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Building Envelope Design Guide - Masonry Wall Systems

Richard A. Weber, RA, PE
Wiss, Janney, Elstner Associates, Inc.

Last updated: 02-19-2013

Introduction

Masonry has been used in building construction for thousands of years in construction. It can be used to form a durable cladding system and to achieve various aesthetic effects. The masonry units can be oriented in different positions to create different patterns on the exterior wall. In addition to forming the exterior cladding, masonry walls can serve as a portion of the structural framing for the building. Masonry walls also typically increase the fire resistance of the wall system or structural elements.

Masonry walls can be single or multi-wythe. A wythe of masonry refers to a thickness of wall equal to the thickness of the individual units.

Description

Masonry is typically site constructed (laid) using manufactured masonry units and site mixed mortar. The units are laid in mortar to various heights, with the strength of the assembly being achieved during curing of the mortar. Masonry can form structural elements (typically bearing walls, columns, or pilasters) and/or the finished cladding system.

Masonry Units

Several different types of masonry units are commonly used. Common masonry unit types include clay and concrete units, which may be solid or hollow, and glazed or unglazed. Other masonry unit types include cast stone and calcium silicate units.

Clay Units

Clay brick units are typically used in brick masonry construction. Depending on the clay used and the method of forming the units during manufacturing, clay units have various colors, sizes and textures. Other types of units include glazed brick (both clay and concrete) units, concrete brick, calcium silicate brick, and hollow clay tile (typically used in older masonry buildings).

Clay masonry units are typically formed of soft clay extruded into the required shape in the manufacturing plant. Several different finishes can be formed on the exterior surface of the brick such as wire cut or sand finished, depending on the method used to form the brick into the desired shape. Brick units are then heated in a kiln (fired) to a temperature of 1100 to 1200 Fahrenheit degrees in order create the structural properties of the units.

The units can be hollow (cores occupy greater than 25% of unit) or solid. Units categorized as solid typically contain cores for handling and to allow more uniform firing. For most exterior walls, units categorized as solid are used.

The standard for clay masonry units is ASTM C216 (Standard Specification for Facing Brick (Solid Masonry Units Made from Clay or Shale). In this standard, and in building specifications, clay units are categorized by grade (NW, MW or SW) and type (FBA, FBS and FBX). The masonry grade depends on the required durability of the units. Typically, Grade SW (severe weathering) is recommended in most areas of the US. These units are much more resistant to freeze-thaw cycling. MW (moderate weathering) units should only used in areas where freezing cycles are not anticipated. NW (negligible weathering) units should only be used in interior conditions where the interior air is conditioned and there is no exposure to moisture.

The type of unit depends on the required dimensional tolerances. Typically Type FBS is specified unless unusually tight tolerances are required. Where tight tolerances are required, Type FBX should be specified. Type FBA units are typically used to create a rustic appearance with a high dimensional tolerance.

Glazed clay masonry units should meet the requirements of ASTM C126 (Standard Specification for Ceramic Glazed Structural Clay Facing Tile, Facing Brick, and Solid Masonry Units).

Concrete Masonry Units (CMU)

Concrete masonry units (CMU) are made from a mixture of portland cement and aggregates under controlled conditions. The units can be made to various dimensions, but typically have face dimensions of 8 inches high by 16 inches wide (nominal). Concrete masonry units are typically made in forms to the desired shape and then pressure-cured in the manufacturing plant. The units are often used when masonry is to form a load-bearing wall or an interior partition between spaces within a building. Concrete masonry units can be manufactured in different sizes and with a variety of face textures.

Concrete masonry units should meet the requirements of ASTM C90. The units are categorized based on the weight (lightweight, normal weight and heavyweight). Structural masonry units are either normal weight or heavyweight. Lightweight units are used for non-load-bearing conditions or as veneers.

Since these units are typically larger than brick units, the construction time required for laying the units is typically less than that for brick. The units can be solid or hollow (two or three cores) and can have solid or flanged ends. The cores provide continuous vertical voids that are often reinforced. Steel bars are placed in the cores with grout installed surrounding the bars. In this fashion, the wall acts similar to a reinforced concrete element.

