Masonry Technical Library — Types of Flashing for Masonry

Masonry Technical Library - Types of Flashing for Masonry

Reducing water penetration is one of the primary goals of designers and builders in the effort to protect the structures they create. It not only forestalls owner-designer-contractor disputes, it also ensures full utilization throughout the building’s projected life. Flashing is a critical component of successful projects, as it is intended to divert any moisture that enters the exterior building surface.

Desirable Attributes

There are certain attributes that through-wall flashing materials must possess. The most obvious requirement is water imperviousness. But it must also be resistant to damage during construction by puncture, tears, burns, and sunlight. The flashing material must not be damaged by corrosion in fresh mortar or have adverse reactions with adjacent materials. It should be easy to form and join, and retain its given shape. Flashing must be compatible with adjoining adhesives and movement joint sealants. A properly selected flashing material should not cause discoloration of the brickwork. Additionally, it should have an expected lifetime that matches the anticipated life of the building. Due to the prohibitive cost of replacement, flashing is one construction material that should never be selected solely for having the lowest installed cost.

Materials

A variety of flashing materials has been developed and promoted to meet these demands. Although several variations exist, flashing materials fall into three basic categories: sheet metals, composites, and plastic or rubber compounds. Due to fragility and lack of durability, asphalt impregnated felts (tar papers) and plastic sheeting intended for use as moisture or vapor barriers should not be used as a through-wall flashing.

Stainless steel and copper flashings are the most durable but most expensive flashing materials. They are typically specified for projects that are intended to last for a hundred years or more, such as publicly funded projects. All of the sheet metal flashings have to be bent and connected by soldering or adhesives and rivets to conform to the intricacies of the building, adding to the in-the-wall costs.

Stainless steel provides the greatest durability, and is most resistant to corrosion. It does not pose staining problems. However, it is also the most expensive flashing material and the hardest to work. The minimum thickness allowed should be 0.01 in. (0.25 mm). It should comply with ASTM A 167, and solder should comply to ASTM B 32.

Copper sheet flashing is highly durable, corrosion resistant, and more easily shaped than stainless steel. It has a tendency to deposit green stains on brick and mortar adjacent to the flashing if exposed; lead coated copper sheet reduces staining substantially. Chloride-based mortar additives can be corrosive and should not be allowed when copper flashing is used. Sheet copper is commonly available in 10 to 20 ounces per square foot (3100 — 6100 g/m2), and 10 ounce (3100 g) copper is the minimum that should be used. Copper flashing should conform to ASTM B 370.

Galvanized steel is less expensive than stainless steel and impervious to moisture. It is difficult to form and to solder laps effectively. The galvanized coating may crack at bends in the flashing, lowering the corrosion resistance. Galvanized steel is subject to corrosion in acidic or salty conditions. These limitations must be considered when specifying galvanized steel as a flashing material.

Composite Flashings

In an effort to offer more easily installed and moderately priced durable flashings, manufacturers have developed many composite materials for flashing purposes. The most common variation is a thin metal sheet such as aluminum, copper, or lead, coated with a flexible compound. In some instances, the metal is sandwiched between layers of material such as an asphalt-impregnated kraft paper, fiberglass fabric, or plastic film. The laminations may separate due to moisture and movement, and some laminated films degrade outside the wall if used to form a drip edge.

Copper laminated flashings arc popular due to the known durability and desirability of copper as a flashing material. Copper weighing 2 to 7 ounces per square foot (610-2100 g/ m’) is available. Durability should be expected to be greater as the copper weight increases.

Aluminum laminate flashing is also available, typically coated with a polyester film and reinforced with fiberglass. This product can be cut with scissors and applied with mastic. It is susceptible to on-site tears and punctures, though they are said to be easily patched using a polyester film tape. Long term adhesion of such tape is questionable.

Plastics and Rubber Compounds

Plastics are highly popular due to their relatively low initial cost and characteristic high flexibility which makes them easy to install. Plastics have undesirable side effects of degradation in ultraviolet (UV) light, lack of durability in a caustic environment (fresh mortar has a pH of 12.5 -13.5), and brittleness or fracturing at cold temperatures. Therefore it is especially important that the manufacturer be requested to provide documentation that their product meets the owner’ s requirements.

Polyvinyl chloride, or PVC, is one of the most common plastic flashing materials. The addition of plasticizers have made this material flexible during the manufacturing process, but the plasticizers used in the past have not always been stable. This has caused PVC flashings to be known for shrinking and eventually becoming brittle. However, not all PVC flashings have experienced this fate. The use of PVC with a thickness greater than 30 mils (0.8 mm), from a well recognized manufacturer, should prove adequate for shorter-life buildings. Since PVC cannot be exposed to sunlight, a metal drip should be adhered to PVC flashing in order to project from the face of the wall.

Ethylene Propylene Diene Monomer, or EPDM, is a synthetic rubber that is used as a single ply roofing membrane as well as flashing. It has better low temperature performance than PVC, and better weathering resistance than butyl rubber. It is commonly available in a thickness of 40 mils (1.0mm) or greater, reducing concerns of fragility during construction. Dimensional stability may be a concern.

A recent boon to speed the installation of flashing is the development of self-adhering, rubberized asphalt. This material easily adheres to itself, requiring less effort to seal laps or corners, and is self-healing in the event of punctures. However, it should not be applied to damp. dirty, or dusty surfaces, and has a lower limit installation temperature of 25? F (-4? C. )It also cannot be stored at temperatures in excess of 100 ?F (38? C). Because it degrades in the presence of sunlight and sealants will not adhere to it, it requires a metal drip edge to penetrate the face of the wall. No long term studies have been completed to determine the longevity of its adhesion.

Conclusion

There are many options available when specifying a through-wall flashing material. The design life of the structure is important in assessing the required flashing durability. Due consideration must be given to the issues of durability, compatibility, and ease of installation as well as initial cost. A satisfactory balance between budget and performance can be achieved.


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