Flashing and Drainage

It has become increasingly difficult to cut through the clutter when choosing products for flashing and drainage systems in cavity wall construction. Manufacturers now are trying constantly to reinvent the wheel with new and improved materials, driven by the notion of holding the only acceptable product in a specification. It is no longer function or development that drives new products to the marketplace, but the race to beat one’s competition to the idea.

Performance, warranties and even aesthetics need to play an integral role in the products we choose. While mostly tucked away behind the face of the wall, your flashing system is the first line of defense against problems associated with moisture penetration. Mold, efflorescence, cracking due to freeze/thaw cycles, and general degradation are all effects of inferior flashing materials, systems or installations.

Synthetic flashings, while considered cost effective, typically offer little, if any, long-term protection. Costing pennies per square foot, Polyvinyl Chloride, or PVC membranes, are often suggested as a substitution for more expensive flashing materials. With this in mind, it is important to realize the chemical composition of PVC. In its natural state, PVC is actually a rigid material. It is made flexible by the addition of plasticizers, which, in a short period, will migrate to the membrane’s surface. The plasticizers then will evaporate naturally, rendering the flashing material rigid and brittle. This causes surface cracks that allow present moisture under and behind the flashing membrane.

The ever-popular rubberized asphalt peel-and-stick flashings provide the installer with another inexpensive and flexible flashing membrane. While these flashings are, for the most part, 40 mils in thickness, they are not necessarily offering 40 mils of protection. Products vary mostly from a four- to 15-mil thick polyethylene sheet with the rubberized asphalt filling the difference. It is important to realize that rubberized asphalt, commonly known under many different names, is a viscous material that also acts as the adhesive for these peel-and-stick membranes. It should not be confused as a protective layer. Should the polyethylene membrane degrade, the rubberized asphalt is left on its own to be self-supporting as a flashing within the cavity wall. It should not be mistaken that these types of flashings can be merely “stuck” onto a backup wall and left to act as a flashing system for years to come.

Gravity will, of course, eventually prevail, separating the membrane from the backup substrate it was originally set upon. For any of these flashings to be in any way beneficial, it is imperative that the membrane be fastened to the backup wall with a termination bar and a caulked top ledge, preventing moisture from entering from behind the flashing. Of more concern, however, is the viscosity of the rubberized asphalt material should the temperature of the veneer unit’s rise. As the wall face temperature rises during sun exposure, the rubberized asphalt becomes thinner and will seep out the joint from underneath.

This results in unsightly stains, recurring cleaning visits, and a compromised flashing membrane. There have been a few products recently released into the marketplace to address this issue. Manufacturers are now marketing true 40-mil membranes with an applied adhesive on one side, promising no more staining issues. While these are a promising replacement for their rubberized asphalt kin, it is important to consider what the makeup of the sheet membrane actually is. For example, putting an adhesive on an already faulty material like PVC or polyethylene is not a solution to a failure, but the probable cause of another one.

Preformed sheet metal flashings are a sound defense against any moisture penetration inside the cavity wall. Stainless steel and copper, most commonly in .015-inch thicknesses, are an excellent choice for base flashings, pre-manufactured window pans, parapet flashings, and preformed corners and end dams. These metal flashings, while stout in nature, are not without drawbacks. They are usually manufactured off-site, which means workability of the material is limited to its initial profile design. Another consideration has to be the length available for delivery. As these flashings are completely rigid, they are typically available only in eight- or 10-foot lengths, requiring more frequent lap joints and, thereby, providing more opportunity for failure. In spite of this, when installed properly, preformed metal flashing systems are of the strongest materials available and will ensure a lifetime of service inside the wall.

Copper laminate flashings have been in use for the better part of a century, so their operation and performance have been tested throughout the years. Typically comprised of a thin copper sheet that is encapsulated with asphalt, preventing oxidization and providing a primary waterproofing layer, it is also often laminated to a fiberglass reinforcing fabric for additional support. These flashing materials offer exceptional long-term moisture protection, are easy to work with, and remain reasonably priced to the overall scope of the project. While rigid enough to hold their shape, but flexible enough to be bent to form corners, end dams, or to wrap around columns, copper laminated flashings remain user friendly and reliable.

The longevity of a copper-based flashing is unsurpassed by any other flashing materials available in the market today. This is obvious, simply by examining material warranties. A warranty, in most products we purchase, becomes an indicator as to how long the manufacturer will bank on its performance. In the world of flashings, this is no different. In looking at several different synthetic flashing manufacturers, the warranties are extremely limited. All peel-and-stick and PVC warranties I found sit at five years or less, and, surprisingly, I found no warranty by an EPDM manufacturer for longer than the time of shipment. I was also unable to find a warranty on any of the “one size fits all” flashing systems available today (those that use a synthetic membrane with other wall drainage system components glued about them), but I suppose they are far too new to the industry to be able to effectively track performance results.