Mortar

Mortar is typically composed of cement, lime and sand, although lime mortars can also be composed in which no cement is used. Components and proportions of mortars vary depending on the desired mortar properties. Mortars consisting of portland cement and lime as well as sand are most common. Premixed mortars must be carefully reviewed to determine the actual components of the mix.

There are different mortar types depending on the required strength. Mortars for new construction are typically Types N, S, or M. For repairs to existing buildings, some other types such as Type O, or even softer mortars, may be required to replicate the original mortar properties. The most common masonry types and uses in new construction are as follows:

  • Type N—Used in general masonry walls above grade. This is the most common masonry mortar used in non-structural applications in new construction. This has good bond qualities and good resistance to water penetration.
  • Type S—Typically used in structural masonry applications. Has a higher proportion of cement and subsequently can have increased shrinkage of the mortar.
  • Type M—Typically used only in below grade applications.

Mortar proportions and mixing requirements are outlined in ASTM C270 and in the appropriate Technical Notes published by the Brick Institute of America (BIA). Generally, mortars are mixed on site with water to achieve a wet fluid mix, with sufficient water for workability. The mortar is retempered (additional water added to the mix) periodically to maintain workability. After two hours, the bond of fresh unused mortar to new units is significantly reduced. Therefore, mortar that is unused within two hours should be discarded.

Fundamentals

Installation

Masonry must be installed on a solid, rigid base. This is typically a concrete foundation or structural steel or concrete beam system. Most building codes do not allow the weight of the masonry to be supported be wood framing, due to the strength loss of the wood member when exposed to moisture. The support system must be designed for small deflections (typically 1/600th of the span) to avoid cracking of the masonry.

The masonry units are laid in a bed of mortar. The horizontal joints between units are called bed joints while the vertical joints are called head joints. Clay brick masonry should include solid (full) head and bed joints. In concrete masonry it is common to lay the units with mortar only on the face shells (face shell bedding). This is due to the size of the cores and the difficulty in installing mortar in the webs between cores without allowing significant amounts of mortar to fill the cores. Full bedding of concrete masonry units is typically only performed where a portion of the cells will be filled with grout. Where grouting is performed, mortar should be kept from falling into the cells since this will form a weak plane in the grout.

Coursing

Masonry units can also be different sizes and shapes to accommodate specific project needs. The units can also be oriented in various ways to form varying aesthetic effects. Common coursing patterns are as follows:

  • Stretchers—units are oriented horizontally with the full face exposed (most common)
  • Headers—units are oriented perpendicular to the face of the wall with the end exposed (can be true or false headers)
  • Soldiers—units are oriented vertically with the full face exposed
  • Rowlock—units are oriented perpendicular to the face of the wall with the end and face exposed (often used at sills and at tops of walls)

Expansion and Shrinkage of Units

Following manufacture, clay masonry units expand when exposed to moisture. This volumetric change in the unit results in an accumulated growth of the wall system that is irreversible. Concrete masonry units typically shrink following manufacturing. These movements, if not accommodated in the design of the masonry elements, can cause cracking, spalling, and displacements in the masonry. For this reason, expansion joints are required in clay masonry construction, particularly in areas exposed to the exterior in where the units will become wet. Expansion joints are typically required at corners, offsets, and other changes in wall plane; changes in wall construction; and at regular spacings (typically 20 to 30 feet on center maximum, depending on the units). Guidelines for expansion joint design/layout are provided in Brick Industry Association (BIA) Tech Note 18A.

Concrete masonry walls are typically reinforced with joint reinforcement for shrinkage control. Depending on the size and spacing of the reinforcement, the spacing of control joints will vary. However, control joints are required in all concrete masonry walls. Guidelines for control joint placement are provided in National Concrete Masonry Association (NCMA) Tek Note 10-A.

Both clay and concrete masonry also undergo cyclic thermal movements. These materials expand in warm temperatures and contract in cold temperatures. The movement joints must also accommodate these movements.

