Repairing Mortar on Your Historic Masonry Building | Wisconsin Historical Society

Guide or Instruction

Repairing Mortar on Your Historic Masonry Building

Repairing Mortar on Your Historic Masonry Building | Wisconsin Historical Society

If your historic house or building is constructed with brick or stone, you will face some mortar maintenance issues. This maintenance could include repointing of deteriorating mortar. Repointing is the act of carefully removing failed mortar and replacing it with mortar that matches the look, texture, color and density of the original mortar.

In most cases, it will not be necessary to repoint your entire structure. However, even a spot repointing project will require the skills of an experienced preservation mason with years of experience and a proven track record of doing it well. Before you hire a professional for your repointing project, you should become familiar with the process so you can ensure your project is done correctly.

Removing the Failed Mortar

When you solicit bids from masons for a repointing project, ask each bidder to identify the methods and tools he or she uses to remove failed mortar. The use of grinders and air chisels to remove historic mortar can be dangerous to the masonry. These tools can breach even the hard outer crust of bricks and stones. This is especially true with the head joints — the vertical mortar joints on the side of a brick or stone. Grinding wheels are usually bigger than these head joints and will grind into the surface of the bricks from above and below. To ensure that your brick or stone will not be damaged, you should insist that the mason you hire will remove or rake out the failed mortar joints with straight-blade screwdrivers and small hand chisels.

EnlargeRaked mortar joint

Carefully removing deteriorated mortar to a uniform depth is the first step in a successful re-pointing project. Source: WHS - State Historic Preservation Office

The rule of thumb for raking out a mortar joint is to remove the old mortar between 2 and 2-1/2 times the width of the mortar joint. Therefore, if your mortar joint is 3/8-inch wide, your mason should rake the joint out to between 3/4 and 1 inch deep. This practice will ensure that enough new mortar will be placed in the joint so it won't pop out as your brick or stone expands and contracts. Your mason should make certain the back of the raked out joint is square and even. Your mason should always lightly moisten the entire joint before installing new mortar.

Matching Your Original Mortar

If your repointing project is to be successful, the new mortar must match the texture and color of your original mortar.  A good match requires the right sand. However, if your mortar contains Portland cement, the color of the Portland cement can also influence the color match. Portland cement is available in two main colors: white and gray.

Make certain your mason uses the appropriate Portland cement color along with the matching sand to achieve the best results. Your mason should order a mortar analysis of your original mortar from a company that performs this type of test. When you order a mortar analysis, the company will tell you:

  • How and where to remove historic mortar from your house or building.
  • How to identify your mortar.
  • How to package your mortar sample for shipping to the company’s laboratory.

The company will send back the results with the sand separated from the lime and other ingredients in your original mortar. The analysis will specify the original mixture of sand, lime and other components so your mason can match the new mortar with the original.

Your mason should take the sand samples from the mortar analysis to the oldest sand quarry in your region. The quarry will often have piles of sand that match your original sand. The quarry may also have various bagged masonry sands that your mason can compare with your mortar analysis sample.

If your mortar analysis reveals that your original mortar contains lime and sand with no Portland cement, your mason should install the new mortar in layers using two or three applications about a day apart. Lime-based mortar takes longer to cure than the harder Portland-based mortars, and multiple applications will allow each layer of mortar to cure before the next layer is added. If your mason installs lime-based mortar all at once, the back of the joint will not cure as fast as the outside. The uneven curing could cause the new mortar to shrink and fail to bond to your bricks or stones.

If your mortar analysis reveals that a substantial portion of your original mortar has Portland cement, your mason can fill an entire mortar joint in one application.

Matching the Mortar Striking

EnlargeMortar joints

Make note of the type of mortar strike your building has and match that strike style when pointing. Source: WHS - State Historic Preservation Office

Make certain your mason applies the mortar to match the striking of the original mortar joints. The striking is the “shape” of the mortar in the joint. These are the three most common joint profiles:

  • Round joint. A round joint is a concave joint made by striking the mortar with a rounded tool to create a semicircle-shaped joint.
  • Flush joint. A flush joint is a vertical joint that is flush with the face of the brick (no indents).
  • Raked joint. A raked joint is a vertical joint that is set back in the space so it is not flush with the face of the brick.

These other joint profiles can also be found in Wisconsin houses and buildings:

  • Vee joint. A vee joint is a concave joint similar to a round joint, but it is struck with a “V” shaped tool so the joint looks like a “V” turned on its side.
  • Struck joint. A struck joint is an angled joint that is flush with the top brick and recessed on the bottom brick to create a line that angles inward from top to bottom.
  • Weather struck joint. A weather struck joint is the same as a struck joint but reversed. This joint angles outward from top to bottom so the top of the joint is recessed at the top and flush with the bottom brick.
  • Beaded joint. A beaded joint is a convex, protruding joint in which the mortar projects out past the face of the brick. The shape can vary from a simple convex shape to a twisted, rope-shaped bead.  

The information presented here is not intended to provide comprehensive technical advice or instructions on solving historic preservation issues. Any information contained or referenced is meant to provide a basic understanding of historic preservation practices. Read full disclaimer.