All metal flashing materials that I have been able to find warranty information on seem to have a warranty not less than 20 years. These include the stainless steel and heavyweight copper materials discussed earlier. Certainly, these types of flashings will stand the test of time within the masonry wall. Better still, judging by available warranty information, remains the copper laminated flashing materials with a warranty of up to “the life of the wall,” depending upon weight and manufacturer. A bold statement like that can only stem from confidence in the product’s lifelong performance. To me, that kind of assurance would be the simplest to select in the construction of the masonry wall.

I purposefully have not mentioned cost when discussing the metal flashings. Metals, of course, are subject to market volatility, at times making metal-based flashings less desirable than another less-costly alternative. It’s most important to examine the percentage of cost of the flashing systems to the overall cost of the building they are designed to protect (see Table 1).

Table 1.

Preformed Metal Flashing
(Stainless Steel, 28 .015”)

$31,500 .00105%
7-oz. Copper Laminate Flashing $17,500 .000583%
5-oz. Copper Laminate Flashing $12,250 .000408%
Rubberized Asphalt Flashing $3,500 .000116%
PVC Membrane Flashing $1,120 .00003%


As you see in the Table 1, even the most expensive of available protective materials factor only a small fraction of a percentage on the overall project cost. The point to be taken here about the selection or substitution of lesser performing materials is that the cost savings remains insignificant in comparison to the overall cost of the project. We can easily spend $30 million on a high school project, but for the sake of saving a few thousand dollars, then neglect to put a proper collection system in place.

There are other materials and methods to consider, once the flashing membrane is selected and in place. Weep devices play an equally important role, as do flashings, because, of course, if the moisture is unable to find its way to the face of the wall, what’s the point? While it’s the actual void in the head joint that provides the means by which moisture finds its way to the exterior, the weep device provides uniformity across the surface of the wall. This maintaines that they be placed every so often, in the same size and color. They offer a structure by which moisture and air may move freely, though some may do a better job than others.

The industry standard 3/8-inch round, clear plastic weep tube runs itself about four inches back into the cavity wall. These, most often, immediately become new high-end real estate for insects, and a trapping point for dirt and debris. They also are manufactured with a wick and screen glued into either end, which, more or less, clogs the weep before it ever gets onto the jobsite. Inexpensive and available in nearly every masonry showroom to grab by the handful or box, their accessibility and low-cost keeps them as the most widely used weep device on the market. But, are they effective? The larger rectangular version – usually around 1.5 inches in height – does a much better job holding the opening of the head joint, but is still susceptible to insects and debris.

The geotextile-type weep devices are, by far, superior in design, as they absolutely are incapable of clogging when installed properly. They are also available in several different colors, giving options as to how they will look when sitting within the mortar joint. Unfortunately, where these fall short is the very design that makes them so effective. Tiny strands of polyester come together to form these miniature geotex grids, keeping the weeps themselves soft, pliable and, most important, compressible. Without absolute attentive placement in the masonry joint, their compressibility often can lead to disproportionate weeps across the face of the wall. It is because of this that the rigid cellular-type vents are an ideal choice to maintain air circulation and moisture expulsion at the wall face. Honeycomb in design and also available in many colors, their placement across the mortar joints in the wall remains uniform in both aesthetics and performance.

Of course, none of this makes any difference if the weeps are doomed to fail from the beginning. Mortar coming off the back side of the constricting brick joints will fall to the bottom of the cavity wall, risking the integrity of the open weep passage. There was a time when a mason would run a piece of lumber up the back side of the wall as he lay the brick, collecting the mortar droppings and keeping the wall cavity clear. While extremely effective, it does add a secondary process, lowering efficiency in laying bricks. Today, we are fortunate to have simpler methods. Mortar collection devices are an imperative component of the wall drainage system, effectively keeping air and moisture moving through the wall cavity.

Failure to understand the importance of quality products can lead to failure. The library at the University of Connecticut is a prime example of the damage water infiltration can cause when an inappropriate flashing is used. Originally constructed for about $24 million, it was a beautiful structure featuring large granite panels. The problems in the wall began almost immediately. Water often would find its way past the barrier of the interior walls, damaging furniture and flooring. Later, the stone veneer began to come apart, some of it crashing to the sidewalk as a result of the anchoring systems succumbing to moisture. It was later determined, among other defects, that the use of an inferior flashing material was a major contributor to the failure of the wall. Today, the beautiful structure remains shrouded in scaffolding, as it’s determined who will pay for the nearly $20 million in necessary repair.

The most important part of the veneer wall system is the knowledge of the materials going into it, both in design and construction. Without the understanding of how materials work, how they are to be installed or stored, or how they are affected by the elements, the structure’s wellbeing is simply left to chance and assumption. When well constructed, masonry structures are beautiful and strong, yet remain susceptible to extensive damage through moisture penetration. It is never a question of if, but when, moisture will be allowed into a masonry wall, which is why it is so crucial we understand the very components developed to protect it.




Last Updated on Monday, 02 November 2009 19:54