Wall Systems

Masonry walls can be of several different types:

  • Veneer (wall system provides cladding and only resists transfers wind loads to a structural backup)
  • Structural/Load Bearing Wall (can be cladding but also provides load bearing system)

Water penetration through exterior masonry elements exposed to rain should be anticipated. Water typically flows through separations between the mortar and the units. This can be due to bond separations, voids, and cracks. Water penetration can also occur, although typically to a lesser degree, due to absorption through the units and mortar. Systems must be provided in exterior masonry construction to address water penetration into the wall system.

Masonry Veneer

Masonry veneer consists of an exterior wythe of masonry that forms a cladding material only. Lateral support for the masonry veneer is required. This is typically provided by an interior wall. Common interior walls (backup walls) are cold-formed steel framed walls with water-resistant sheathing and concrete masonry.

Critical components in masonry veneers exposed to moisture include:

  • Drainage cavity behind veneer wythe
  • Flashing system at base of veneer
  • Seals for the cavity at fenestrations (window, door, louver frames, etc.)
  • Lateral tie system to anchor veneer to the structural back-up
  • Vertical support system to support weight of veneer
  • Provisions for expansion/contraction of the wall system

Veneer walls are designed as "drainage walls" in respect to their resistance to water penetration. An air space/drainage cavity should be installed behind the masonry veneer to allow water that penetrates the masonry to flow down to the base of the wall, where it can be directed to the exterior. This drainage cavity should remain open to allow water to freely drain. Where restrictions in the cavity exist, flashings are recommended to collect water and drain it to the exterior. This is required at openings in the masonry such as at windows, supports, etc. At the base of the drainage cavity, a flashing system should be installed that consists of a three-sided pan, typically formed by metal and/or membrane materials, to collect water that penetrates into the drainage cavity and direct it to the exterior via drains or weeps. These flashings must be designed to be watertight, particularly at corners, laps, and terminations of the masonry. End dams are required at terminations to prevent water from flowing laterally off the flashing and into the adjacent construction. Common flashing materials are stainless steel, copper, and lead-coated copper. These metal flashings are durable, can be sealed, and include soldered corners and end dams. Membrane materials such as rubberized asphalt and EPDM can also be used in conjunction with metal flashings to seal the top of the metal flashing to the backup construction.

It is critical that a moisture barrier be present on the interior face of the drainage cavity (on the surface of the backup) to prevent the passage of water into the backup construction. The recommended cavity width behind the masonry veneer is 2 inches minimum.

In summer months, the air space behind the brick veneer will typically contain air that is hot and humid relative to the interior. This air can achieve a relatively high vapor pressure relative to the interior. In winter months, this is air space can be filled with relatively cold air in relation to the interior. This is particularly true in northern climates. If this air contacts the interior portion of window frames or interior finishes, condensation can result. For this reason, cavity seals are typically recommended at windows, doors, and other openings to prevent the passage of cavity air (and moisture) to the door/window frames.

Vertical support for the masonry veneer is typically provided at each floor line. For a brick masonry veneer, provisions must be made at each of the vertical supports to accommodate vertical expansion of masonry. This is accomplished by omitting the mortar between the top course of masonry and the underside of the support. This joint should be designed to accommodate the vertical expansion of the masonry, as well as structural deflections of the support. In concrete structures, creep of the concrete frame should also be accommodated.

Metal ties are required to provide the lateral attachment of the veneer to the backup wall. These are typically spaced at 16 inches on center in each direction.

Structural Masonry Walls

Structural masonry walls are typically constructed using concrete masonry. The concrete masonry can be reinforced both vertically and horizontally to achieve the required flexural resistance. Vertical reinforcement that is installed within the cells of the concrete masonry is generally grouted solid. Horizontal reinforcement is typically installed using prefabricated welded wires that are embedded in the bed joints. Although this horizontal reinforcement improves the strength of the masonry, particularly for horizontal spans, but also serves to control shrinkage cracking.

