Reducing Water Permeance in Masonry Walls and Chimneys

Experts agree that the durability of masonry depends primarily on its resistance to water penetration. Water damage may include corrosion, deterioration, dimensional damage, efflorescence, freeze-thaw spalling, staining, damage to interior finishes, and, ultimately, structural failure.

It is well established that water will penetrate a single wythe masonry wall. Even masonry walls constructed according to standard ASTM requirements will permit water penetration through the brick-mortar interface of head and bed joints. Chimney and wall design, material selection, and quality of construction are important factors in determining resistance to water penetration in masonry chimneys or walls. Understanding the causes and mechanisms of water penetration in masonry will enable technicians to better diagnose and solve problems before severe damage occurs.


The main source of masonry water intrusion is driving rains, and the level of penetration is affected by rain quantity and wind pressure. The directional consistency of wind-driven rains can create severe water damage to the affected side of an exposed masonry surface. Damage from wind-driven rain is most severe at the corners and top of a wall or chimney system because of changes in air flow patterns at these locations. Since wind speed and rainfall are such critical factors, a driving rain index for the United States was developed to illustrate climatic patterns. The driving rain index considers the average wind speed and rainfall for the country and creates zones on a scale of one to five, with ‘one’ indicating the least and ‘five’ the greatest exposure. Most severe water damage in masonry occurs in ranges three, four, and five due to frequent freeze-thaw cycles.

Once masonry walls are exposed to hours of driving rains, they usually reach a saturation point. Saturated walls will take from one to several days to lose most of this water. Trees, plants, and micro-organisms like algae can prevent it from drying. With cumulative cycles, a saturation point can be reached in masonry pores that often leads to leaks or damage to the wall system.

Water damage can also be caused by condensation within the wall system itself. Chimney systems are especially vulnerable to condensation because water vapor is a large component of flue gases. Most of the water vapor escapes out the flue, but some will pass through tile liners and the mortar joints between the liners.

The greatest exposure to condensation occurs during the heating season as surface wetting and use of the system produce high humidity in the air cavities surrounding the liners. When the temperature of outer masonry walls of a chimney falls below the temperature of the air in the cavity, condensation often occurs on the inside walls. Masonry units can absorb up to .5 lbs of water from condensation; though masonry is relatively dense, it is also a porous material composed of a network of interconnected pores called capillaries that circulate water by means of suction. Capillary suction is an important factor for openings smaller than 0.5mm. Pores in clay brick generally have a diameter of about 0.01mm, while hairline cracks can range between 0.1mm to 1mm in width.

Water can enter a wall system through pores and cracks in the masonry units and the mortar joints, but very often water seeps through cracks or separations between the masonry and mortar. The water accumulates until it either penetrates to the interior, drains to flashings where it is redirected through weep holes, or simply evaporates through the exterior wall.


A crack comparator for field use can be made by printing this page and photo copying this article onto transparency film. Keep it at hand to help you assess the condition of customers’ masonry chimney systems.

A. Hairline cracks between mortar and masonry unit

B. Incompletely filled mortar joints

C. Cracks through mortar


A crack is defined as a break, fracture, separation, or elongated narrow opening visible on the surface of a masonry unit or mortar joint or in the joint between masonry and an adjacent construction element. Cracks result from strains or stresses which are greater than the masonry can withstand.

Strains may be caused by heavy loads or by volume changes in the masonry materials. Volume changes may be caused by temperature, moisture, water or salt crystallization, or corrosion of metals embedded in the masonry. Stress cracking may also be caused by movements of foundations, structural frames, wood expansions, vibrations, or fire. Cracks in bricks that are parallel to the face and the direction of extrusion are called laminations and are not visible at the surface. All extruded brick laminated to some extent — and the excessive compressive stress caused by heat, moisture, or freezing — may ultimately cause bricks to spall or de-laminate.

Mortar shrinkage is another common cause of facial separation cracks in masonry walls. Both the water-to-cement ratios and sand consistency affect the rate of shrinkage. Generally speaking, too much water or cement in the mortar results in separation cracks. Weathering and sulfate attack from atmospheric pollutants can also cause facial separation cracks. However, it has been found that this type of cracking is most frequently caused by the inadequate tooling of mortar joints during construction.

Cracks in mortar may also be due to differences in movement between brick and mortar. For example, if the thermal expansion of the brick is greater than that of the mortar, vertical cracks may occur in horizontal bed joints.


Proper design is critical to the long-term performance of a wall system. Good design is essential because it is impossible to keep wind driven rains from penetrating a single wythe of masonry, regardless of the quality of the materials or workmanship.

Two types of wall design are commonly used in commercial construction to control water penetration. Barrier walls, which were more common prior to 1950, are typically constructed of 12″ of solid masonry. Newer cavity walls are designed with a back-up wall system.

