Proper Flashing of Cavity Walls
When it comes to flashing of cavity walls, hundreds of conditions affect moisture-management product installation, and one size does not fit all.
By Steven Fechino
Flashing is an important part of many masonry projects. Architects are designing moisture management products into walls from Arizona to Florida, and the ability to properly install the products is paramount to every mason contactor. With the shortage of qualified masonry mechanics, it has become difficult to find experienced personnel who can understand, direct and install flashing without guidance.
The concept of flashing is simple: Run a membrane along the face of the wall to collect any water that gets in, and allow it to drain out. Hundreds of conditions can affect flashing of cavity walls, and one size certainly does not fit all.
Flashing cavity walls
A cavity wall backup wall, or substrate, usually consists of studs and sheathing or concrete masonry units (CMU). The substrate is the interior portion of the wall and usually is constructed before the veneer. The veneer can be comprised of a variety of materials, including CMU, cast stone, precast and brick.
The cavity or air space located on the exterior of the substrate and interior of the veneer provides an area where the weather-resistant barriers or air and vapor barriers are installed. In some cases, rigid insulation and wall ties are attached. These materials do not fill the cavity completely, so an air space always exists.
The open-air space of the cavity does not allow a path for moisture absorbed through the veneer to directly contact and saturate the substrate. It does, however, allow air and moisture that is absorbed in the veneer to follow an open path to the flashings, weeps and soffits, and to evaporate back through the veneer, which keeps walls and buildings dryer.
The biggest change to cavity wall construction during the last few years is a deeper bed depth that can allow rigid insulation to be placed in the cavity. With deeper bed depths, it is important that the person installing the flashing pays attention to a few key details to ensure collected moisture will be drained and not accumulate in the walls.
Basic components of a flashing system
End dams. These are placed under the flashing membrane using the proper sealant at the terminations or ends of the flashing. Installation should include placement of the outside edge of the end dam within ¼ inch from the face of the veneer. End dams can be purchased as one-piece, preformed units – made or tucked using the flashing membrane in the head joints.
Corner boots. The corner boots are placed at inside and outside corners under the flashing membrane using the proper sealant. They should extend to the top of the flashing on the substrate and terminate within ¼ inch of the face of the veneer. It is generally less expensive and more reliable for the mason to purchase corner boot components than to construct them in the field, when labor rates are figured for fabrication.
Termination bar. This component is a plastic (1 ¼ inch wide x 1/8 inch thick) or stainless steel bar (1 ¼ inch wide x 16 gauge) that mechanically attaches the top of the flashing to the substrate wall. A neoprene sealing gasket attached to #14 screws, 2 inches long, are attached at stud locations or directly into the CMU substrate. Sealant must be placed behind the termination bar to create a gasket to prevent liquid moisture that runs down the wall from going behind the flashing. Rubberized asphalt is a popular flashing material that has a self-adhering material on one side. Though it is sticky, the product requires a primer, which is a chemical that enhances the bond. It is always a good idea to create as strong a chemical bond as possible, and a termination bar is a mechanical bond that ensures the flashing will stay attached for the life of the building.
Weeps. This component has been configured in many ways. During the years I have worked in masonry, I have seen metal louvers, cotton ropes, plastic tubes, open head joints, plastic nylon rope, CellVents and high loft non-woven meshes. When considering your choice of weep, here are a few details to consider. Nylon rope will only allow water to pass, but it will not wick or draw water. It is not a strong choice. Cotton rope will deteriorate and rot over time, leaving a black mush in many locations to clog the openings. Ropes also don’t allow any air flow for ventilation. That is why many masons have begun looking for other choices. The high loft meshes and CellVents are the most common weeps used today and are preferred by most installers.
Mortar collection. Mortar collectors were founded on a dovetail shape more than 25 years ago. Prior, the mason shoveled pea gravel into the cavity on top of the flashing to create a drainage plane for moisture to pass through the weep vent. What a mess that was. Multiple testing showed that pea gravel created a mortar dam that prevented water from traveling to the flashing level. Today it is important that the mortar collector fill the distance between the inside face of the veneer and the substrate within ¼ inch. As mortar droppings accumulate, the dovetail separates the droppings on two levels and allows water to pass, unobstructed, through the angles of the dovetails to the flashing below.
Membranes. The membrane of the flashing is the sheet good that is the basic form of the flashing system. Available membranes made with polymer-based materials are typically available in a 40 mil. thickness. A few options include thermoplastic polyolefin, a roofing material that has been adapted for masonry and can be heat-welded when necessary; rubberized asphalt, a self-adhering membrane that is widely used for many transitions, terminations and flashings within a structure; thermoplastic vinyl, a membrane widely used in Europe for roofing; and ethylene propylene diene monomer (EPDM), which is flexible and easy to use.
The use of a polymer-based flashing membrane is common and will produce good results when installed properly. The sealants used in joining the seams, components and terminations must be matched to the products specified. Polyurethane elastomeric and bituminous mastics should be avoided, because chemical compatibility and bonding of the sealant to the membrane can become issues. A good choice across the board would be to use a non-curing synthetic rubber or a butyl to handle connections. Many manufacturers can provide letters of capability for butyl, since they have been on the market for many years and have offered consistent performance.
Metal flashings are available as laminated sheet goods and cold rolled products. Laminated sheet goods can be either copper (available in different weights) or stainless steel. The protective coatings laminated to the metal are usually fiberglass or polypropylene and resist punctures that can occur on a project site. Cold rolled products are rare these days. Purchased as 26-gauge or 16-ounce materials, labor costs are high for this type of flashing. Pre-forming at a sheet metal fabricator and extensive planning are required to assure the product meets the actual building’s tolerances.
Drip edge. Generally made of stainless steel, options include Kynar-coated steel (for options with color) or copper. The purpose of the drip edge is to remove the water that accumulates at the face of the building and prevent it from rolling back under the flashing through capillary action. The drip edge has a turn down that usually extends 3/8 inch and has a hemmed edge. The hem allows the accumulated moisture to be pulled away from the face of the veneer and drip off away from the structure. Recently, a turned-under drip edge became popular for those who did not want to see the hemmed edge. I do not recommend this detail as it can lead to water entering the structure from the underside of the flashing and can allow water to accumulate at the back of the cavity, due to the increased thickness of the bend.
Steven Fechino is an engineering and construction manager with 35 years of experience in masonry restoration, moisture management and hardscape construction. He can be reached at firstname.lastname@example.org.