From Mortar Snots to Perpends: The Basics of Through-Wall Flashing

Detailing, specifying, and communicating basic through-wall flashing details avoid moisture damage to cavity walls

Celeste Allen Novak, AIA, LEED AP

The calls are unexpected. A few years after the successful grand opening, the client on the other end of the phone is complaining about water damage. Over the past few years, the lack of oversight on the cavity wall details during construction allowed water, drop-by-drop, to migrate into the building. Instead of allowing the building to breathe, these cavity walls created the perfect environment for mold—moisture, warm temperatures, and darkness.

Corrective remediation for even small drops of water in cavity walls can include everything from the replacement of surface finishes to structural systems. When the building failures occur in a high-rise masonry building, the damage is even more difficult to repair. Masonry buildings located near oceans are particularly vulnerable to damage due to the high salinity of rainwater. The list of common problems found by building experts, in damaged cavity wall systems include:

• Damage to interior finishes, including carpets, drywall, and cabinetry.
• Rusting of galvanized steel studs.
• Corrosion of metal wall fasteners.
• Corrosion to steel relieving angles and structural failures around openings.

Water in buildings is invasive, costly, and avoidable. Architect and senior project manager at Bowie Gridley Architects, Robert Allen, AIA, recalls the destruction of almost the entire wood structural framing system of a stone-veneered home he investigated for mold and structural damage in Virginia, due to recurring water penetration into the framing cavities. Remediation was both costly and time consuming. In his experience in commercial projects, flashing details are a critical element to the durability of a high-performing building: “Experience teaches us that it is not a question of IF an exterior surface will allow water penetration, but rather, how do we address it WHEN it will happen. Flashing and membrane details are key to extending the life of building wall and roof systems, and maintaining a healthy living environment within a structure.”

Design professionals provide the lead in detailing wall systems and they are responsible for communicating clearly to the contractor and subcontractors the best methods for constructing durable masonry cavity wall systems. The problem is that there is a lack of knowledge regarding through-wall flashing and masonry construction. Today's architects are exploring many new computer-generated building designs. These may include unusual terminations of wall, floor-plates, and windows that provide challenges to masons who are familiar only with traditional wall cavity construction. The art and science of designing a durable cavity wall with proper through-wall flashing requires the collaboration of all of the team members on a project and the use of new high-performance membranes.

Cavity Walls and Through-Wall Flashing

There are two common types of masonry cavity wall construction. A cavity wall is constructed of two “wythes” of masonry separated by an opening of varying dimension. The term wythe is from old English as are many of the words used to describe masonry systems. Although masonry walls were used by the ancient Chinese, Greeks, and Romans, according to the Masonry Advisory Council, “Sometime in the early part of the 19th century, the cavity wall was probably reinvented by the British. Plans dating as early as 1805 suggest a type of construction, featuring two leaves of brickwork, bonded by headers spanning across a 6-inch cavity.”¹ Early British publications suggest that cavity wall construction was a means to protect buildings from moisture penetration.

There are two types of cavity walls, the first using brick with a masonry block back up. The second type of cavity wall system uses metal ties to hold the walls together. Sometime in the mid-1800s, wrought iron ties were introduced in Southern England as part of a composite wall system. Cavity wall design came to the U.S. from Europe in the 1920s. Before that, barrier walls or composite walls with numerous layers of brick were used to construct most masonry buildings. In the 20th century, the second type of wall construction typically included an exterior brick wall connected by metal ties to a metal or wood stud wall.

Conceptually, moisture control in masonry walls is passive aggressive. Moisture that penetrates masonry veneer is allowed through the wall to run down its back face, dropping to the flashing and out through the weep system. Building paper, vapor barrier membranes or fluids are brought down the face of the back-up wall in a shingle fashion to shed any water to a waterproof flashing membrane. Wood studs are usually used in residential construction and steel studs for commercial work. “Through-wall” waterproof flashing membrane closes the gap between the two segments of the wall. The flashing provides a path for moisture to drain along the face of the back-up wall to the exterior of the building.

Gregory A. Jones, AIA, preservation architect with Hopkins Burns Design Studio, comments, “Historic buildings often totally lack flashing in locations where it would normally be used in modern construction.

“A common flashing issue in historic buildings is the lack of flashing and weeps over steel lintels. Since non-galvanized lintels were typically used in buildings into the mid-20th century, the lack of flashing frequently contributes to a condition known as rust jacking. Moisture that enters walls above lintels has no flashing to protect the lintel and no weeps to direct moisture out of the wall. As a result, moisture collects on the lintel causing it to rust and expand several times its original thickness. This results in expansive forces that can literally lift thousands of pounds of masonry veneer and introduce significant cracking into walls and/or cause spalling of brickwork, as well as deflection of the lintel. Correction of this condition usually consists of replacement of lintels with new galvanized lintels and addition of flashing and weeps to protect the lintel.”

