In light of recent weather fluctuations, I thought it prudent to review the maintenance and repair of historic mortar for residents contemplating spring projects. What’s the big deal about mortar? Read on to find out…
What is it and where does it come from?
The use of mortar goes back thousands of years. It was easily obtained, being primarily comprised of sand and lime. Often the lime putty was mixed with shell fragments, pigments and even animal hair to add to its bonding capabilities.
In 1872, long after many German Village homes were constructed, Portland cement was first produced in the United States, having its origins in Dorset, Great Britain in a town called Portland. There it was named after Portland stone due to its similar color. Portland cement was added to lime putty and sand to hasten the time it took to set. The construction date of your home can be a good guide to determining the composition of your mortar. Homes built prior to 1873 generally have sand and lime mortar. Homes built between 1873 and 1930 can possess various ratios of lime, sand and Portland cement. After 1930, additional types of cement were introduced in the US, all intended to accelerate the building process. .
How do I know my mortar needs repair?
The cause and appearance of deteriorated mortar is usually easy to detect. Disintegrating mortar, cracks in mortar joints, loose bricks or stones, damp walls, or damaged interior plasterwork are all indications of failing mortar. It is particularly important to address failed mortar joints around window frames and door jams as failed mortar can destabilize lintels, sills, and frames.
It is prudent to make sure the root cause of the issue is appropriately addressed prior to your mortar repair. Look for leaking roofs or gutters, differential settlement of the building, capillary action causing rising damp, or extreme weather exposure as possible causes. Luckily, many German Village homes possess stone water tables that prevent capillary action in the abutting brick above from moisture due to rain splash or puddling at grade. Form follows function, now you know why you have that beautiful course of stone around the base of your exterior walls!
Given the high sand content, and little or no Portland cement additive in older Village homes, mortar failure simply due to age is just as common a cause as entrapped moisture.
How do I determine what type of mortar to use?
What is key to the successful relationship between brick and mortar is the density of the mortar vs. the brick. Mortars for re-pointing should be softer or more permeable than the masonry units and no harder or more impermeable than the historic mortar to prevent damage to the masonry units. Professional masonry contractors know this and will do a mortar analysis prior to ensure no further deterioration to the brick occurs as a result of replacement mortar.
The National Park Service gives an excellent guide to choosing the correct mortar for your historic home:
“In creating a re-pointing mortar that is compatible with the masonry units, the objective is to achieve one that matches the historic mortar as closely as possible, so that the new material can coexist with the old in a sympathetic, supportive and, if necessary, sacrificial capacity. The exact physical and chemical properties of the historic mortar are not of major significance as long as the new mortar conforms to the following criteria:
- The new mortar must match the historic mortar in color, texture and tooling. (If a laboratory analysis is undertaken, it may be possible to match the binder components and their proportions with the historic mortar, if those materials are available.)
- The sand must match the sand in the historic mortar. (The color and texture of the new mortar will usually fall into place if the sand is matched successfully.)
- The new mortar must have greater vapor permeability and be softer (measured in compressive strength) than the masonry units.
- The new mortar must be as vapor permeable and as soft or softer (measured in compressive strength) than the historic mortar. (Softness or hardness is not necessarily an indication of permeability; old, hard lime mortars can still retain high permeability.)
Remember, mortar repair should not be done during a Polar Vortex! The exterior wall temp should be somewhere between 40 degrees and 90 degrees for a successful repair.
To learn more about repair and maintenance of historic mortar, check out this Preservation Brief from the National Park Service entitled Repointing Mortar Joints In Historic Buildings:https://www.nps.gov/tps/how-to-preserve/briefs/2-repoint-mortar-joints.htm