Someone recently asked me a great question. It was related to a mid-1800’s brick home that needed some repointing and brick repairs. The question was something like:
What kind of mortar would you use to repoint? Would Type O mortar still allow the walls to breath?
First, this is an impressive question coming from someone without experience in either historic preservation or masonry. It shows a high level of understanding of the needs of structural masonry walls and the importance of moisture-release in mortar!
You hear a lot about the “Types” of mortar in restoration circles. Let’s dig into this a bit.
The Types of Mortar (K, O, N, S, and M) are based on ASTM standards for compressive strength. Compressive strength is how engineers measure the amount of load (weight, or pressure) a material can hold without compressing or distorting in any way. Compressive strength is measured in pounds per square inch (psi).
The strength of Type O mortar is specified as a minimum of 350 psi. Type K is minimum 75 psi.
To put this in perspective, a structural masonry home (solid brick or stone walls that carry the whole weight of the house, contents, and roof) puts between 40 and 80 psi of pressure on the ground. The low end would be a large house with a slate roof and a typical footing, and the high end would be the same house with the maximum amount of people and belongings that the flooring would be able to handle.
So a typical large brick house built before 1920 needs a mortar of roughly 60-80 psi to more than handle the weight without the mortar failing, and Type O is 350+ psi !
You see, the compressive strength of the mortar is really not the issue. The real issue is how well does the mortar release water?
Unfortunately, nobody in the building trades is talking about or knows about the moisture-releasing capacities of materials.
The modern perspective is that moisture should be sealed out, which is contradictory to the very principles with which historic homes were built: the masonry can be in contact with the ground because the mortar releases the moisture without any damage to itself.
Not so with portland cement.
Here’s what you have to remember: mortar that is made with portland cement—in any amount—does not release moisture easily. Even a Type O mortar (way too hard for your bricks) is going to hold onto the moisture because it is made with cement.
The beauty of lime-sand mortar, such as what we use from Lancaster Lime Works, is that it has plenty of compressive strength for structural masonry, but releases moisture like your cotton underwear drying on a clothesline.
The problem in masonry restoration today is that most masons were trained to open a bag of mortar and mix with sand and water. But the bags are all just different percentages of cement and additives. Most know little about different kinds of lime.
You cannot even buy high-calcium lime-sand mortar at the masonry supply yards. Nor can you buy a lime there that will react with carbon dioxide and get hard enough to be a stand-alone binder in mortar.
There are only a couple companies nationwide making it, and it is a different installation process than cement mortar.
So the answer to the question is:
no, Type O mortar doesn’t offer any more breathability than any other cement-based mortar. Type K is the closest to correct because it is softer, which is better for your bricks, but still not breathable (vapor permeable). It just has a lower cement content.
We would use a non-hydraulic, high-calcium, lime-sand mortar (contains no cement), which is not classified with ASTM specs, but would fall in the range of Type K in compressive strength.
So if you are talking to a masonry company about repointing your pre-1900’s brick or stone house, insist they use Lancaster Lime Works mortar. LLW can provide them with training on how to install it.
Or, call Brick and Lime.
I have been working with building conservators for several years, learning about the characteristics of mortar and how to install it properly. I have overseen 4 projects for the federal government which involved lime mortars and cement mortars, including work on the Blair House and the Winder Building in Washington, DC.
Specifically, I was trained in lime mortar theory and installation by John Greenewalt Lee, and have worked with the conservators in the White House Service Center and the National Park Service in completing their projects with lime mortar and stucco.
They would all agree: your pre-1920’s house needs high-calcium lime mortar to perform as it was designed (release moisture without damaging the brick) and stand the test of time.