Technical Notes 7
Water Resistance of Brick Masonry, Design and Detailing, Part 1
Feb. 1985 (Reissued Feb. 1998)

Abstract: Proper design and detailing of brick masonry walls to allow minimum water penetration into or through a wall system are absolute necessities. Careful evaluation of several items is a requirement in order to obtain proper resistance to water penetration. The selection of the proper type of wall is of utmost importance in the design process as is the need for complete and accurate detailing. In addition to discussing various wall types, this Technical Notes illustrates suggested details which have been found to be acceptable methods of achieving walls resistant to water penetration.

Key Words: barrier, brick masonry, design, detailing, drainage, flashing, installation methods, rain, wall types, weep holes.

INTRODUCTION

Water penetration is responsible for many of the problems encountered in masonry walls today. If a wall is saturated with water, freezing and thawing may cause cracking, crazing, spelling and disintegration. Water can cause masonry to experience dimensional changes, metals to corrode, insulation to lose its effectiveness, interior finishes to deteriorate and efflorescence to appear on exterior surfaces.

Water is abundant in many forms. Rain and snow contact building materials, wetting them. Water vapor is present in the air from many sources. As a result, since water cannot be completely eliminated, water penetration must be controlled. When water passes through brick masonry walls, it invariably does so through minute separations between the brick units and the mortar joints. Under normal exposures, it is virtually impossible for significant amounts of water to pass directly through the brick units or through the mortar. Highly absorbent brick will absorb some water, but certainly do not contribute to an outright flow of water through a wall.

Before brick curtain wall systems became popular, masonry walls usually functioned as both the structural system and as the exterior skin of the building. As a result, these masonry walls were quite massive, ranging in thickness from 12 in. (300 mm) up to 6 ft (1.83 m) of solid brick. These masonry walls, both because of their thickness and their being in constant compression due to the structural loads, worked quite well in keeping water out of the interior of the building. Many older masonry walls were built with cornices and other ornamentation which helped to protect the faces of the buildings from excessive water rundown and subsequent water penetration to the interior.

Walls used today are much less massive, and the masonry may be only 3 in. (75 mm) in thickness. In many cases, they have minimal overhang at the top, allowing sheeting of the rain water from the roof or parapet down to the ground. As a result of these newer wall systems, rain water is allowed to be in contact with the masonry in larger quantities and for longer periods of time, thus leading to more opportunity for water penetration problems.

The successful performance of a masonry wall depends on limiting the amount of water penetration and controlling any water that enters the wall system. If water penetration can be limited, for all practical purposes, the wall will remain dry.

Water resistance of a masonry wall depends on four key factors:

1. Design, including detailing.

2. Materials.

3. Construction.

4. Maintenance.

Attention to all four factors is necessary to produce a satisfactorily performing wall. Failure to properly address any one factor can result in water penetration problems.

This Technical Notes Series addresses water resistance of brick masonry. The first three Technical Notes in this series provide detailed guidelines in the above areas of design, materials and construction. Other Technical Notes in the series provide information on condensation analysis, and the use of colorless coatings on brick masonry. The subject of "Maintenance" will be covered in a future issue of this Technical Notes Series.

This Technical Notes deals with proper design of brick masonry to resist water penetration.

DESIGN

The first factor to evaluate in the control of water resistance of masonry is that of design. Proper design of masonry does not mean just proper structural design. Design includes fire resistance, heat transmission, structural integrity, material compatibility, sound reduction, aesthetics and water resistance. Other Technical Notes provide guidance on all of these different design factors.

Design for water resistance requires evaluation of several items, including: (1) sources of moisture; (2)selection of wall type; and (3) flashing and weepholes. Each of these items will be addressed separately.

Sources of Moisture

Moisture is present almost everywhere in various forms, i.e., rain, snow, condensation, ground water, construction water, etc. Some of these lend themselves to control; some do not. This section deals with wind-driven rain. Interstitial condensation and its control are discussed in Technical Notes 7C and 7D.

Wind-Driven Rain
The exposure to which a masonry wall will be subjected is very important to the proper design of the wall. No single standard design can be expected to perform equally well under all exposures.

Exposures vary greatly throughout the United States, from severe on the Atlantic Seaboard and Gulf Coast, where rains of several hours' duration may be accompanied by high velocity winds; to moderate in the Midwest and Mississippi Valley, where wind velocities are usually lower; to slight in the arid areas of the West. See Figure 1. Exposure areas may be defined roughly in terms of wind pressure and annual precipitation as follows:

FIG. 1

Severe: Annual precipitation 30 in. or over, wind pressure 30 psf or over.

