Learning how to repoint historic mortar is an important part of maintaining any masonry structure. Whether it’s built with brick, stone, or block any historic masonry building with lime mortar will need some amount of repointing during its life.
When repointing it’s also important to understand the differences between lime mortars and portland cement mortars because they are very different and cannot be used interchangeably. In this post I’ll walk you through a brief history of the different types of historic mortar, how to pick the right products and tools, and finally the nuts and bolts of how to repoint historic mortar.
Lime Mortar vs Portland Cement
Up until 1871 in America nearly every masonry building was made using lime mortar. Lime mortar is a slow curing, easy to work with, very versatile mortar that is made very simply of lime putty and sand. Lime mortars and natural cements were used in a variety of ways until portland cement came on the scene in 1871 when the first manufacturing plant was opened in the Lehigh Valley area of Pennsylvania.
Portland cement could achieve much quicker setting times than lime mortar, though in the early years its strength was not much more than the natural cements available so the market for it was initially small. From the early 1870s through the 1920s portland cement proportions grew rapidly in mortar mixes eventually replacing lime completely. Eventually, manufacturers were able to improve the workability and strength of portland cement in major ways making it up to 10 times as strong as traditional lime mortar!
These interim years of 1871-1929 are associated with a mortar that had progressively more portland cement and less lime as time progressed making mortar analysis important if you want to get the same mix that was originally used. If mortar analysis is not feasible then it’s best to err on the side of lime mortar so that mortar isn’t too hard for the brick.
Its increased strength and fast set time helped make portland cement the standard for construction today so that we can build the huge skyscrapers (and parking garages) that are a part of our modern lives. While it is an extraordinary material for modern construction it is a terrible choice for historic buildings for a couple big reasons.
Old Masonry & New Mortar?
Just like mortar has changed from soft lime mortars to harder portland cement mortars over the last century, masonry units like bricks and blocks have gotten harder as our kilns have gotten hotter. The rule has always been this:
“The masonry unit must be harder than the mortar that holds it in place.”
Using portland cement mortar with historic masonry does not follow this rule and here’s why.
- Too Hard: Portland cement is too hard for historic masonry. These soft bricks can be completely destroyed by hard, unwieldy mortar. Pointing old brick with portland cement mortar can cause the faces of the brick to crack and fall off and literally turn to powder, a condition known as “spalling.” The extra hard mortar doesn’t allow the brick to expand and contract with seasonal changes and its tight grip is what causes the spalling.
- Not Breathable: Historic masonry is not a waterproof material. It takes in moisture and lets it back out again as does the lime mortar. But portland cement is not breathable. Repointing with portland cement will trap moisture in the wall and cause efflorescence and premature failure of the brick. Not to mention the building is not able to sufficiently breathe to expel the moisture that inevitably builds up indoors in the winter.
So now that you understand why you should only use lime mortars on historic masonry let’s get into the nitty gritty of how to repoint historic mortar.
How to Repoint Historic Mortar
First thing you need to do is find a suitable mortar. You can make your own mortar with slaked lime putty and sand by mixing in a 1:3 proportion of lime to sand if you want to be super old school. You don’t need any water in the mix other a slight amount to “bulk” the sand initially.
Usually it’s most efficient to buy a pre-package mortar like Ecologic Mortar from Limeworks.us. They make several different colors and textures to help you find a match for you project since color matching is so important. You can even send them a sample and have them make you a custom blended mortar that matches yours exactly.
For repointing your mortar mix should be very thick. Your mortar is mixed properly if you can take a handful of the mortar and ball it up in your hand and it will hold its shape.
Step 1 Remove Damaged Mortar
Lime mortar is usually soft enough that unless you are doing large projects removal is best done with a cold chisel and hammer. For larger jobs it may be beneficial to get an Arbortech Saw like in the picture. These make short work of lime mortar removal and can even allow you to remove whole bricks without damaging the surrounding brick. Check out this video.
The mortar should be removed to a depth of 2 to 2 1/2 times the width of the mortar joint. For example, a mortar joint of 3/8″ wide should have the old mortar removed back to between 3/4″ and 1″. All mortar in that area should be completely removed from the brickwork so that you are down to clean masonry with a flat squared off back in the cleaned mortar joint.
Step 2 Vacuum Joint Clean
Once the mortar removal is complete throughly vacuum out the joint so there is no more debris. Clean is the word here.
Step 3 Wet the Joint
Using a spray bottle gently wet the joint. Don’t drench it, but every surface should be misted. Some old bricks are particularly thirsty for moisture so you may need to add a lot of water initially and let it soak in. Just make sure that the surface is damp but not glistening wet when you begin repointing.
Step 4 Fill the Joint
With you mortar mixed and ready to go it’s time to pack the joint with mortar. Using a pointing trowel that is slightly smaller than the joint width you are working with fill the joint completely and pack the mortar tightly all the way to the back. If you have trouble getting the mortar to stay on you trowel you can hog it into the joint right from the hawk like in the picture. This method can be much easier for filling very large joints.
It’s very important to pack the mortar tightly and actually overfill the joint just a bit. Also be sure to keep the mortar off the face of the brick to avoid staining it with the lime.
Step 5 Tool the Face
After the mortar has had time to setup and is thumbprint hard it’s time to tool the face of the mortar. Thumbprint hard means that when you press your finger on the mortar it doesn’t mar the mortar or leave wet residue on your thumb. Once it’s ready use your pointing trowel or putty knife to lightly scrape the joint back to almost flush with the surrounding mortar.
Your goal is to gently get the new mortar to a level just a hair fuller than the existing mortar in the area so we can finish it flush and allow it to blend in as closely as possible. It’s helpful to take a pass on each edge and then come back and scrape the center flush with the edges.
Step 6 Finish the Face
Using a churn brush slap the brush against the face of the mortar a few times. This serves to compact the mortar even more, bring more aggregate to the surface, and age the surface so it looks similar to the other aged mortar in the area.
Step 7 Proper Curing
Lime mortar cures by a process called carbonation which means it pulls carbon dioxide from
the air as it expels water. Over the next 30 days the mortar will gradually harden, but in the first 3 days it’s imperative to mist the mortar at least 3 times daily to aid in the curing process. If this step is skipped the mortar may not cure properly and may develop stress cracks.
In warm weather or direct sunlight misting more often may be required. You can also attach burlap over the surface to help slow the curing process in hot and sunny conditions.
I hope this post has helped you understand your old masonry a bit more. There is also a great Preservation Brief on History Masonry co-written by my teacher John Speweik you can read for more details and ideas.