August 14, 2019
by Kenneth Itle, AIA, Mike Ford, AIA, and Timothy Penich, AIA
Contemporary masonry veneer construction consists of a single wythe of masonry such as brick or stone mechanically anchored to a backup structure, typically either wood or steel framing or concrete masonry units (CMUs). The veneer is supported laterally by the backup structure and bears on the foundation or a shelf angle. Veneer construction incorporates an air space to deter water penetration into the building and reduce heat transmission. Water-resistant barriers (WRBs), flashings, weeps, and drainage systems are also installed to facilitate drainage. Masonry veneer wall systems are composed of many different materials. Thus, masonry veneer construction must be designed with the goal to accommodate differential movement.
Code requirements for masonry veneer walls are outlined in the Masonry Society (TMS) 402/602-16, Building Code Requirements and Specification for Masonry Structures. Additional guidance for specifiers based on the code is provided in the Masonry Designers’ Guide, published by TMS. The Brick Industry Association (BIA) has authored various technical notes on brick construction to provide guidance on individual materials and general construction of masonry veneer systems. They serve as useful references. Finally, ASTM standards define the various characteristics of masonry materials.
Brick are manufactured from naturally occurring substances that are fired. Typically, test data including absorption, saturation coefficient, compressive strength, efflorescence, and initial rate of absorption can be obtained from brick manufacturers and suppliers. Brick testing is conducted in accordance with ASTM C67, Standard Test Methods for Sampling and Testing Brick and Structural Clay Tile. ASTM C216, Standard Specification for Facing Brick (Solid Masonry Units Made from Clay or Shale), can be used to evaluate the testing data. This standard also defines two grades of facing brick. Grades refer to a brick’s resistance to damage caused by freeze-thaw cycles. Grade SW (severe weathering) brick is meant for areas where high resistance to potential damage caused by freeze-thaw cycles is necessary. Grade MW (moderate weathering) brick is intended for use where only moderate resistance to damage caused by freeze-thaw cycles is needed. Almost all of United States requires Grade SW brick for veneer construction.
The physical requirements for each grade include minimum compressive strength and maximum water absorption and saturation coefficient. The saturation coefficient is a ratio of the rate of absorption by 24-hour submersion in cold water to five-hour submersion in boiling water. ASTM C216 provides an absorption alternate that states the saturation coefficient requirement does not apply if the 24-hour cold water absorption of the five tested brick units does not exceed eight percent.
Initial rate of absorption and efflorescence are also included in the test data. Initial rate of absorption data should be requested from the brick manufacturer, as this property is used to assist in mortar selection as well as the installation process. For example, brick with a high initial rate of absorption may potentially experience increased levels of efflorescence. This can be avoided by wetting bricks during installation to prevent absorption of moisture from the mortar.
Results from efflorescence testing are also included in manufacturer’s data. The specified rating for efflorescence should be “not effloresced.”
Three brick types are outlined in ASTM C216: FBS, FBX, and FBA. FBS is brick for general use and is the most commonly specified type. FBX is for projects requiring a high degree of precision and low variation in size, and FBA is meant for producing architectural effects resulting from non-uniformity in size and texture of individual brick units. ASTM C216 outlines physical requirements for each brick type.
While selecting brick with the appropriate grade is important, it is also crucial to minimize the volume of moisture to which the brick is exposed by proper detailing to ensure a successful veneer wall.
Mortar is typically composed of cement, hydrated lime, and sand, mixed with water. Admixtures and pigments are used as well. ASTM C270, Standard Specification for Mortar for Unit Masonry, outlines standards to reference when specifying mortar.
Cement-lime mortar, masonry cement, and mortar cement can be used in mortar. Portland cement is blended with hydrated lime to produce cement-lime mortar. Masonry cement contains Portland or blended hydraulic cement and limestone or lime. Mortar cements are similar to masonry cements in content but there are controls on air content and minimum strength requirements in the former. These are outlined in ASTM C1329, Standard Specification for Mortar Cement. Both masonry and mortar cement may contain additives specific to each manufacturer. Therefore, careful review of product data and safety data sheets is necessary.
