhaps give you a better understanding of what to look at when you inspect stone masonry.
Stone construction represents a high level of skill in shaping and assembling a difficult natural material. Before the industrial revolution, transporting this material was also a factor in which type of stone was used in a particular locality. Different parts of the country have different rock formations and geological conditions. The stone mason and the quarryman would use the most suitable stone that was close at hand. After all, transporting stone from quarry to job site was usually done by horse and wagon.
Sandstone was a popular material in Wisconsin in the 1800s because it was plentiful and readily available. Therefore you would see buildings made of this material in the Madison area for example. In Michigan, you are likely to see buildings made of a much harder igneous stone, the type of boulders that a farmer would gather when getting a field ready to plant. In fact, they are called field stone, usually very irregular in size and shape.
Generally, all stone masonry is either rubble or ashlar. Rubble is composed of loose stones used as is in its natural state or split to create roughly square edges while retaining a natural outline. Ashlar masonry is made up of stone that is squared so that its exposed surfaces are uniform. The regular dimensions of ashlar stone makes specific or repeating patterns possible and in this respect is much like brickwork. Within these basic categories, the character of the stonework is often further defined with labels such as coursed, split, broken or random.
Prior to roughly the middle of the 19 th century, most stone houses were built with solid stone walls. These walls consisted of outer facing stones that created the exterior appearance of the building inner facing stones that were usually less refined and infill of smaller or broker stones that made up the space between. Larger through stones or tie stones then bonded together the full width of the wall at strategic places so it would not buckle under the load. Other details included wood strips mortared in with the inner facing stone to create a nailing surface for anchoring plaster lath and cavities created by incompletely mortaring the infill to limit moisture condensation. In the latter 19 th century, the construction of most stone house walls gradually changed from solid stone to using brick infill with a stone exterior.
Let’s examine a stone building.
Begin by looking at the overall appearance of the exterior. In a well constructed stone building, the walls are straight without bowing or leaning. This would be the same for any type of construction but in a stone building built in the early 19 th century, if you do see a failure of this type, you would look for reasons associated with the manner in which the stone was laid. A poorly laid foundation for example. You may also find that, over the years, exposure to the elements has caused failure of the mortar joints. This would result in some of the units being loose or missing. You will also see that in some buildings made with a softer type of sandstone, an erosion pattern has formed from rain and wind at a predominant location. Some of the more ornate features such as carvings in the soft stone may have lost their sharpness due to the elements.
It would not be unusual for some kind of repair to have been done on an 1800s stone building. After more than 150 years of exposure to wind and rain, re-pointing of the mortar joints would have most likely been necessary.
A common problem when re-pointing historic mortar joints is that the modern materials used are often not compatible with original materials.
In a masonry wall, mortar not only fills the voids and distributes the load of the structure; it acts as a buffer between the stone or brick units, particularly in older masonry where gravity plays a major role in holding the system together. From a perspective of restoration, mortar is either “soft” or “hard” according to its compressive strength.
The earliest soft mortars were made with clay. In some colonial buildings, walls were completely mortared with clay, sometimes mixed with straw or animal hair for strength. When used to build chimneys, clay mortar was baked hard by heat. The most common soft mortar binder used until the late 1800s was made from lime. Lime mortar sets in several hours but the final hardening is a very slow process as the lime re-forms into calcium carbonate. This can take months or years to achieve maximum strength. The result is a mortar that expands slightly as it sets, (making shrinkage cracks unlikely) and relatively stable through temperature changes. Lime mortar is also porous enough to allow water vapor to migrate out of the masonry and water soluble enough to enable it to re-deposit itself and “heal” hairline cracks that appear due to settling or other stresses.
Lime mortar is soft because it gives and takes or “flexes” as masonry units expand and contract keeping the bond intact.
By and large, it’s a safe bet that mortar in a pre 1800s building is straight lime mortar. This is important to know because mortars for re-pointing or repairs must always be compatible with the original type used. Introducing a hard high Portland cement mix in soft lime mortar masonry can cause problems over time such as cracking of the hard mortar or spalling and disintegration of the softer materials. If you are not sure about the nature of the existing mortar, brush vinegar on representative samples. The vinegar will immediately show fizzing if the mortar is lime based no action if it is Portland based.
Don Randazzo has been a member of A S H I since 1992. He has a background in historic preservation. Retired from The Henry Ford museum in 1996 and has been doing home inspections since the mid 1980s.