This usually consists of a concrete masonry unit and, more recently, metal studs covered with some type of sheeting or membrane to prevent water penetration. A minimum 2″ air cavity is recommended between the back-up walls and the exterior brick facade. Flashing is installed at the base of the cavity to redirect water outward through weep holes.

Unfortunately, good design components are not always utilized in residential construction; residential brick walls are often laid directly on the foundation with no base flashing or weep holes. The lack of proper wall, crown, and flashing design in masonry chimneys invariably creates water penetration problems.

According to the Brick Institute of America (BIA), base flashing should be used at the joint between the foundation and the brick work. The flashing should extend through the brick wythe, turn up behind the exterior of the brick, and be imbedded into the back-up wall.

Above the roof line, counter flashing should be lapped over the base flashing a minimum of 3″, extended through the chimney wall, and turned up into the air space between the flue tile and the chimney.


After design, workmanship affects water permeance more than any other factor. The most critical elements of workmanship to consider in constructing masonry include proper mortar materials selection, proportioning/mixing, brick laying, and joint tooling. The selection of compatible brick and mortar is the first step. Once materials are selected, the bond strength and water resistance of the wall will further depend on the proportions of the mortar mixture and the timeliness of the brick laying. All bed and head joints should be completely filled with mortar. Concave tooling of mortar joints reduces water permeance, while rough cut, raked, or flat-tooled joints have been found to increase water permeance.

Separation cracks caused by improper tooling are common. Research confirms that completely filled joints, including the vertical head joints, greatly reduce water permeance. Research also shows that as the time interval between spreading mortar and laying brick increases, the water resistance of the wall system decreases. An interval longer than one minute is considered excessive.


The selection of masonry units is another important consideration in preventing water penetration. Although most leaks occur at the mortar joints, a good weathering grade of brick can help prevent the passage of water. However, masonry units that are cracked, dirty, or nonuniform in shape and size are more susceptible to water penetration. In addition, the larger the vertical height of a masonry unit, the greater the problem of properly filling it with mortar. As a result, walls constructed with larger masonry units are usually less water resistant.


Masonry walls constructed with Portland cement lime-based mortars are generally accepted to be more water resistant than walls constructed with masonry cement mortars. Sand gradation in mortar is known to have an effect on the water permeance of masonry walls; however, masons typically use sand that is locally available, which varies in grade by region. As sand fineness increases, more water is needed in the mortar to moisten fine particles. This can cause excessive shrinkage of the mortar and a loss in bond strength.


The compatibility between brick and mortar is a critical factor in evaluating the water resistance of a masonry wall. Very often leaks occur in masonry walls that appear to have few or no visible cracks. Many leaks in masonry wall systems originate at the head joints, or vertical joints, because often they are not completely filled with mortar.

Although bed joints, or horizontal joints, are usually completely filled with mortar, a poor bond between the brick and mortar can contribute to leakage. The points on a masonry wall most susceptible to water penetration are at the junctures of the brick and mortar, referred to as the brick-mortar interface.

For maximum performance of a masonry wall, the properties of the mortar should be matched to the properties of the masonry units. The bond between the brick and mortar is critical to performance. A poor bond may result from certain combinations of brick and mortar simply because the two materials are incompatible rather than because of a defect in either.

During moderate construction temperatures of about 80°F, a mortar mix of 1:_:4_ (Portland cement:lime:sand) by volume is recommended for use with dense brick having water absorption of less than five percent. For medium-absorption bricks having absorption between five and ten percent, a 1:1:6 mortar is recommended; a 1:2:9 mortar is recommended for use with bricks having absorption greater than ten percent.


The following list highlights important considerations for evaluating masonry wall systems.

1. Pay close attention to wall design and detailing of copings, sills, and wall terminations.

2. Check to see whether base and counter flashings are missing or improperly installed.

3. Determine whether weep holes are missing or plugged.

4. Inspect chimney crowns, which are often poorly designed and improperly constructed.

5. Check condition of gutters and down spouts to protect walls from excessive exposure to water.

To prevent water damage, a certain amount of general maintenance during the life of the structure is usually required. Sometimes cleaning or restoring masonry is necessary before repairs are begun, but it is important to consider the potential consequences of various cleaning methods. For example, washing walls with strong acids can attack and weaken mortar joints, leaving them more susceptible to water penetration. Sandblasting, another common restoration method, has been found to greatly increase facial separation cracks between brick and mortar, which also increases water permeance.

The best choice is to strike a balance in selecting a method or agent that will allow you to achieve the desired results with the least amount of damage to masonry units and mortar joints. Proper maintenance and timely repairs are essential for reducing water permeance and extending the life of masonry walls and chimneys.

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