Flashing: Rules Never To Be Broken

Words: Bronzella Cleveland

By Steven Fechino,
Engineering and Construction Manager,
Mortar Net Solutions

Unitized flashing installed on a CMU wall.
Unitized flashing installed on a CMU wall.

The masonry industry is a proud group of professionals who build extraordinarily durable structures using time-honored techniques that have spanned generations. Discipline and mentoring are the keys to learning the rules that make up the complex trade.

As with all things, masonry has rules that will and will not change over time.

  • Rule No. 1—Masons get paid for what they lay on the wall, not for effort.
  • Rule No. 2—Completing a job incorrectly will cost the mason four times more than doing it correctly the first time: One: removal of defective work. Two: replacement of defective work. Three and four: the two jobs that were not completed while the repair work was being performed.
  • Rule No. 3—A wall without properly completed flashing has a high chance of leaking, while a wall that is flashed correctly has almost no chance of leaking.

Now let’s address some rules that are changing. Sealants and flashing technology have recently seen some positive process changes. Products designed to improve the workmanship and quality—compared to what has been available in the past—are now available to both residential and commercial masonry contractors.

For example, new sealants, flashing fabrics, membranes, and composites give masonry and roofing contractors better options to serve their building owners.


Early in my career, prior to the sealants that had evolved into what we see used today, bituminous mastics were commonly used to lap flashings, patch around dowels, and seal wall penetrations. Bituminous mastics, when used now, should be checked for compatibility with today’s polymer-based flashing membranes and components. It is typically the responsibility of the manufacturer to determine the compatibility between sealants and other wall components. Simply call the manufacturer and ask. Once the question is answered, it is common to request a letter stating the status of approval or disapproval of the product.

Sealants fall into several general categories depending on chemical make-up and formulations by the individual manufacturers. The characteristics listed in this article are based on a wide range of chemical formulations and not specific products, with the understanding that each product can be formulated to achieve different results. Specialty conditions that exist under difficult jobsite restrictions can in in some cases be formulated to offer longer or shorter tack times, more forgiving installation temperatures or cure ranges, as well as abilities to adhere to products that exhibit higher-than-standard surface energy ratings.


Mortar Net butyl sealant
Mortar Net butyl sealant

Developed in 1937, butyl has had a long run in the construction industry. A product that has been around for that long either delivers consistent performance or it does not stay active within an industry. Butyl is successful primarily due to its adaptability to the needs of the roofing and masonry industries. It is an uncured synthetic rubber and is available in sealant tube form as well as convenient tape rolls. It is typically more economical for the contractor to use in the correct application than some of the other sealants on the market.

When making a flashing lap or sealing around dowels, butyl is a smart choice. However, it should not be used for general joint sealing, expansion joints or any exposed joints, as this material remains mostly in a “tacky” state throughout its life. Butyl is compatible with today’s polyvinyl chlorides (PVC), thermoplastic polyolefin (TPO), ethylene propylene diene monomer (EPDM), laminated copper fabrics, and most rubberized asphalts (RA) with excellent adhesion to all of these products.


Polyurethane elastomeric sealants are the sealant of choice by many architects and specifiers for masonry construction because they provide quality expansion, control, jamb and head joints found in masonry-to-masonry and masonry-to-dissimilar material joints. Polyurethanes typically are one- or two-part sealants that have a large variance in tack time, and they offer a wide range of colors. Polyurethanes do not perform well as a lap sealant with many of today’s flexible flashing membranes, as the high surface energy ratings will not allow for anything but a very short-term adhesion.

Polyurethanes offer a life span ranging from six to 10 years, depending on exposure and geographical location. Their working temperature is between 40 and 95 degrees Fahrenheit, a range that is truly within the useable “gunnable” range for the product. Single-part polyurethane elastomerics typically are moisture cured. Moisture curing begins when the material is released from the packaging tube in a viscous form and reacts with the humidity. Two-component sealants begin as separate, individually packaged products that are mixed thoroughly just before application. Components consist of a base, an activator (sometimes called a hardener), and an optional color pack. The product can be applied several different ways, but the most common ways are to use a bulk gun or a knife—equally important methods depending on the location of the joint to be sealed. Curing begins immediately once the materials have been combined, but tack free and cure times are based on the formulation, and individual product technical data sheets will provide a range of expected cure times.

Modified Polyether (Polyether)

Modified polyethers have become a new option in the joint sealant and flashing industry. Polyethers are modified silicones that have the gunnability of silicone and the best characteristics of polyurethanes when installed within the proper temperature range. Polyethers do not migrate oils into porous masonry so they won’t cause staining, and they can be installed in harsh climates at temperatures down to 15 degrees Fahrenheit. With low VOCs and an available quick, tack-free time, polyethers could one day replace today’s polyurethanes because of superior joint material performance and their ability to bond to a wide variety of membranes and substrates. Polyether used with a primer will seal thermoplastic polyolifins with excellent results and can replace heat welding when necessary to meet installation restrictions.


Flashings have seen the biggest change in the market during the last few years with the introduction of time-tested and proven roofing industry products into the masonry industry.

Copper Laminate

The tried-and-true copper laminate we all used for years still is on the market, and two manufacturers have made their products much more durable and desirable by laminating reinforced fabrics on both sides of the copper. One of the manufacturers utilizes a fiberglass mesh and the other offers a polymer fabric. These products are durable, nearly impossible to cut with a trowel, and as good a material as any that can be found on the market. They are premium flashing materials that will perform well for years to come. Many of the copper laminates of today do not contain asphalt as a binder, making them compatible with most building products.

