The Importance of Wall Flashings
By Michael C. Tolson, Sr. & Rick Staton
Observing new building construction is always exciting and at the same time unnerving. Exciting because it indicates that our economy is growing. Unnerving because of the design techniques, construction practices and technical skill being incorporated into many projects. It seems that more and more buildings are being constructed without attention to the details that make a real difference when it comes to keeping water out.
With all of the attention given to fungal bioaerosols in today’s media and people’s awareness of mold, one would assume that careful design, installation techniques and inspection of wall construction would be of the foremost importance. But it’s not! It would take an article the length of a short novel to cover what contributing effect this has had to building owner’s headaches, the additional cost to the contractors for repairs and design professional’s insurance premiums. You can only imagine…. But for these purposes, the focus will be on preventing “future leakers.”
Rain, snow, ice, water vapor and condensation cannot be prevented, and all are sources that infiltrate building envelopes. Water is the universal solvent and it is responsible for many problems. Water causes deterioration in building materials through several mechanisms, including dimensional changes in construction materials; cyclic wetting and drying; freezing and thawing action; corrosion and many others. Since water cannot be eliminated, its penetration must be controlled. One important component in controlling water in wall systems is the proper design and installation of flashings.
Flashings are water impervious membranes or metals installed in wall systems that collect water penetrating the exterior and facilitate directing the water back to the exterior. The material used for flashing must be able to withstand the abuse of construction following the installation. Building felt and plastic sheeting should never be used as a flashing material. Due to its importance, the flashing should have an anticipated life at least matching the expected life of the building. As often is the case, flashing material should never be selected based upon the lowest cost.
While nearly all wall construction should have flashings, they are often missing, or more often, are improperly installed. Wall flashings are often the most important component in a wall system.
For a wall system to be successful it must incorporate four basic elements: good design, proper detailing, quality materials and good workmanship. All cavity walls and nearly all other wall systems permit some moisture penetration that must be controlled with flashings in the overall design. It should be expected that wind-driven rain would penetrate the exterior face of the wall, windows or other penetrations. The wall system must be designed to control this water and direct it to the exterior with flashing and proper weeps.
Proper design should include flashing at the wall bases; windowsills and heads of openings; spandrels; gravity support angles; projections and recesses; tops of walls and at roof transitions. Continuous flashing with properly designed weeps should be installed at the base of all drainage wall systems and as secondary protection in barrier wall systems. Flashings must be installed above grade level to prevent blockage of water drainage and the entrance of accumulated ground storm water into the wall system. Flashing installed at penetrations, support angles and other areas above the base of the wall that are not continuous must be installed with end dams and weeps to direct the water to the exterior. Projections, recesses and the tops of walls collect substantial water, ice and snow. Proper design of these areas require slope away from the wall and flashing.
Continuous flashing, installed in pieces, must have splices lapped and sealed with compatible sealant or adhesive. All flashing should be fully adhered to prevent water migration under the flashing. Flashings that are not continuous should be extended beyond the opening and both ends should be turned up several inches to form end dams. Flashing should extend completely through the wall and form a drip beyond the face of the wall. Flashing should extend completely around corners and any penetrations or supports going through the flashing must be sealed. The top edge of the flashing material should rise above the shelf angle, slab or other building element six to eight inches and must be permanently adhered or mechanically terminated to the substrate. Where dampproofing or building wrap is utilized to protect the backup wall, it should be installed to counterflash the flashing material a minimum of four inches. All flashing must be installed with weeps to allow water to exit the wall system.
In addition to improper installation of flashing material, one of the most commonly found construction deficiencies is the accumulation of mortar tailings within the wall cavity. The accumulated mortar bridges a cavity wall system and circumvents the flashing and weep system by blocking the drainage of moisture from the wall cavity. Several mortar netting materials and pea gravel are available for installation within the cavity at the flashing line in the wall to prevent blockage caused by mortar tailings. However, these materials do not take the place of good workmanship and a conscious effort to keep the cavity clean of tailings.
The bottom line is that flashing is critical to the water resistance and sustainability of nearly all wall systems. All designs, conditions and flashing considerations cannot be covered here. That said, the design considerations and information presented, if followed with good judgment, common sense and considerations to the specific building design requirements, should result in wall systems that are more resistant to water penetration. And that’s a good start.
About the Authors:
Mike C. Tolson, Sr. has been working in the A/E/C industry for more than 25 years and is a Senior Design Consultant for Exterior Consulting Innovations, Inc. His expertise is frequently called upon by design firms, general contractors and clients with in-house facilities management and maintenance staffs.
Rick Staton also has some 25 years experience in the industry. Mr. Staton serves as Principal for Exterior Consulting Innovations, Inc. His varied experience has enabled him to establish a solid reputation for quality work and cost-efficient project management.