Masonry Magazine April 2007 Page. 46
FLASHING AND DRAINAGE
Flashing must extend through the full thickness of the wall and terminate beyond the face of the wall with a drip edge. Terminating flashing within the exterior wythe of masonry could allow water to travel back beneath the flashing, into the cavity, and possibly back to the interior wythe of the wall system. Use of self-adhesive rubberized asphalt flashing should include separate, stainless steel drip edges adhered directly to lintels and shelf angles, beneath the self-adhesive flashing. This is because rubberized asphalts will degrade with UV exposure. Therefore, when used with a stainless steel drip edge, the rubberized asphalt does not need to extend beyond the face of the exterior wall.
The interior vertical leg of flashing should extend up a minimum of eight inches. The top edge of the flashing must be terminated properly. The two most common termination methods are embedment in a bed joint of the masonry backup or with a termination bar. Termination bars should be fairly stiff to provide a uniform clamping force between the fasteners and resist bending during installation. A "[" shape often is best for this purpose. Termination bars should also be corrosion resistant.
Details of seams should include a minimum lap length of four inches. Although metal flashing often is soldered at the seams, sometimes it is better to adhere the lapped areas with flexible sealant. This will accommodate thermal expansion and contraction where soldered seams will not.
Flexible flashing must be fully supported across the full width of the wall cavity below. At shelf angles and lintels this is always the case. However, flexible flashing placed at floor lines without shelf angles will form a trough over the width of the cavity if supplemental support is not provided. This trough will retain water and can cause large leaks where there are any small imperfections in the flashing seams. Water will then flow through the seams, into the cavity space below and possibly down the face of the interior wythe, exposing interior areas of a building to water penetration. Supplemental support typically is provided through installation of a sheet metal angle attached to the masonry backup wythe and spanning across the full width of the cavity.
Penetrations through flashing materials, such as at coping stone anchors, must be detailed to ensure the penetration is watertight. Most often, this is accomplished with a flexible sealant or mastic.
Weeps must be placed in the mortar joint immediately above the flashing at a maximum of 24 inches on center. In general, the larger the weeps, the better. Cell vents installed in head joints, typically the same size as a head joint, will quickly remove more water than cotton rope or small plastic tube weeps.
Flashing details are often best presented in isometric views. In particular, details at corners and end dams are difficult to show clearly in traditional, two-dimensional details.
Although not specifically the topic of this article, it is important to note that flashing cannot serve its intended purpose if the wall cavity is too narrow and not kept clean during construction. For cavity walls to function as intended, they must be designed with a minimum inch-inch wide cavity. This dimension is considered the minimum width necessary, as recommended by the Brick Industry Association, to prevent the cavity from being bridged by mortar or other materials and to allow water to cross over to the interior wythe. Narrower cavities are far more difficult to keep clear during construction. However, it should be noted that mortar bridging can still occur with a wider cavity if the masons do not exercise care to keep the cavity clear during construction.
In addition, be aware that a weather-resistive barrier (WRB) is required by all three versions of the IBC. A WRB also can serve as a vapor retarder and/or an air barrier depending on its properties. Design and installation of flashing for masonry cavity walls will require close coordination between WRB installers and flashing installation crews as these two components will serve as parts of the moisture management system within the