ASTM C207, Standard Specification for Hydrated Lime for Masonry Purposes, outlines four types of hydrated lime:
It should be noted some building codes prohibit the use of air-entraining materials in mortar due to the resulting reduction in bond and compressive strength. Therefore, Types NA and SA should not be specified for masonry mortar. The special types of hydrated lime (S and SA) offer the advantages of higher early plasticity and water retentivity. For these reasons, Type S
is the most commonly specified hydrated lime type for masonry mortar. Depending on the amount of lime used in the mortar mix, bond, workability, water retention, and elasticity can be adjusted.
Sand or aggregate is a filler material and help to reduce shrinkage in mortar. Different sands or colored aggregate can also be used to achieve a specific mortar texture or color. ASTM C144, Standard Specification for Aggregate for Masonry Mortar, specifies aggregate for use in mortar shall consist of natural or manufactured sand, which can be crushed stone, gravel, or air-cooled iron blast-furnace slag. Admixtures can result in a change of properties such as increased workability or a decrease in setting time. They can also be used to achieve a specific color. Water-repellent admixtures are commonly specified for mortar when concrete masonry is used for the veneer. They need to be coordinated with other water-repellents employed in CMUs. Cold-weather admixtures to allow installation of masonry at temperatures below 4 C (40 F) can be used with caution, although specifiers should carefully review the effect on future durability and performance. If any type of admixture is to be used, the same material should be employed consistently all across the project for consistency of appearance and performance.
Pigments could be employed to achieve a certain color, although colored aggregate may be preferable for distinctive or bold hues. The use of an excessive proportion of pigment can also reduce strength or durability.
In addition to mortar components, ASTM C270 outlines four mortar types and recommends in what conditions they should be used. The four mortar types are M, S, N, and O. All of them have different minimum strength, water retention, and air content requirements depending on the type of cement used in the mix. Table 1 in ASTM C270 outlines these requirements and should be referenced when preparing construction documents. Type N mortar is recommended for loadbearing walls. For non-loadbearing walls such as veneer construction, Type N or Type O mortar should be used. Type O mortar can be employed if the wall is unlikely to be exposed to freezing temperatures when saturated or subjected to high lateral loads.
The mortar ingredients (Portland cement, hydrated lime, sand, and any pigments or admixtures) can be blended onsite, although quality control (QC) to ensure consistency between batches can be challenging. Alternately, custom preblended dry mixes with all ingredients for the project are readily available. These mixes only require the addition of water onsite. (Product data should be reviewed to understand the admixtures in the mix).
In addition to selecting the appropriate mortar based on how or where it will be used, it is also important to evaluate how the selected mortar will interact with the selected brick when specifying a mortar type. For example, mortar typically bonds best with masonry units with a moderate initial rate of absorption.
Veneer ties and anchors
In a veneer wall system, the ties are intended to connect the backup wall construction to the outer wythe of masonry, transfer lateral loads, and accommodate in-plane differential movement. An effective veneer anchor system should be securely tied to the backup construction and veneer masonry, have sufficient stiffness to accommodate lateral loads, and be corrosion resistant.
There are many types of veneer tie and anchor systems for new construction including:
Corrugated metal ties are not recommended unless used in low-rise, wood-framed construction with masonry veneer. Due to the width of the corrugated ties, they may provide an avenue for moisture to bridge the air space. They are more susceptible to corrosion than wire ties and sometimes experience insufficient bond with mortar at joints.
Rectangular, ladder- and truss-type veneer ties are best suited for multi-wythe masonry construction. With these masonry tie systems, the CMU backup wall and veneer are built simultaneously with ties installed at the bed joints. These ties provide strength and stiffness to accommodate lateral forces, but have the potential to restrict expected in-plane movement or deform under service loading.
Adjustable veneer anchors are a two-piece system, and include eye and pintle, dovetail, and slotted systems. Adjustable systems generally allow for construction of the backup assembly first, followed by later installation of a masonry veneer. Adjustable ties can also accommodate larger in-plane differential movement and construction tolerances.