If structural masonry walls are to serve as the exterior walls, a second wythe of masonry is typically recommended. In this construction, the masonry can be built as a composite wall (both wythes act as a unit to resist loads) or as a non-composite wall (individual wythes act independently to support loads). Since water penetration through the exterior wythe of masonry is to be expected, the reliance on a single wythe of masonry as the exterior wall system is typically not recommended. If single wythe exterior walls are to be installed, a barrier should be provided on the exterior surface, such as a fluid-applied, breathable masonry coating or over-cladding (EIFS, metal panels, stucco and similar) to prevent water penetration into the masonry. Admixtures can be used in the fabrication of concrete masonry units to reduce water penetration due to absorption into units themselves. However, the admixture must also be mixed into the mortar in order to achieve proper bond. These systems can be effective in reducing the amount of water penetration into the masonry; however, they should not be relied upon to eliminate water penetration.

Thermal Performance

Masonry is typically a large thermal mass that can be heated and cooled by it's exposure to the sun and the exterior temperatures. Masonry exposed to sunlight can achieve temperatures well in excess of 100 degrees Fahrenheit. The masonry absorbs heat and will radiate the heat to the surrounding components of the wall system. During cold temperatures, masonry will be cool, particularly in shaded exposures. In design, the thermal performance characteristics of the masonry are typically based primarily on the insulation placed in the wall cavity or within the backup wall. The masonry is typically assumed to provide little insulating value.

Fire Safety

Masonry provides a significant improvement in fire safety for building walls. Concrete masonry is typically used for firewall construction. The fire resistive characteristics are based on the thickness of the masonry.

Acoustics

Because of the mass, masonry wall systems can provide better sound insulation than lighter wall systems such as metal panels. To improve acoustical performance, concrete masonry is typically filled with insulation to eliminate the voids in the cores.

Maintenance

When properly constructed, masonry wall systems require relatively little maintenance as compared to other wall systems. The service life of the masonry can be 100 years or more, depending on the detailing and maintenance. The most frequent maintenance is the regular replacement of sealant in expansion joints, perimeter of openings (windows, doors, etc.) and at through wall flashings. The time frame for sealant replacement depends on the sealant used but usually ranges from every 7 to 20 years.

Repointing of the mortar joints in exterior masonry is typically required between 20 to 30 years after installation, depending on the type and quality of the original masonry installation.

Applications

See Appendices for climate-specific guidance regarding building enclosure design.

Details

DWG iconDWF iconPDF icon

The following details can be downloaded in DWG format or viewed online in DWF™ (Design Web Format™) or Adobe Acrobat PDF by clicking on the appropriate format to the right of the drawing title. Download Autodesk® DWF Viewer. Download Adobe Reader.

The details associated with this section of the BEDG on the WBDG were developed by committee and are intended solely as a means to illustrate general design and construction concepts only. Appropriate use and application of the concepts illustrated in these details will vary based on performance considerations and environmental conditions unique to each project and, therefore, do not represent the final opinion or recommendation of the author of each section or the committee members responsible for the development of the WBDG.

Clay Brick Inside Corner  PDF

Clay Brick Outside Corner  PDF

Clay Brick Through-Wall Flashing  PDF

Emerging Issues

New developments in masonry wall design include the use of pre-stressed masonry. This consists of building a concrete masonry wall with cables within the cells, similar to a pre-stressed concrete element. After the wall is built, the cables are tensioned and anchored to the masonry. This can greatly increase the resistance of the masonry wall to flexural loads and bending.

The necessity to make building envelopes blast-resistant has forced consideration of reinforced masonry façade design options with respect to water integrity and thermal performance.

Relevant Codes and Standards

Additional Resources

WBDG

Design Objectives

Functional / Operational—Ensure Appropriate Product/Systems Integration

Products and Systems

Section 07 92 00: Joint Sealants, See appropriate sections under applicable guide specifications: Unified Facility Guide Specifications (UFGS), VA Guide Specifications (UFGS), DRAFT Federal Guide for Green Construction Specifications, MasterSpec®

Publications

Organizations