Moderate: Annual precipitation 30 in. or over, wind pressure 20 to 25 psf.

Slight: Annual precipitation less than 30 in., wind pressure 20 to 25 psf, or annual precipitation less than 20 in.

More recently, a Driving Rain Index (Grimm, C.T., "A Driving Rain Index for Masonry Walls", Masonry: Materials Properties and Performance, ASTM STP 778, J.G. Borchelt, Ed, American Society for Testing and Materials, 1982, pp. 171-177), see Table 1 and Fig. 2, has been proposed, which is based on the assumption that the likelihood of rain penetration is proportional to the product of annual average rainfall and annual average wind speed. The wall exposure (severe, moderate or sheltered) is then determined by correlating the Driving Rain Index with the wall relative to its surroundings, using Table 1.

Driving Rain Index

FIG. 2

Selection of Wall Type
The selection of the proper wall type to use in any given situation is very important. Under normal conditions, it is nearly impossible to keep a heavy wind-driven rain from penetrating a single wythe of brickwork, regardless of the quality of the materials or the degree of workmanship used.

Therefore, a major concern is to control the moisture once it begins to penetrate the wall. Two basic wall systems are used for this purpose: the drainage wall and the barrier wall.

Typical Brick Cavity Wall

Fig. 3

Drainage Wall Systems - Drainage wall systems include cavity walls (metal-tied and masonry-bonded hollow walls), and anchored veneer walls. See Figs. 3 through 6. The basic concept behind the drainage wall assumes a heavy, wind-driven rain may penetrate the exterior wythe of brick. When it does, the moisture migrates inward to the cavity or air space between the wythes. Here it flows down the back face of the outer brick wythe, is collected on the flashing, and is directed out of the wall system through the weepholes. Properly designed, detailed and constructed drainage wall systems are rated excellent with respect to water penetration resistance. Specific detailed information on all aspects of cavity wall systems can be found in Technical Notes 21 Series. Technical Notes 28 Series addresses anchored veneer wall systems.

Barrier Wall Systems - Barrier wall systems, shown in Fig. 7, include multi-wythe walls (including composite brick and concrete block walls), reinforced brick masonry walls and adhered veneer walls. The basic concept is that when a wind-driven rain penetrates the exterior wythe of brick masonry it migrates inward toward the interior wythe. When this migrating water reaches the filled collar joint, the joint acts as a barrier to prevent further inward movement. The water then flows back out of the wall system. The key item is that the collar joint must be completely filled with grout or mortar to be an effective barrier. Placing mortar in the collar joint with a trowel after the individual wythes are laid, commonly referred to as "slushing", does not result in completely filled joints, and is not recommended. Properly designed, detailed and constructed barrier wall systems are rated very good with respect to water penetration resistance.

Single-wythe masonry walls can be considered a special case. In single-wythe walls, the masonry wythe is usually much thicker than a nominal 4-in. (100 mm) thick exterior brick wythe, and as a result, the added thickness helps to prevent water from penetrating to the interior of the wall system. Single-wythe walls are not as good in preventing water penetration as are drainage wall systems or multi-wythe barrier wall systems, but with careful detailing and good construction practices, they can perform very well. Single-wythe brick masonry construction can be designed with either solid or hollow units.

Flashing

Flashing is a membrane, installed in a masonry wall system, which collects water that has penetrated the exterior wythe and facilitates its drainage back to the exterior. Flashing is essential in a drainage wall system, and is recommended as a second line of defense in a barrier wall system. Various types of flashing materials which may be used in the design of brick masonry and composite walls are covered in Technical Notes 7A Revised.

Locations
Proper design requires flashing at wall bases, window sills, heads of openings, spandrels, shelf angles, projections, recesses, tops of walls and roofs.

Wall Base - Moisture which does enter a wall gradually travels downward. Continuous flashing must be placed above grade at the base of all walls to divert this water to the exterior. In addition, base flashing prevents water from rising up into the wall system due to capillary action. Once the designer has determined the level for placing flashing in the wall in accordance with the grading plans, care should be taken that field modifications do not result in any section of flashing being below grade.

Window Sills - Through-wall flashing should be placed under all sills and turned up at the ends to form dams. Soffits and deep reveals may require special flashing considerations. Technical Notes 36 Series contains further details and information.

Steel Lintels - Through-wall flashing should be installed over all openings. An exception may be those completely protected by overhangs. The flashing should be placed directly on top of the lintels and turned up at the ends to form dams.

