How to Remove Mortar Before Repointing

The biggest challenge in restoring masonry walls and buildings is replacing Portland-cement-based mortars with a natural lime mortar. The difference between the two is easy to determine. Using a pointing chisel to remove lime mortar is quite easy; the mortar is soft and breaks easily in front ofthe chisel. Portland-based mortars are quite hard, and hand-chiseling takes many blows from the hammer to remove just a couple inches of joint.

This hard mortar is very damaging for brick and soft stone. One reason they are so damaging is that Portland-based mortars (Type N, Type S, and most mortars available today) are not water permeable, so they do not allow moisture to escape from the wall. Also, Portland-based mortar is not flexible, but brittle, so the masonry units are stressed by freezing temperatures and imperceptible building movements. This can take the faces of brick right off of buildings that should be preserved and protected. Natural lime mortar will wick moisture away, and flex with the small movements of temperature changes and settling that occurs in every building over time.

To remove Portland-based mortars efficiently, a grinder should be used to cut a groove in the center of the joint. Usually a thin blade works better than a thick one, and a 4" blade will cut through the Portland-based pointing back to the original lime-based mortar. Cutting the center of the mortar joints prevents slipups from damaging the original brick or stone. Special care should be taken on head (vertical) joints to cut only as deep as the grinder can without cutting into the masonry units. This can take strength and focused attention because the grinder is harder to control when removing very hard material. Center-cutting the joints relieves the pressure that the hard mortar puts on the faces of brick and stone.

The second step is to hand chisel the joints out. Usually the best method is to place a flat chisel right where the mortar meets the edge of the brick or stone, aiming toward the center groove that was cut with the grinder. A pointing chisel can also be used to go across. Pick a 2"or 3" section of joint and aim the pointing chisel toward the area that has already been removed. The chisels will break the bond between the Portland-based mortar and the brick or stone.

Usually the repointed joints are less than an inch deep, so removal is safe as long as care is taken with the faces of the masonry units. Remove mortar at least 2x the height of the joint. A 3/8"-wide joint should be chiseled back to at least 3/4". This will ensure an adequate bond between the new mortar and the brick or stone.

Once the mortar is removed, the joints should be vacuumed, or blown out with compressed air. A low pressure jet of water, such as from a nozzle on a garden hose, can also be used. If the joints are vacuumed or blown out, it is a good idea to dampen the wall before applying the new mortar.

Never use a pressure washer on soft brick or stone. High-pressure water can damage walls of even hard stone (like granite), so the nozzle should be kept well back from the face of the stone. Pressure washers often remove the beauty of the weathered faces of stone, including the green moss that can make a building look old.

As a company, we are committed to the challenge of using sustainable building methods in our historic restoration projects. Sometimes we are asked specific technical questions as to how these two objectives are reconcilable--how can we use historic methods over modern techniques to stand the test of time? Recently, we received just such a question: "I am committed to using the sustainable and historically accurate natural hydraulic lime mortar on my chimney cap, but how will the natural lime mortar hold up in the long run?"

This is a valid question. After all, Portland is harder than natural lime mortar, right? So although it is historically accurate, how is it sustainable to use a softer material on a chimney, one of the hardest to reach places and one of the most vulnerable spots on your historic house? Shouldn't your chimney, of all places have a durable and water proof cap? Beautiful homes, that have been meticulously maintained, have often been found to have small plants growing out of the chimney.

First let's consider a few of the attacks a chimney has to endure. The most obvious is the extreme exposure to all forms of moisture: rain, snow, dew and frost. Constantly varying temperatures throughout the year, from cold winter nights to scorching hot summer days add an additional assault on the chimney--the constant expansion and contraction takes its toll. In the dead of winter, when the furnace kicks on and 400 plus degree air suddenly hits a 15 degree chimney flue, then what happens? The resulting pressure has an effect much like putting an untempered glass into a hot oven: it cracks. This expanding and contracting phenomenon has another destructive effect; the clay liner which your chimney is built with will expand upward as it heats up then contract back down as it cools. This effect may further compromise your chimney's structural integrity. As if this was not enough, a surprising attack comes from another part of nature. A chimney is a natural resting place for birds. We have seen the corrosive effect that bird droppings have on the integrity of a chimney cap. While we won't attempt to address all of these issues here, we will address the basic way that we apply a chimney cap.

Needless to say, the chimney has a very difficult and challenging life and it would be advisable for the responsible homeowner to inspect a chimney for integritymuch as he would inspect his gutters for leaks. Just as a leaky gutter will ultimately wreak havoc on any masonry construction, a leaky chimney will do the same. Add the chimney cap to the schedule of five year maintenance checks. Eventually, you may have to repair it, or even replace it completely. As I like to say, a chimney cap replaced in time can save nine.

So, now let's go back to the historical use of hydraulic lime mortar on the chimney cap instead of Portland. Portland is harder and water proof right? Doesn't that mean it will last longer, and need fewer repairs? It's a good theory. But there are more destructive forces at work on a chimney cap than just moisture.

Recall our description of one of the most destructive attacks: thermal expansion and contraction from the flue gasses as they heat a very cold chimney. Portland may be harder, but hardness doesn't handle that kind of expansion very well, it cracks easily and once it cracks it doesn't heal. Earlier, we raised a concern that natural lime mortar is softer, right? But now we see that the softness is a positive thing because it will permit more movement than Portland. If it does crack, it heals itself! This is how it works chemically: the content of natural free lime actually attracts to itself and thus it self-heals, sealing the crack.

The tried and true method we use to apply a chimney cap is as follows:

We apply three coats of mortar. The first two coats consist of two parts course concrete sand, one part 3.5 natural hydraulic lime mortar, and one handful of natural hair or fiberglass. The third and final coat we apply is proportioned as follows: two and one half parts sand, one part 3.5 natural lime mortar, this final coat has a bit more sand; no fiberglass or hair. This method will allow the first two coats to act as a stronger mixture--the fiberglass acts as a rebar of sorts-and the final layer seals the fibrous ends so they won't act as a wick for water absorption; we don't want a conduit for moisture. For a few weeks after the new chimney cap has been applied, it should be covered from moisture-depending on the temperatures-and should be kept damp in hot temperatures.

While the exact method of applying a new chimney cap may vary extensively, we have found that our method works quite well and lasts for many years. It is our commitment to balance the use of natural and sustainable products, while enabling the use of historically accurate products and procedures.

Brookline Builders will repoint and rebuild your old chimney with Natural Lime Mortar. The reason most chimneys fail is because of water penetration. If a cap of either flagstone or metal is maintained and installed on your chimney, the water will not penetrate and your chimney will outlast the roof. If water is not kept out of your chimney it will deteriorate extremely fast, especially in the winter during freeze thaw cycles. If your chimney needs to be rebuilt don't ever use portland based mortar, especially on a chimney.

Portland will not withstand the expansion and contraction of an exposed chimney, it will crack quickly then fall apart in just a few years. This is because portland based cements are water proof, so once the water penetrates into the chimney, and it will, the portland does not allow the moisture to escape. This moisture that is trapped in the exposed chimney will expand when it freezes and thaws. Over a few years the effect of this cycle the chimney will blow apart.

The key to repointing an historic chimney is to not use any portland cement. Rather use natural lime mortar. Natural lime mortar will allow trapped moisture to quickly escape and will self heal any cracks. The pyramids in Egypt were built using natural lime mortar. Portland cement wasn't widely used in the US until the early 1900's.