Introduction to Historic Masonry

There are major fundamental differences between historic structural masonry buildings and modern buildings. Understanding the difference between these two is paramount to be able to properly renovate, repair, repoint, and care for these older buildings.

Architects, engineers, builders, project managers and workmen must have a thorough understanding of what a structural masonry building is. Understanding the mortar they were built with, how water moves, how salt affects old masonry, the types of mortars that should be used, and how the brick and stone foundations were designed is important to building longevity. It is asking for failure to apply new construction techniques and materials to old structural masonry buildings.

What is Structural Masonry Construction?

A solid structural masonry building, also referred to as load bearing masonry, is where the exterior supporting walls are built entirely of solid masonry. Structural masonry supports the building and all structural components.  This is not to be confused with a masonry building using wood or steel as the supporting structure with a single layer of masonry veneer brick or stone applied to the exterior. Historic brick walls built using the solid structural masonry construction method are at least two wythes of brick thick. Buildings that go multiple stories high often have 4-6 (or more) wythes of brick at the lower levels of the building. As the walls of the building extend progressively higher, the number of wythes of brick in them will decrease. For example, a typical two and half story brick farmhouse will have three wythes of brick in the first-floor exterior walls and only two wythes of brick in the exterior second floor and attic walls.

Structural stone walls are generally 22” – 24” wide at the foundation levels and will sometimes narrow to 18” or so in attic gable ends. These solid structural masonry walls handle moisture and temperature swings in totally different ways than veneer masonry walls. Solid masonry walls hold onto large amounts of moisture. Solid structural masonry walls will also hold temperatures for greater amounts of time when compared to a veneer masonry wall. Remember that in a load-bearing masonry building, we want the mortar to act as a cushion to evenly distribute the load. It is not a glue that glues bricks and stones together.  

Brief Description and History of Lime Mortar

Lime is not Lime is not Lime

This brief phrase expresses the complexity of lime. There are hundreds of types of lime in use in countless industries. Lime is used in the manufacturing of chocolate and steel. It is used to scrub waste air in factories to meet EPA requirements. Lime is used in wastewater, drinking water treatment, and soil stabilization. There is garden lime for yards and farmers’ fields. There are types S and N lime for adding to Portland cement. There are also Natural Hydraulic Limes, and Carbonating limes which is what we specialize in.  For lime to act as a sole binder when mixed with sand in the making of mortar or plaster, it must have very particular characteristics. Some of these characteristics are:

• High pH
• High surface area
• High calcium
• Highly reactive carbonating lime. 

The Law of Lime:

The basic overarching law of chemistry that applies to historic masonry lime mortar structures, is that lime slowly breaks down when it stays wet or damp. This law is as irrefutable as the law of gravity. If you let go of a rock, it will fall to the ground. If lime stays wet and damp, it will eventually dissolve.

The law of dissolving lime is observed often in Lancaster County. Sinkholes open when the lime bedrock dissolves from underground water. It is because of this law of chemistry that solid structural masonry buildings stay dry . When these walls do get damp, they must dry quickly, or the very mortar they were built out of will dissolve. The amount of time in which the mortar dissolves depends on many things. However, it is not a matter of “will” the mortar fail if it stays wet/damp. It is a matter of “when” it will fail.

The abbreviated history of using lime as a primary binder in mortars can be summed up:

• When man first started using mortar to build with, lime was the only material used. There is a high probability that a building built in the USA before 1900 was built using a carbonating lime putty mortar.
• Structural stone walls before 1900 usually used earth mortars in the interior of the stone wall with just the exterior being pointed with lime mortar.
• Portland cement was widely and commercially introduced in the USA in circa 1900. Although there are a few instances where Portland used in masonry in the US before 1900, it was extremely rare.
• Between 1900 and 1930 we find a mix of historic lime and Portland cement being mixed in mortars used in the USA.
• From 1930 to present day, Portland cement is the primary binder that has been used in the USA. It is rare to find a building built in the USA with lime as the binder in the mortar after 1930. 

Lime mortars gain most of their structural strength and longevity from the sand. The sand needs to be sharp, and pack firmly and tightly together. The lime in a true lime mortar, only acts as a binder to hold sand in a packed interlocked state. The binder itself does not contribute structurally other than to hold the sand in place. The sand itself must have a wide variety of particle sizes, be sharp, angular and be able to structurally interlock tightly of it's own accord. Add lime in just the right quantity to fill the void space between the particles of sand. 

How "strong" does my mortar need to be?

For a load-bearing masonry building, not very strong at all. The guidance from the Brick Industry Association is that mortar should always be weaker than the masonry units it binds. Use the lowest compressive strength mortar that meets performance requirements. In other words, “stronger” or higher psi mortars are not better. 

The study of historic lime mortars in vast. This is only a summary and continues in the learning center under "Types of Binders", "Salt and Historic Masonry" and "Moisture and Structural Masonry". Thank you for reading!