Flashing Types

Although the choice of flashing material is often governed by price, other factors to consider are the ease of installation and the expected life of the material once it is embedded in the wall system. Metal flashings are often used on commercial jobs, but the two most common types of flashing for most residential jobs are EPDM and self-adhering rubberized asphalt. The BIA recommends that most nonmetal flashings should be at least 30 mils thick, which means polyethylene, housewrap, and #15 asphalt felt are not acceptable flashing materials. Aluminum is unacceptable because it corrodes when embedded in mortar, which becomes very alkaline when exposed to moisture. PVC flashing has been known to deteriorate as well, raising concerns about its expected life. Remember that the drainage wall system only works if all the materials are properly installed, and if the flashing is designed to last as long as the wall itself. Consult the flashing manufacturers for recommendations on which product to use in a specific wall system.

End Dams

If through-wall flashing is not continuous around the whole building - for example, above a window, or wherever stepped flashing is needed - the flashing must include end dams (Figure 8).

Figure 8.

End dams are required in any through-wall flashings that are not continuous, such as step flashings or window heads and sills. To create an end dam, turn the flashing up sideways, against the adjacent head joint, to create a pocket. An end dam will prevent moisture from traveling off the end of the flashing and into the adjacent air space. The BIA recommends that end dams should be at least 2 inches high.

Weepholes

Most building codes require weepholes above all through-wall flashings. The sole purpose of a weephole is to allow water that has been collected by the flashing to exit the wall system. Since it is impossible to eliminate all mortar droppings between the brick and backup, the weepholes must allow for drainage even when some mortar has dropped onto the flashing. There are three common types of weepholes: open head joints, rope wicks, and weep inserts. Open head joints are formed by omitting the mortar from the vertical joints immediately above the flashing every 24 inches (Figure 9).

Figure 9. The most dependable type of weephole is the open head joint.

They're simple, they're cheap, and they work. In standard-size brick, it takes 2-1/4 inches of mortar droppings before an open head joint gets clogged. (A 2-1/4-inch chunk of mortar creates more problems than just a clogged weephole; the odds are mortar has also clogged the air space above, impeding the proper drainage of the wall system.) Rope wicks are the next most popular type of weephole. Although they do not allow as much flow as an open head joint, they are often chosen to avoid the appearance of an open hole or the shadow created by an open head joint. The best type of rope for this purpose is cotton rope, because nylon, polypropylene, and polyester do not wick. Wicks should be spaced 16 inches on-center - more closely spaced than open head joints, to make up for their smaller size. The rope should be a minimum of 10 inches long, so that when the outer end of the wick is installed flush with the exterior face of the brick, the excess can be draped in the cavity or attached to the backup wall. The extended length of wick in the air space will reduce the likelihood of mortar droppings covering the entire wick. There are also several different types of weep inserts available. These are typically made of plastic, and are designed to fit into a head joint. The smaller the insert, the more easily it can be clogged. Plastic tubes should not be used, because the openings on either side of the tube are only 1/4 to 1/2 inch, and are too easily clogged. Weepholes must always be above grade to allow the wall to drain. Remember that the grade can change over time. For instance, when homeowners install several applications of mulch, the grade may end up above the weepholes, preventing proper drainage of the air space. In most cases, through-wall base flashing and weepholes should be 4 inches to 6 inches above grade. The air space below the base flashing should be filled solid with grout or mortar.

Through-Chimney Flashing

Most people are familiar with the visible base flashings and counterflashings that are installed where the roofing meets a chimney. These visible roof flashings prevent water from traveling between the roofing and the brick - but they don't stop any water that is already in the chimney, such as water that may enter through the brickwork or the chimney cap. Through-chimney flashing is designed to prevent moisture that has penetrated the chimney from traveling below the roof. Through-chimney flashing operates on the same principles as through-wall flashing. Through-chimney flashing has to be installed above the roof flashing, even if that means putting it a few courses up, so that the collected water is led to the exterior (). Flashing should also be installed under a concrete chimney cap. This prevents moisture from entering the top of the masonry, and also prevents staining or contamination of the brick. The flashing can be laid in place above the top course of bricks, before the concrete cap is poured. A mortar wash does not form an adequate chimney cap. A mortar wash will shrink and crack as it cures, especially when the mortar is brought to a feather edge (see "Troubleshooting Common Chimney Problems," 7/98). Mortar is very similar to concrete, only with smaller aggregate. Remember, concrete usually needs something to give it some tensile strength and to prevent cracking - for example, 6x6 mesh or other reinforcement. For chimney applications, choose a flashing material that can hold up to the potentially high temperatures expected when the flue is in use. Use metal flashing at the chimney cap.