Preservation by Prevention What is the right way to repoint historic masonry?

When done correctly the new mortar should blend seamlessly with the old. In this picture, the repointed sections of the wall are very obvious.

When done correctly the new mortar should blend seamlessly with the old. In this picture, the repointed sections of the wall are very obvious.

Now that spring has arrived, complete with warmer temperatures and blooms in the yard, we’ve been spending more time outside. Lots of things need repair, but most immediately worrisome is step cracking in our brick rear stoop. There is unattractive evidence of an attempt to repoint the mortar joints to close the cracks which has failed because the settlement problem that caused the cracking was never addressed. In any case, the repointing job looks bad because it is messy and the mortar does not match the original. And, that brings me to this month’s topic – proper masonry repointing.

Before beginning a repointing project, it is important to understand how historic mortars and masonry units (i.e. bricks, stones, or terra cotta) work together to form a wall. The mortar is usually “sacrificial,” being softer and more vapor permeable than the masonry unit. It will fail before the masonry unit fails. Repointing mortars must also be sacrificial, and therefore softer and more vapor permeable than the historic masonry unit and as soft and vapor permeable as the historic mortar.

As the wall contracts and expands because of thermal changes, or settles, the mortar compresses or expands, removing physical stresses from the masonry unit and preventing it from cracking or spalling. Repointing mortars that are stronger than the historic masonry units and mortars will force the historic building materials to break apart instead.

Historically, vapor-permeable lime mortars allowed the wall to breathe, providing moisture with a path through the wall. A modern mortar that does not breathe, or allow vapor transmission, will force moisture through the masonry unit instead. When moisture passes through the masonry unit, it may deposit salt crystals in the unit, which often causes spalling. Vapor-permeable lime mortars have a second advantage in that they are self-repairing; lime dissolved out of the mortar during water movement through the mortar will deposit itself into small cracks in the mortar and seal them.

Repointing historic masonry has three critical stages, preparing the joints for work, matching the new mortar to the historic mortar, and filling the joints with new mortar.

Preparing the Joints

To get the most benefit from repointing, the old mortar should be removed to a significant depth to expose sufficient bonding surfaces for the new mortar. Simply applying a shallow band of new mortar over old mortar is a low-quality repair that will fail quickly. The old mortar should be removed to a depth 2 to 2 ½ times the width of the joint. For brickwork, this generally means that the mortar should be removed to a depth of ½ to 1 inch. If there is loose or deteriorated mortar deeper than the specified depth, it should also be removed.

For lime-based mortars used with soft bricks, it is always best to remove the old mortar by hand using a chisel and hammer. This drastically reduces the chance of damaging the edges of the bricks, as compared to the risk of power tools. Power saws may slip when used on horizontal joints, chipping the bricks above and below the joint. Similarly, when used to cut the short vertical – or head – joints, power saws inevitably overcut the joint and damage the hard-fired face of the bricks above and below the cut. After the mortar is removed, the surfaces at the back and sides of the joint should be square.

To remove modern, portland cement mortars, power tools may be necessary. The best practice, in this case, is to cut a straight line down the center of the horizontal joint with a power saw and then use chisels to remove the mortar left between the brick and the cut. To remove mortar from vertical joints, a power chisel may be used if it is done very carefully, but in no case should power saws be used on the vertical joints.

After removing the old mortar, the wall and joints should be washed down. This removes debris, and it also limits the absorption of water from the new mortar by the dry masonry units. This is particularly important with lime mortars because they do not cure properly when they dry too quickly. Although the joints should be damp when they are filled, there should not being any standing water visible.

Matching Historic Mortar

Matching historic mortar is a science and an art. The science can be accomplished by having a sample of the mortar analyzed at a laboratory. The analysis determines the components and proportions of the original mortar. The most important component identified is usually the color, shape, and size of the sand because it comprises the bulk of the mortar and gives mortar its color and texture.

Historically, lime was used as the binder for mortars, in a 1:3 ratio of binder to sand. After being mixed with water, lime hardens as it absorbs carbon dioxide from the air, converting itself to calcium carbonate. Nowadays, most mortars are made with portland cement or masonry cement. These extremely strong mortars are generally stronger than historic masonry and are inappropriate for use on historic structures because they put the masonry units in the sacrificial role. However, small amounts of portland cement are sometimes added to lime mortar mixes for use in high stress areas and to improve the speed and strength of initial setting.

