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Dane County. Cracks often follow mortar joints - here the cracking is occurring at a stone foundation wall. Source: WHS - State Historic Preservation Office.

If your historic house or building is constructed with stone, your maintenance routine should include an annual inspection to evaluate the condition of your stones. By making this annual effort to review your building’s masonry exterior, you can identify potential issues that could turn into significant problems if not addressed. The most common problems you may encounter with your stones are described below.

Mortar Deterioration

If your house or building has existed for 100 years or more, you will probably find mortar missing or deteriorating between your stones. Mortar deterioration is caused primarily by moisture penetration. Other factors can include wind erosion or  stones that have been coated with paint or sealants that do not allow moisture vapor to escape.

If your mortar has deteriorated significantly or is missing altogether, you will need to replace the mortar by repointing. Repointing is the act of carefully removing the failed mortar and replacing it with mortar that matches the look, texture, color, composition and hardness of the original mortar. (The original act of installing the finish layer to a mortar joint is referred to as “pointing.”) You should hire an experienced preservation mason to repoint your mortar.

Cracked or Spalling Stones

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Columbia County. The face of soft stones can spall leaving the masonry unit vulnerable to water infiltration. Spalled stones may be repaired either using a specifically formulated mortar, reversing the stone or replacing it with a new stone to match the original. Source: WHS - State Historic Preservation Office.

During your annual masonry inspection, you might find that some of your stones are cracking or spalling. These problems usually occur under the following conditions:

• Excessive moisture enters the stones and freezes in the winter. Stones naturally take in and let out moisture, but excessive moisture can enter through failed mortar joints, exposure to cascading water due to a lack of effective gutters or sealants that do not allow normal water vapor to escape.
• Repointing work is done with mortar that is harder than the stones. As a stone contracts and expands by taking in and letting out moisture, it puts pressure on the adjacent mortar. If the mortar is harder than the stones, this pressure will crack and spall the surface of the stones.
• Foundation settling puts downward pressure on the stones. Downward pressure from foundation settling can cause your stones to crack.

You can solve a problem with a damaged stone in two ways:

• Replace the entire cracked or broken stone.
• Widen the crack to a uniform width and fill the crack with an appropriate mortar.

Stone Erosion

Your annual inspection could reveal that some of your stones are eroding. Some historic houses and buildings were built with stones that were soft or prone to delamination and erosion. The hardness of your stones depends on the location where your stones were quarried and the depth at which they were extracted from the stone deposit. Your stones could contain lots of veins that can delaminate.

Stone erosion can also occur if your house or building was treated with an abrasive cleaning technique such as sand blasting or high-pressure washing. These cleaning techniques are so damaging to stones and mortar that both practices are banned in all preservation best practices guidelines. If your house or building is listed on the National Register of Historic Places, it is against the law in Wisconsin (WI Statute 101.1215) to use an abrasive blasting technique on the exterior of your historic building.

Generally, the softer the stone, the more subject it is to erosion, delamination and fracture. Three basic types of stone are typically used in house and building construction:

• Sedimentary rock. Sedimentary rocks include limestone, sandstone and shale. Sedimentary stones are soft when they are quarried. After these stones are cut into blocks or rubble stones, the air mixes with the cut surface and the stones begin to develop a harder protective exterior shell. You can even think of a sedimentary stone as a piece of pastry. Pastry has a hard crust on the outside, while the inside consists of multiple layers of soft, flaky bread. If you remove the hard outer crust, the pastry is not able to shed water. Instead, it absorbs water like a sponge. This water absorption in a sedimentary stone leads to the stone’s deterioration. If your sedimentary stones are laid vertical with the bed joints standing on end and water seeps between the layers of stone, the stones can delaminate. Sandblasting and high-pressure washing can remove the hard crust on stones and expose them to excessive erosion until a new shell is naturally formed on the freshly exposed surface.
• Igneous rock. Igneous rock includes granite and basalt. The word “igneous” is derived from the Latin word ignis, which means fire. Igneous rock is formed through the cooling and solidification of magma, or lava. Because igneous rock can form with or without crystallization and under many conditions, there are several hundred different types if igneous rocks. It is generally solid all the way through but occasionally it can have impurities in them if resulting from partial melting of existing rocks mixing with magma. These rocks tend to be very hard due to how they are made. In fact, basalt is harder than diamonds.
• Metamorphic rock. Metamorphic rock is formed through the transformation of existing rocks that are subject to heat and pressure. The heat and pressure causes profound physical and/or chemical transformations in the rock. For example, limestone subjected to heat and pressure is transformed into marble; shale subjected to intense pressure is transformed into slate; and sandstone subjected to heat and pressure is transformed into quartzite. Metamorphic stones are harder than their original rock. Consequently, marble is harder than limestone and slate is harder than shale.   

Structural Failures in Your Stone Walls       

Your annual inspection could reveal that sections of your stone walls are bowing inward, outward or even collapsing. These problems are considered catastrophic structural failures. These problems will typically be caused by excessive foundation settling or heaving, or massive amounts of water entering your stone walls. This water problem most commonly occurs from a roof leaking over a long period of time.

You will have to address the underlying problem or problems to remedy a collapsing stone wall. When you have solved the underlying problem, you should hire a preservation mason under the guidance of a preservation-oriented structural engineer to stabilize or take down and rebuild your stone wall.

Stair-Step Mortar Cracks

You might find cracks in the mortar between your stones that look like stair steps. These mortar cracks are primarily caused by foundation settling or upward heaving. Often when stair-step cracks happen, they never get any worse. Old houses and buildings can settle a bit over many years, and if no other moisture or structural problems exist, you can repair cracked mortar joints by repointing them. If a crack does get worse, you will have to address the structural foundation problems with a preservation mason under the guidance of a preservation-oriented structural engineer.

Stones Covered with a White Powdery Substance

During your annual inspection, you might spot efflorescence, a white powdery substance on your stones. Efflorescence happens when salts in the mortar or stone are dissolved by excess moisture and migrate to the surface of the stones. Efflorescence indicates there is too much moisture in your stone walls. Salt-laden groundwater can rise into your masonry structure (called damp rising) or water can enter through weak mortar joints, uncaulked wood, stone joints, ineffective gutters or a leaking roof.

You should hire a masonry expert to help you fix the moisture problem first. When you have solved the moisture problem, clean the efflorescence off your bricks with this two-step procedure:

1. Scrub off the white powder with a stiff scrub brush and water.
2. Clean the stone surface with a non-ionic detergent and a stiff nylon or natural bristled brush.

Stones Containing Surface Debris: Dirt, Moss and Mold

You might find stones that are dirty or have moss or mold growing on them. Dirty stones are usually the result of exposure to airborne pollution and particulates in the air that attach to the stones and mortar. Dirt can also accumulate in areas protected from the rain. Hire a preservation professional to help you identify and fix any moisture problems and then clean your stones. Preservation masonry supply companies sell a variety of products designed to kill and clean mold, mildew and moss.

CAUTION: Never remove dirt, moss or mold with high-pressure washing or any kind of high-pressure blasting, such as sand blasting. The use of higher-pressure power washing will erode the harder surface of your stones and cause permanent damage.