Through-wall flashing is usually adhered to the back-up wall whether it is drywall, insulation, or wood. The cavity between the exterior and back-up wall systems is the “rain plane,” the place where smart design controls moisture penetration.

Moisture, Perpends and Mortar Snots

Building scientists state that all of the forms of water: solid, liquid, vapor and adsorbed are culprits in how water enters a brick wall cavity. Once in the building wall system: “Water always changes its behavior, because its form is never constant. Evaporation, condensation, capillary suction, gravitational flow, vapor diffusion and mass flow of moist air are all happening at the same time inside building cavities and inside materials.”² With a good understanding of the principles of water migration, a building science investigator can analyze the solution to most problems caused by poor wall construction.

Most moisture enters a wall cavity through a badly filled perpend. Derived from old English, perpends are head or vertical mortar joints between bricks. Head joints are more difficult to fill than the horizontal bed joints on a brick wall and the perpend must be buttered properly to prevent openings through to the backside of a wall system. Unfortunately, water runs through the path of least resistance and will find even the smallest opening in mortar a means to enter an interior wall cavity. If the cavity is blocked with excess mortar droppings, these “mortar snots” allow the wall to become a reservoir for water to pool and breed bacteria.

Another bridge that can block the interior cavity of a wall is an inward sloping, metal wall tie. Metal ties can collect mortar snots and contribute to moisture blockage. In worse cases, mortar droppings have filled cavity walls above flashing materials forcing water over the top of even the best laid through-wall flashing systems.

To help decrease the amount of mortar droppings, specify that the mason slope the backside of the mortar bed so that there is less mortar extruding as he lays the bricks. The Brick Industry Association also asks for a minimum of 2-inch clear space for the cavity wall to be constructed correctly.

Damproofing Course

The British refer to through-wall flashing as a damproofing course or DPC. When considering moisture penetration into a wall, all of the sides of the wall are important, top, bottom, inside and outside. There are different treatments for flashing at all of these four sides of a masonry wall. The top of a masonry wall is covered by a coping system designed to keep water from entering the top of the wall. The obvious source of moisture into wall cavities is from wind driven rain. Wind driven rain can cause both vertical and horizontal penetrations into a wall cavity. In some areas of a building, rising ground water and/or moisture will travel from below grade up into a wall.

A hidden source of moisture damage and one of the most common culprits is the migration of water vapor traveling through the wall assembly. Water vapor at the dew point is transformed into water. Water can fill a badly designed wall assembly to cause moisture damage if it is not properly drained back to the exterior with a damproofing course or through-wall flashing. Often, through-wall flashing may have the proper details but improper installation and/or attachments to the back-up wall. This causes water to go around the flashing material. In some cases, flashing manufacturers compound the problems by recommending that flashing be held back a half inch from the face of the brick without the caveat that this detail requires the additional step of adding a drip edge.

In an article by David Nicastro, P.E., from the March 1996 Construction Specifier “Magazine Masonry cavity walls – flashing not extended to face of wall”³ provides a good example of this type of flashing failure. Nicastro, a building science investigator was contacted because water was getting through the walls although every detail appeared to be correct. His evaluation of the through-wall flashing system determined that the flashing material was in good condition and it was attached to or built into the backup wall correctly. There were good details at the corners, but water was still getting into the building. He concluded that water was entering the building by going around the flashing. The flashing had been held back a half inch from the face of the brick and there was not an additional drip edge to complete the flashing system. The Brick Industry Technical Notes 21B Drip Edge/Flashing Extension provides the correct solution to this problem.⁴

Design Principles

There are several important factors to consider when designing a flashing system for high performance buildings. Design professionals should consider specifying through-wall flashing that has the following attributes. The flashing should be constructed of a high performance membrane that is long lasting, compatible to sealants and UV stable. Although the cheapest flashing, PVC is not a recommended material for long lasting flashing.

Through-wall flashings rarely leak through the membrane however, they can be damaged during installation. To prevent puncture, a durable high performance membrane similar to high end roofing membranes should be specified. All membrane flashing comes to the job site in rolls, and all manufacturers have accessories that are used to join the pieces together correctly.

Flashing systems should incorporate preformed shapes or “cloaks” which a mason can easily install. Most leaks occur at areas that require special detailing. Using a preformed shape will allow the mason to be responsible for only the watertight laps around what might otherwise be a difficult flashing detail. Cloaks eliminate the need for the cutting and pasting required with other flashing membranes. The basic design principle is to understand the basics of water migration and design an integrated moisture barrier and drainage plane throughout the entire wall cavity.