Spandrels and Shelf Angles - In concrete or steel frame buildings, the entire faces of the spandrel beams may be flashed or the flashing may be inserted in a continuous reglet installed in the spandrel beam. When the entire face of the spandrel is flashed, layered flashing (counter-flashing) should be used. See Fig. 13.

Steel Shelf Angles

Fig. 13

Projections, Recesses and Caps - Projections, recesses and caps tend to collect rain water and snow. They should be sloped away from the wall to drain and be flashed where possible. Other details and information can be found in Technical Notes 36 Series.

Tops of Walls - The tops of all walls and parapets should have an adequate cap or coping, and there should be flashing beneath the coping. Drainage-type parapet walls are recommended as the best wall system for resistance to water penetration. Technical Notes 36 Series provides more details and information on these subjects.

Roof Flashing - Because roof flashing occurs at very vulnerable points, it must be designed and installed with great care. Roof flashing design may depend upon the type of roofing used. Where the roof flashing is metal, the counter-flashing should also be metal, extending into the wall and overlapping the roof flashing a minimum of 3 to 4 in. (75 to 100 mm).

Installation Methods
In addition to specific location information, there are other considerations regarding installation of flashing which the designer must address. These are discussed in detail in this section.

Continuity - Flashing is not usually installed in one long, continuous sheet. As a result, pieces must be fitted together on the job. Flashing pieces should be lapped at least 6 in. (150 mm) and the laps sealed with mastic or an adhesive compatible with the flashing material.

End Dams - Where the flashing is not continuous, such as over and under openings in the wall, the ends of the flashing should be extended beyond the jamb lines on both sides and should be turned up into the head joint several inches at each end to form a dam.

Flashing Around Corners - When flashing around corners, the flashing should be continuous. To achieve this continuity, the pieces of flashing may need to be cut, lapped and sealed to conform to the shape of the structure.

Flashing at Vertical Supports - In some cases, vertical support angles make it necessary to cut, puncture or otherwise interrupt the flashing. When this occurs, it is important to make sure that all openings in the flashing are tightly sealed, and that the flashing is attached to these supports with mastic.

Gravel Beds - It may be desirable to provide a layer of gravel several inches deep on top of the base flashing. This will help keep mortar droppings from falling on the flashing and clogging the weepholes. Care must be taken in the choice of size and shape of the gravel to avoid blocking the weepholes and puncturing the flashing. It is recommended that a bed of mortar, conforming to the curve of the flashing, be placed under the flashing for additional support of the gravel bed. Note that gravel should not be placed on top of flashing which covers bolted shelf angles as the weight of the gravel on the flashing may cause tearing or puncturing at the bolt head.

Bond Breaks - It must be remembered that mortar bond to flashing is not as good as mortar bond to masonry units. Where mortar is placed immediately above and below flashing, flexural strength of the wall may be reduced about 30 to 70 percent. Where flashing is placed directly on masonry, without mortar, the flexural strength should be considered as zero.

Weepholes

In order to properly drain any water collected on the flashing, weepholes must be provided immediately above the flashing at all flashing locations. The practice of specifying the installation of weepholes one or more courses of brick above the flashing can cause a backup of water and is not recommended. In general, weepholes should be at least 1/4 in. (6 mm) in diameter, and should be spaced no further apart than 24 in. (600 mm) o.c. horizontally. In other cases, such as where a wick material is used in the weephole, the spacing should be reduced to 16 in. (400 mm) maximum. See Fig. 24.

Flashing and Weepholes

Fig. 24

SUMMARY

This Technical Notes is the first in a series on water resistance of brick masonry. It provides the basic information required to properly design and detail brick masonry to avoid water penetration problems. Obviously, this Technical Notes cannot cover all designs or all conditions. The details are provided to illustrate the principles involved, not as standard details.

The information contained in this Technical Notes is based on the available data and experience of the technical staff of the Brick Institute of America. This information should be recognized as recommendations which, if followed with good judgment, should result in masonry walls that are resistant to water penetration.

Final decisions on the use of information, details and materials as discussed in this Technical Notes are not within the purview of the Brick Institute of America and must rest with the project designer, owner or both.

REFERENCES

More comprehensive information on specific design procedures and details for various types of walls beyond those items discussed in this Technical Notes is contained in the following publications:

Efflorescence Part 1• Efflorescence Part 2 • Brick Classification
Moisture Control • Condensation Analysis • Colorless Coatings
Water Resistance • Return to UKC Tech Notes