When specifying and detailing lateral anchors, hot-dipped galvanized steel in conformance with ASTM A153, Standard Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware, are required by code for exterior wall systems. In more corrosive environments or areas where wall systems are susceptible to excessive water penetration, stainless steel ties in conformance with ASTM A580, Standard Specification for Stainless Steel Wire, are recommended. Masonry wall ties should be sized to fit the width of the mortar joints and wall construction and spaced to accommodate lateral loads. Standard wire diameters for masonry ties are W1.7 (9-ga or 4-mm [1/8-in.]) and W2.8 (5-mm [3/16-in.] diameter wire). According to TMS 402/602-16, the allowable wire diameter for masonry ties is half the mortar joint width. However, given the tolerances in joint width, the smaller diameter wire may be recommended to allow for sufficient mortar coverage in a typical 9.5-mm (3/8-in.) joint. Veneer ties and anchors should be sized so that they span the cavity space and extend a minimum of half the depth of the masonry veneer unit. BIA suggests a minimum embedment of 38 mm (1 1/2 in.) with a minimum cover of 16 mm (5/8 in.) at the exterior face of the masonry veneer.
Although masonry is a highly durable material, it is not inherently waterproof. Masonry materials will absorb moisture from precipitation and only gradually dry as ambient conditions change. For this reason, a masonry veneer must be isolated from the backup construction by a cavity.
The cavity in a veneer wall system provides an avenue to drain water entering the wall system. It consists of an air space and typically a WRB, and often includes rigid insulation and a drainage mat or mortar net. Veneer ties span across the drainage cavity. The width of the air space is measured as the distance between the back of the masonry veneer to the face of the backup wall or insulation. The International Building Code (IBC) requires a specified 25-mm (1-in.) wide air space, noting the cavity may potentially be less than 25 mm, given construction tolerance. However, a 50-mm (2-in.) air cavity is recommended.
Depending on the construction of the backup wall and commonly available options for veneer ties, the maximum width of the drainage cavity can range from 114 mm (4 ½ in.) for wood and steel-framed backup walls to 168 mm (6 5/8 in.) for masonry backup walls. For wider drainage cavities, more robust and frequent spacing of veneer ties is required to transfer lateral loads.
The effectiveness of the drainage cavity depends on the continuity of the air space and WRB. Mortar bridging, resulting from excessive mortar droppings, interrupts the air space and can create avenues for water to reach the backup construction. A narrow air cavity, changes in the depth of the veneer masonry, or misalignment of insulation in the drainage cavity may promote the formation of mortar bridges. It is recommended to mechanically fasten or self-adhere the rigid insulation in the drainage cavity to the backup.
WRB is installed between the exterior wall sheathing and air space and may be liquid-applied, self-adhered, or mechanically fastened. The barrier should be free of defects such as wrinkles and open joints that may promote mortar bridging or allow water to infiltrate to the interior. If self-adhered or mechanically fastened, the edges of the membrane should be shingle lapped at least 152 mm (6 in.). Ideally, screw heads and penetrations in the WRB should be filled with a compatible sealant to maintain the continuity of the barrier. While not preferred, water-resistant facing of sheathing or rigid insulation may be used if all penetrations, edges, and joints are properly taped and sealed.
For narrower air spaces, where there is a greater probability of mortar bridging within the cavity, drainage systems such as drainage mats or mortar nets are recommended. These systems are most effective if installed the full height of the wall and can provide a clear path for movement of air and water in the wall cavity. For drainage mats, the drainage portion of the mat should face the cavity face of the masonry veneer.
Flashings and weeps
At all horizontal terminations or discontinuities in the masonry veneer construction, flashings must be provided to ensure moisture in the masonry and drainage cavity is directed to the exterior. Common flashing materials include metal (stainless steel or copper) or membrane (rubberized asphalt or butyl-based materials). Metal flashings can be made continuous by soldering adjacent sections or by lapping and sealing with butyl. Rubberized flashings are lapped and sealed with a compatible product. Although other materials, such as laminated copper flashing, asphalt-coated copper fabric flashing, ethylene propylene diene monomer (EPDM) rubber, thermoplastic, or polyvinyl chloride (PVC) are available for flashings, these may be more prone to damage during construction, and it may be more challenging to form watertight seams between sections of flashing.
Flashing materials in a veneer system need to bridge across the cavity to integrate with the WRB at the back of the cavity or to terminate into the substrate. At the point where the flashing crosses the cavity, flexible materials may be prone to sagging or puckering, creating valleys where water can collect. Also, it can be challenging to make the seams between flashing pieces watertight at the cavity. For these reasons, rigid metal flashings are preferred, especially as a flashing pan to span the cavity.