Flashing Placement

Through-wall flashing should be placed at grade, above, and below any wall openings, at the top of all masonry walls and any location where the downward flow of water is interrupted. A complete flashing system includes not only the flashing, but also the protective waterproofing membranes that overlap flashing materials as well as the knowledge as to where that membrane is placed based on the humidity levels of the project as seen in the case study for the National Museum For The American Indian.

It is important that the flashing is installed to a suitable substrate. The flashing should be installed to a solid surface that is dry, smooth and free from protrusions. As preservation architect, Randy Case, AIA, LEED AP, principal of Architecture + Design in Battle Creek, Michigan has discovered that “Many older buildings were constructed without proper through-wall flashings, or details, and correcting those conditions can be a challenge when meeting the Secretary of Interiors Standards for rehabilitation and guidelines. Be sure to engage the State Historic Preservation office when designing flashing systems that are being proposed to correct original design flaws when they are going to effect the physical appearance of an historic property.”

At Grade

Beginning at the lowest point of a building, flashing at grade can be built into a block back-up wall, brought across the cavity and through the veneer to form a drip edge. Reviewing the materials of the flashing with the manufacturer is critical to assure that the selected flashing material will not “fishmouth” or create a wave along the drip edge. Through-wall flashing is attached to the back-up walls and then brought through the veneer 4 to 6 inches above grade so water can drain out through the weep system.

Since grade is not always level; step flashing will require careful detailing. Step flashing is placed at each different course of masonry maintaining a constant 4 – 6 inches level above grade to capture any moisture that is running down the backside of the brick. At each change in level, the through-wall flashing is turned up to form end dams and two weeps per flashing course. Two weeps per flashing course should also be provided. Some specifications call for the through-wall flashing to be left exposed by as much as 2 – 3 inches until the architect directs the contractor to trim it off. The advantage of this specification it to assure that the through-wall flashing is not installed a half-inch back from the face of the building at a juncture where it is critical.

Above and Below Wall Openings

Through-wall flashing is installed above and below any wall opening. Windows, doors, and ventilation systems all require flashing that is turned up on three sides so that water is directed out the weep system. A typical two dimensional detail provided to the contractor shows the location of the flashing course, but unless the image is projected in three dimensions, the detail doesn't show the end dams that capture the water to remove it from the building.

Roofs, Copings and the Top of Walls

Roofs, copings and the top of walls are areas that require the installation of both the through-wall flashing and the roofing counter flashing. If these are not done at the same time the roofer will return later to saw cut the mortar joint to install counter flashing, blocking or breaking weeps and through-wall flashing. Coping stones are less porous than brick and do not absorb much moisture. However, water will enter through mortar joints. The detail for this condition should show raked mortar joint, with backer-rods and a quality sealant. This area is one that may need constant maintenance even with the most careful detailing. Because of severe exposure to moisture, the joints will open up early in the building's life.

Metal copings can also be penetrated by wind driven rain. Metal copings come in sections from 10 to 14 feet and water can pool and enter in the overlaps. Through-wall flashing can be installed at the top of the masonry wall by the mason. As an alternative, the roofer can run his roofing membrane up and over the parapet to stop water entry through the top of the wall.

This illustration is an example of “copper almost through the wall flashing.” The specifications and details for this project clearly showed that the flashing was to be brought through the wall on both sides of the wall. This section of coping was opened up because of severe leaking to the windows below. Again, the water got through the mortar joints, between the coping stones, and easily got around the flashing because it was not brought through the wall. Maintaining communication between the various subcontractors to integrate elements at each stage of the project is critical to successful detailing.

Flashing should be placed at the top of all masonry walls. This includes walls that are outside the building, some of which are part of an entry sign with the company's name. Others are used to hide electrical equipment and garbage dumpsters. If flashing is not placed there, efflorescence will occur on the wall. Efflorescence occurs when water moving through the wall brings salts from the cement mortar to the surface causing powdery white streaks. Efflorescence can both be a cosmetic as well as a structural problem. Without water, efflorescence will not occur. Staining can also be caused because there is no drip edge to the through-wall flashing The purpose of the drip is to allow the water that is collected by the flashing to run out the weeps and drip away from the building, not run down the face of the building.

Communication and Isometric Drawings

The power of additional isometric drawings in construction drawings is that the design professional can clearly demonstrate design details correctly. A three-dimensional drawing of a single wythe construction wall is illustrated as an example. Assuming this detail is for a 12-inch wall, the course above the opening is broken into an 8-inch and a 4-inch block, installing the flashing in a “Z” type configuration. Single wythe walls receive flashing at all the same places: at grade, above and below wall openings, at the top of the masonry wall and/or any place that an obstruction to the downward flow of water would occur, bond beams.