The art of matching a historic mortar is properly selecting all of the new mortar components – binder, sand, water, and any additives – to match the color, strength, and texture of the historic mortar while creating a workable and durable substance.

To make the repointing mortar, the dry ingredients should be measured by volume using a standard measure – not a shovel. After the dry ingredients are thoroughly mixed, water should be added in small portions. Experts recommend starting with half of the required water and then gradually adding more until the proper consistency is reached. Properly-mixed mortar should stick to a trowel held upside down, but it should also release easily if the trowel is shaken. Mortar that will not stick has too much sand, and mortar that will not release has too much binder.

Filling the Joints

Unless you are working with portland cement, you should never use power tools when repointing masonry.

Unless you are working with portland cement, you should never use power tools when repointing masonry.

Mortar should only be applied when the temperature is between 40 and 90 degrees Fahrenheit to prevent freezing of the mortar, or excessive evaporation of the water in it.

New mortar should be inserted using a pointing tool in successive layers no deeper than ¼ inch or so. The mortar should be well packed and allowed to dry to thumbprint hardness before the next layer is applied. This minimizes gaps in the joint resulting from shrinkage because each successive layer accounts for earlier voids. Once the final layer has dried to thumbprint hardness, it should be tooled to match the historic mortar profile. If the mortar is tooled when it is too soft, it can develop hairline cracks. If it is tooled when it is too hard, it may become streaked with “tool burn” or be more weakly bonded to the masonry units.

If the historic masonry units are no longer square and have rounded edges, you should consider slightly recessing the surface of the mortar in the joint. This prevents the mortar joints from appearing to be wider than they actually were historically and also avoids the creation of a thin edge where the mortar is weak and covers the corner of the masonry unit.

After the joints have been tooled, excess mortar should be removed from the surface using a stiff, non-metal brush. Lime-mortars should be allowed to cure for approximately three days before being exposed to the weather. Protecting the wall during curing essentially means preventing it from drying too quickly. This is usually accomplished through a combination of misting and protection with burlap or plastic sheeting. Once the mortar has cured, and if the job has been done well, the repointed wall should remain in good repair for at least thirty years.

 

For Additional Information

Allanbrook, Timothy and Kyle C. Normandin. “The Restoration of the Fifth Avenue Facades of the Metropolitan Museum of Art.” APT Bulletin 38, no. 4 (2007): 45-53.
http://www.jstor.org/stable/40004811 (accessed April 13, 2009).

Erlin, Bernard and William G. Hime. “Evaluating Mortar Deterioration.” APT Bulletin 19, no. 4 (1987): 8-54. http://www.jstor.org/stable/1494143 (accessed April 13, 2009).

Fontaine, Lyne, Margaret L. Thomson, and Gary T. Suter. “In Search of Durable Repair Mortar for Historic Masonry in Northern Climates: A Prologue.” APT Bulletin 29, no. 2 (1998): 39-40. http://www.jstor.org/stable/1504517 (accessed April 13, 2009).

Howard, Murray. “Craftsmanship at the University of Virginia.” APT Bulletin 18, no. 4 (1986): 30-40. http://www.jstor.org/stable/1494229 (accessed April 13, 2009).

Mack, Robert C. and James T. Askins. “An Introduction to Repointing.” APT Bulletin 11, no. 3 (1979): 44-60. http://www.jstor.org/stable/1493799 (accessed April 13, 2009).

Mack, Robert C. and John P. Spewik. “Repointing Mortar Joints in Historic Masonry
Buildings.” National Park Service Preservation Briefs 2 (1998). http://www.nps.gov/history/hps/tps/briefs/brief02.htm (accessed April 16, 2009).

Palousek, Danny. “How to Repair Mortar in a Brick Wall: Overview.” thisoldhouse.com.
http://www.thisoldhouse.com/toh/how-to/intro/0,,20268891,00.html (accessed April 16, 2009).

Rist, Curtis. “Repointing Brick.” thisoldhouse.com. http://www.thisoldhouse.com/toh/article/0,,232787-3,00.html (accessed April 16, 2009).

Shellenbarger, Michael “Tuck Pointing History and Confusion.” APT Bulletin 23, no. 3
(1991): 38-47. http://www.jstor.org/stable/1504338 (accessed April 15, 2009).

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