BMJ Stone
EZG Manufacturing
Federated Insurance
Fraco USA, Inc.
Hohmann and Barnard, Inc.
Hydro Mobile, Inc.
iQ Power Tools
Kennison Forest Products, Inc.
Mortar Net Solutions
Non-Stop Scaffolding
Pullman Ermator
Tradesmen's Software, Inc.
February 19, 2001 1:32 PM CST

Masonry Troubleshooting: Flasing

Proper installation of flasing


For many years, designers have known that moisture trapped in walls causes significant damage to the integrity of buildings. In fact, providing for proper elimination of moisture from the exterior walls of buildings is one of the most important aspects of proper building design. In the last decade, masonry repair and restoration has become one of the fastest growing segments in the marketplace. Forensic engineers frequently find that improper flashing installation or sub-standard flashing materials are the root of the problem in masonry walls that have failed.

Additionally, once moisture penetrates a building's moisture barrier, it can damage insulation, drywall and other building materials. The deterioration of these building materials caused by moisture penetration, sometimes results in the "sick building" syndrome, whereby, bacteria seeps into the ventilation system and is carried into a building causing people to become ill with various maladies such as allergic reactions and headaches.

Many years ago, the industry created the concept of the "rain screen principle" for masonry buildings. This building method involves the use of a cavity wall system, where multi-wythe walls use the veneer as a rain screen to protect the rest of the building from moisture penetration. This building design method assumes that moisture will penetrate the veneer and provides for its swift elimination via flashing and weeps.

This article will discuss proper installation of flashing and accessories as well as typical problem areas in cavity wall flashing.

Flashing should be installed at the base of the wall, over all openings, at every shelf angle and at parapet walls.

The type of flashing being used determines its proper installation:

  • Non-Adhesive: Composite or sheet flashing that does not have an adhesive backing should be extended from the outer most face of the veneer through the mortar joint, across the cavity and up the substrate wall a minimum of 8 inches. Testing has shown that moisture driven by winds of 90 mph will splash about 6 inches high so bringing the flashing 8 inches up the substrate wall provides more than adequate coverage. It should be inserted into the outermost face shell bed joint of the masonry substrate. If the substrate is concrete, reglets can be formed into it to receive the flashing, or a termination bar can be screwed into the wall through the flashing to hold the flashing up. If a termination bar is used, the screws should have a washer. If the back-up wall is steel or wood stud, the flashing can be run behind the sheathing and screwed to the stud. Regardless of the type of substrate, a bead of mastic should be run across the top of the flashing to prevent migration of moisture into the substrate.

  • Adhesive-Backed: Composite flashings such as rubberized asphalt composites are usually manufactured with an adhesive backing covered with kraft release paper. So instead of having to install the flashing when the back-wall or substrate is erected, the contractor can wait until they are ready to build the veneer to install the flashing. Once the paper backing is peeled off, the flashing can be brought up the wall eight inches and literally stuck to the wall. Again, a bead of mastic should be run across the top of the flashing.
It should be noted that most of these adhesive-backed flashings will not adhere to a dirt or mortar specked wall, or at temperatures below 40 degrees, unless a primer has first been applied to the wall. Most manufacturers stock a primer for use with their adhered flashing.

Problem Areas
Where the flashing extends through the face shell of the veneer, it should be turned down to form a drip to ensure that moisture is flashed outward and down. When flashing is stopped short of the face shell, a potential portal for moisture to seep underneath the flashing is created.

Depending on the type of flashing material, extending the flashing through the veneer may not be possible. For instance, rubberized flashing deteriorates when exposed to UV rays from the sun, so it must be stopped one-half inch back from the face shell of the veneer. Plastic flashings can also be adversely affected by the sun and probably should not be exposed. One way to overcome this problem is to use a stainless steel drip edge flashing ranging in depth from 1-1/2 inch to the full depth to reach the face of the substrate. This highly corrosion resistant flashing sits on top of the shelf angle or footer and extends through the face of the veneer where it turns down to form a drip. The rubberized or plastic flashing is then adhered to the edge flashing and a through-wall flashing installation is accomplished. This is an extremely effective flashing method that allows for the combination of stainless steel with economical rubberized flashing. Plastic flashings comprised of poly-vinyl chloride (PVC) are not recommended in any through-wall flashing application.

Laps in flashing are another area that can cause problems when not sealed properly. Flashing should be lapped a minimum of four inches and a bead of mastic run along the entire seam. Flashing inside and outside corners usually involves some type of cutting, so it is crucial that proper lapping and sealing occurs at these locations. At these areas, pay particular attention to sealing the lap all the way into the corner.

Where flashing terminates such as at doorframes, end dams should be formed to prevent migration of moisture into that area.

Flashing Accessories
Flashing is only as good as the accessories that help it do its job. Proper weep vents must be installed in the veneer wall to assure that moisture migrates to the exterior of the building. The BIA recommends an open head (vertical joint) in the veneer. Although this creates the air space necessary for moisture to flow out of the cavity, it can become clogged with debris or bugs might nest there, effectively closing off the weep area. It also creates a dark hole. Many designers choose to specify that a plastic weep vent (sized to fit the head joint) be used instead. This keeps out bugs and debris while allowing the same amount of airflow as an open head joint. The weep vents come in translucent and typical mortar colors, which eliminates the problem with dark holes. Weep vents should be placed at twenty-four inches o.c. wherever flashing is located and beneath the shelf angles at every floor level of the building. This creates airflow through the cavity that aids in quickly pulling the moisture out of the cavity.

Mortar droppings in the cavity can clog up the weeps and prevent the migration of moisture out of the cavity. The best masons in the world will still drop some mortar into the cavity. It is nearly impossible to prevent it and quite impractical to attempt to clean out the cavity. Using methods such as plywood in the cavity to catch droppings requires extra labor for pulling the board up and out every sixteen inches and is counter-productive from a production standpoint. A layer of pea gravel on top of the flashing helps but still may create a problem with freeze/thaw potential when combined with moisture and mortar. A much more effective and economical solution is to use mortar-catching devices that catch and suspend the mortar above the weep vents which prevents clogging of the weeps. Because of the special open mesh design of products, moisture is allowed to flow freely through the cavity and out the weep vents.

While these are some of the more frequently noted problem areas in constructing a suitable moisture control system, there are many items not covered in this article. Associations such as MCAA, BIA and NCMA, have numerous technical papers and reference documents that more properly and thoroughly cover proper flashing installation

About the Author

Karen Flynn is the Director of Marketing for Dur-O-Wal, Inc.


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