The Brick Industry Association (BIA) is “the national trade association representing distributors and manufacturers of clay brick and suppliers of related products and services. Since it's founding in 1934, the association has been the nationally recognized authority on clay brick construction and represents the industry in all model building code forums and national standards committees.”⁵ They provide a library of technical notes with three dimensional images of many through-wall flashing details. Isometric details are one of the best ways to convey information for three-dimensional forms to masons along with full-scale models for difficult details. They can convey all of the parts and location of elements that comprise correct through-wall flashing details, weeps, stubs, end dams and the shingle overlapping of membranes. Most trades build from drawings or examples, not from specifications that are more than likely to be left in the office.

Through-Wall Flashing Components

Two contractors are engaged in the construction of a masonry wall, the mason and the sealant contractor. The components of through-wall flashing may include the following:

• One quarter inch or 5/16 inch steel relief angles.
• Flashing which is either brought through to form the drip edge or held back one-half inch from the face of the building.
• Metal drip installed to bring flashing through the wall and accept sealant
• Sealant
• Bonding Tape of backer-rods that allow the sealant to be adhered on two sides.
• Compressible filler that keeps moisture absorptive brick from growing into metal angles and causing a brick face to shear off.

In the field, the contractors work within the narrow confines of a wall cavity that can range from 3/8 – 1/2 inches. This skilled labor requires both knowledge and precision and any shortcuts can lead to building failures that can appear both immediately or after many years.

Membranes should be selected that are compatible with silicones and urethane sealants. They should also be stable when exposed to ultra-violet rays. Many synthetic materials will crack, discolor, harden or deteriorate when exposed to sunlight or harden as they age. High alkaline environments that are typical of masonry environments should not affect flashing materials and the manufacturer should supply verification of the materials chemical stability. Flashing can be specified to match mortar colors to reduce the visual impact of drip edges on a wall. Established manufacturers will provide design assistance, technical data, flashing details, installation instructions, and job site assistance to architects, engineers, specifiers and contractors.

Corners and Odd Shapes

Corners and odd shaped openings that are not properly flashed may be the first place that shows moisture damage. Mortar joints that crumble, efflorescence, cracked brick are the evidence that there is a problem. Special prefabricated shapes are available that can be used either in new construction or when repairing wall failures. These shapes can be specified for a variety of conditions. These include outside and inside 90 and 135 degree corners, level changes with built in end dams, window corners, internal and external corner caps, nut covers and many more. Depending on the manufacturer, the supply of prefabricated shapes can provide many benefits to speed project delivery.

Weep Holes

Solid cavity walls require weep holes that allow moisture to drain. The least expensive form of weep holes is the open head joint. They should be spaced every 24” or a minimum of two weeps above any opening. Some masons also use cotton cords as weeps. The BIA recommends that these should be placed a minimum of every 16” on center. Plastic tubes are not recommended because mortar droppings can dam up the openings in the tubes and they make perfect homes for insect nests.

Economics

Review the square foot costs of flashing before selecting a flashing material. The cost depends on the design of the building. The number of inside and outside corners, windows, doors, balconies and grade conditions will determine the number of special forms that will be required.

High performance membrane flashing systems with preformed shapes are comparable in price to both 5 oz. copper or rubberized asphalt flashing with a metal stainless steel drip edge. Any additional cost to use this product is equalized by advantages that include labor savings and durability. Keeping mortar snots from causing damage to an entire wall system is a matter of providing the correct “cold medicine.” Design professionals need to develop and participate in collaborative and integrated design and construction teams that include site problem solving throughout the length of the project. In addition to specifications, isometric drawings and site models provide better communication methods of difficult flashing details. The prescription for a successful project includes several preventative measures as well as the specification of a high performance through-wall flashing membrane systems.

Endnotes

1. Cavity Walls. http://www.maconline.org/tech/design/cavity2web.pdf. 3/18/2015.
2. Building Science Digests. http://www.buildingscience.com/documents/digests/bsd-108-investigating-and-diagnosing-moisture-problems 3/18/2015.
3. Nicastro, P.E. “Magazine Masonry cavity walls; flashing not extended to face of wall” Construction Specifier, March 1996.
4. Drip Edge/Flashing Extension. http://www.parkerblock.com/pdf/divisions/brick/contractors/tech-note-21b-brick-masonry-cavity-walls-detailing.pdf March 20, 2015.
5. http://www.gobrick.com/Resources/About-BIA-Regions/Mission-Services. March 22, 2015.

Architect Celeste Allen Novak, FAIA, LEED AP, specializes in sustainable design and planning in Ann Arbor, Michigan. She is the author of “Designing Rainwater Harvesting Systems: Integrating Rainwater Into Building Systems.” www.celesteallennovakarchitect.com

Hyload is part of IKO, a family business operating for more than 60 years providing global manufacturing and supply of quality roofing products. Hyload products include commercial roofing, waterproofing, and masonry through-wall flashing products. www.Hyload.com www.Hyload.com