The front edge of the flashing should extend beyond the face of the veneer with a drip edge, so water draining from the flashing is not allowed to run back onto the masonry or other materials below. Since rubberized flashing materials are generally unstable in ultraviolet (UV) light, these flashing materials must be held back at least 12 mm (1/2 in.) from the exposed face of the veneer, and a stainless steel drip edge provided to form the termination of the flashing system. The back edge of the flashing system should be secured to the backup, typically with a termination bar and/or mechanical fasteners. If a WRB is used, the barrier materials should be lapped into the flashing assembly and fully sealed.
An end dam must be provided wherever a flashing terminates horizontally, such as at the end of a window lintel or shelf angle. Without an end dam, water on the flashing may drain from the back corner of the flashing, creating a concentration of water within the cavity that can saturate adjacent materials.
To ensure proper drainage from flashings, weeps or vents are provided. Cotton or synthetic rope wicks, either with or without plastic tubes, may be used. These types of wicks should be placed by the mason within the head joints in the first course of masonry above the flashing, and should be long enough to extend from the outer face of the veneer to the back of the cavity. Cellular plastic or aluminum cell vents sized to fit exactly within a single head joint are another option—these vents are available in a variety of colors to minimize their visual impact. Weeps should be spaced at 400 mm (16 in.) on center (o.c.).
Control and expansion joints
For masonry veneers, movement related to thermal and moisture changes are key considerations. The coefficient for thermal expansion of clay masonry as defined in TMS 402 equates to about 12 mm (½ in.) for a 30-m (100-ft) long wall experiencing 38 C (100 F) of temperature change. The equivalent value for concrete masonry is slightly higher. As a result, differential thermal movement between the materials of 1.6 to 4.8 mm (1/16 to 3/16 in.) is possible over a 30-m length.
Moisture-related masonry movements are mostly a concern related to long-term, irreversible volume changes. Clay masonry units are at their smallest size when removed from the kiln and first installed. The units will slowly and irreversibly expand over time as they absorb ambient moisture. The expansion is most rapid for the first year in service and decreases to a negligible rate after 10 to 20 years. A challenge in masonry design is that other commonly used building materials, particularly concrete and wood, experience irreversible shrinkage while in service, so the relative increase in size in a clay masonry veneer compared to the size of the underlying wood or concrete structure may be significant. For example, concrete masonry will experience long-term irreversible shrinkage due to drying.
Control and expansion joints are used to accommodate movements resulting from these volume changes. Joints are also needed at changes in the architectural or structural geometry where cracks are likely to form. In veneers, vertical joints should be spaced at approximately 8-m (25-ft) intervals in walls without openings, at offsets or near building corners, door openings and window jambs, and at any joints in the underlying structure. Horizontal joints should be provided at every floor level in multistory construction. Design of control and expansion joints should extend the full depth of the masonry veneer and include a highly compressible fill material to prevent mortar and debris from clogging the joint. The minimum size of the joint needed can be determined by reference to the coefficients of thermal and moisture expansion for the various materials, the expected temperature change for the façade surface (which may be 50 C (120 F) or more), and the size of the masonry panel.
Contemporary masonry veneer wall systems offer flexibility of design, means of addressing water penetration, and ease of construction. They incorporate a wide range of materials and systems including masonry, mortars, masonry ties, WRBs, insulation, flashings, and control and expansion joints. As such, the effectiveness of masonry veneer wall systems is dependent on material specification, attention to design and detailing, and proper construction procedures.
Kenneth Itle, AIA, is an architect and associate principal with Wiss, Janney, Elstner Associates (WJE) in Northbrook, Illinois. He specializes in historic preservation and the assessment and repair of masonry systems. Itle can be reached at email@example.com.
Mike Ford, AIA, is an architect and senior associate with WJE’s Northbrook, Illinois, office, specializing in historic preservation and the assessment and repair of existing buildings. He can be reached at firstname.lastname@example.org.
Timothy Penich, AIA, is an architect and senior associate with WJE’s Northbrook, Illinois office, specializing in historic preservation. His work also includes condition surveys and the preparation of repair documents relating to masonry and roofing and waterproofing. He can be reached at email@example.com.
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