Rubberized Asphalt

Rubberized asphalt is the most common flashing on the market today, with several major brands offering products in the United States. Commonly installed “Sticky Back” rubberized asphalt is a good, stable material that performs well when installed properly. Primers are sold by some manufacturers as part of every application, and other companies will allow the omission of the primer if the membrane is installed with a termination bar. Installation details for this membrane should be discussed with the project contractor and manufacturer prior to installation.

Unitized flashings are factory-assembled flashing systems that can be manufactured with any of the different flashing membranes that have been discussed.
Unitized flashings are factory-assembled flashing systems that can be manufactured with any of the different flashing membranes that have been discussed.

It is critical that the mason contractor read the fine print for each product, as warranty coverage can depend on the use of primers and full adherence to the substrate. Rubberized asphalt is UV sensitive, so extended exposure to sunlight is not recommended. Compatibility between rubberized asphalt and PVC-molded corner boots and end dams should be investigated prior to construction as plasticizers can migrate from the PVC and reduce the plasticity of the rubberized asphalt over time. This causes the potential for a leak down the road. Metal drip edges typically are used at the face of the wall when installing rubberized asphalt, as many manufacturers require that the membrane be held back from the brick face +/- 1/2 inch.

Polyvinyl chloride (PVC)

Today’s PVCs combine old and new technology to create strong, durable and flexible materials, thanks to the addition of non-migratory plasticizers, which will not leach out of or evaporate from the material. Modern PVCs stay flexible and will not break down and become brittle, which would allow cracks and create leaks. PVCs are as good a membrane as any on the market. A crossover from the roofing industry, PVC membranes have proven their durability, workability and economy on roofs all around the world for many years.

Thermoplastic Polyolifin (TPO)

TPO is another product that we inherited from the roofing industry. It is a flexible membrane that has a 30-year life span when installed on a roof, and an unlimited life span when placed in the cavity of a building. TPO has an advantage over all other membranes on the market because laps can be sealed using a butyl sealant or primed polyether, or they can be heat-welded. Heat-welding eliminates the need for separate sealants at the heat-welded locations and uses nothing but a hot air gun and a roller to produce excellent, leak-free results.

Low-cost equipment for heat-welding can be purchased for under $100 and reduces the sealant requirements for installing flashing. Unprimed polyethers and polyurethanes cannot be used for sealing this membrane, as surface energy of the membrane will not allow a durable, long-term bond.

Ethylene Propylene Diene Monomer (EPDM)

EPDM is yet another roofing crossover material that has entered the masonry industry during the last several years. Comparable to the tire tube material we are all familiar with, EPDM is flexible and easy to cut, and installs easily as it has little memory to the roll in hot or cold temperatures. EPDM is a rubber that bonds well with certain polyethers and butyls when used as a lap and general installation sealant. It is not compatible with PVC-molded corner boots or end dams. EPDM must have similar EPDM components for terminations and corners applications, and it is compatible with most air barriers on the market.

Brick installation over Unitized Flashing System.
Brick installation over Unitized Flashing System.

Unitized Flashing Solutions

Another improvement in the flashing industry is the introduction of unitized flashing solutions. Unitized flashings are factory-assembled flashing systems that can be manufactured with any of the different flashing membranes that have been discussed. The systems include a membrane, pre-designed membrane laps, screws, weep tabs, termination bar, drip edge, and a mortar collection device that has passed the requirements of the ASTM E-514 water test.

Unitized flashing solutions also come with manufacturer-supplied, pre-engineered flashing take-offs, and custom-cut panels for window and door heads delivered to the jobsite. Custom cutting gives the contractor a fully assembled masonry opening head flashing for each exterior wall opening right out of the box. Each custom-cut flashing section is installed as a complete unit, reducing the costs of labor time, equipment rental and general conditions.

Technical supervision and training also is provided, with video-based instruction, and live streaming, phone and onsite visits available to assist the architect and contractor. Unitized flashing has a higher initial material cost than field-assembled pieces-and-parts, but with the labor cost savings, plus the reduction in material waste, equipment rental and general conditions costs, unitized flashing solutions are proven to save the contractor time and money over the course of the project.

Roll flashing with the mortar collection system factory-assembled to the membrane, but with no attached termination bar or drip edge, also is available. Any of the membranes and composites discussed earlier are offered for this system, which allows the design or construction team to choose the correct membrane for the project.

All of the flashing membranes mentioned in this article are compatible with all of the high-loft, non-woven, and unilateral mesh products used in mortar-collection devices and mesh-wall fabrics that cover the entire inner wythe of the structure.

Unitized Flashing System—close up of weeps and stainless steel drip edge.
Unitized Flashing System—close up of weeps and stainless steel drip edge.

It is critical that the wall mesh not be placed behind the termination bar. The mesh should extend over the termination bar and lap about one inch over the flashing.

A final major improvement in the sealant and flashing market is that contractors can now obtain answers to their technical questions about nearly anything related to masonry wall construction and design. For example, Mortar Net Solutions maintains a specialized staff who can answer masonry-related questions posed by architects, specifiers, and masons.

The flashing and sealant industry is constantly evolving, and the rules determining what makes the best building design and the best building techniques also are evolving. It’s important to keep up with the changes this evolutionary process produces, but all the changes are being made with the intention of helping construction industry professionals prevent every leak, every day for the life of each building.




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