Every roof can be reasonably expected to protect building occupants and contents from moisture penetration, as well as from all applicable atmospheric conditions, for its warranted service life. The focus of this article is flashing-related failure as it applies to low-slope commercial roof systems.
Since flashings are the most common point of low-slope roofing failure, it makes sense to approach their specification and installation with the same care and technical proficiency that you would apply in choosing the roof system itself.
The four steps required for successful flashing performance are:
- Analysis-driven design
- Quality materials
- Meticulous craftsmanship
- Regular inspections and appropriate maintenance
The first step toward a high-quality flashing system is evaluating what is currently on the roof. If a mature roof can be successfully restored, it is typically advisable to replace existing flashings prior to restoration. Vertical flashings exposed to a Southern direction are particularly vulnerable to deterioration from ultraviolet rays, and should be given special attention. When restoration is not an option and the roof needs to be entirely removed and replaced, new flashings should be installed according to the manufacturer’s evaluating what is right and wrong with existing flashings is a important aspect in designing new systems. A few areas of flashing design that should be taken into account are:
- Flashing heights
- Number of plies
- UV Protection
Poor quality materials, inadequate attachment, improper placement, and improper measurement of metal edge details are the primary cause of metal edge failures.
Proper design of high-performance metal edge details is a critical and complex process. Following the procedures established in FM Global Property Loss Prevention Data Sheet 1-49 is strongly recommended. If the roof of a building is to blow off, it starts at the edge and the roof is right behind. The perimeter zone the corners are considered zone 3.
The metal edge detail in the above photo was a victim of different coefficients of expansion between the stripping ply and the metal itself. The metal movement will cause failure in membranes even with 400% elongation. The high-performance metal edge detail in the bottom photo is mineral-surfaced and was installed using FM 1-49 guidelines, providing the rigidity necessary to withstand wind uplift pressures.
Improper Flashing Height Termination
Another area frequently overlooked is flashing height. Curb details and flashings lower than eight inches are particularly vulnerable to water penetration. Low flashings allow water to get up and over the termination and enter the roof system or worse, the building itself. The NRCA has established a minimum height of eight inches for base flashings, but areas with heavy snowfall often have requirements for 12-inch flashing heights. The maximum height for flashings is 24 inches. If your walls are higher than that, it is wise to consider a high-performance metal wall panel. In any case, always strap your flashings so the width of the material is the width of the flashing ply. Also, stagger all plies between the base and surface flashing membrane.
This properly installed flashing detail, which is just at the 24-inch maximum height uses multiple plies that have been strapped up and over the wall and will eventually be covered with coping.
Since this wall is higher than the 24-inch maximum, the flashing termination has been shielded from wind and weather through the installation of a metal wall panel and sill system, ensuring lasting high performance.
Flashing failure can also be caused by poor substrates or improper attachment methods. Prior to construction, you need to verify the structural integrity and conditions of the wall. For example, rotted wood, corrosive metal, brick spalling, or missing mortar must be remedied prior to flashing installation. Typically, those procedures are included in the bid document, but there is always the possibility of unforeseen conditions. For attachment purposes, all adhesives must be compatible with the flashing and the substrate, and be designed for use in vertical applications.
As with metal edge procedures, all guidelines for proper attachment can be found in FM Global Property Loss Prevention Data Sheet 1-49 for Perimeter Flashing. Use the information found on this data sheet to make certain that the material the fasteners are made of is compatible with the material they are being fastened to, and that the spacing/pattern of fasteners is appropriate for anticipated wind uplift pressures. Engineering service providers can calculate anticipated wind uplift based on geography, weather, height, and other site-specific factors.
Improper Termination Techniques
Surface adhesion alone cannot prevent water penetration. In fact, heavier membranes have a constant force of gravity working against their adhesion. Prior to the installation of a counterflashing, the membrane must be attached to the substrate with an extruded termination bar. That bar should compress the flashing to the substrate using a two-sided butyl tape as the gasket. The flashing termination fasteners should be appropriate for the substrate whether it be wood, concrete or metal. Once the termination bar is in place underneath a through wall counterflashing reglet or a wall panel sill, the counterflashing component should be snapped into place.
Redundancy in Design
The termination technique described in the paragraphs above is only one example of the redundant design that is essential to successful roofing. Just as rooftop materials are designed with a safety factor that ensures continued protection as organic materials inevitably degrade over time, details so critical to rooftop performance must be designed with an eye towards redundancy. Engineering individual details is of critical importance because they all have a specific job to do. Failure at these locations results in water damage in the facility and damage to the surrounding waterproofing and insulation systems.
Materials, Craft, and Oversight
Based on the understanding of the basics that need to be considered prior to creating a proper flashing, it is wise to consider that the best designs only perform if they are implemented properly. Proper implementation depends on quality materials, quality workmanship, and oversight by a party with a vested interest in the successful installation.
Understanding the mechanical, thermal, and waterproofing-related properties of flashing materials is essential to success. Materials perform differently; some are better suited for certain situations. Manufacturer data sheets should provide you with physical properties, such as tensile and tear properties but common sense should also play a role. If you have a flashing with southern exposure, it should have exceptional UV resistance. If there is a lot of mechanical work on a roof, multi-ply flashings are a good choice. Additional coatings on finished membranes play a large role in the longevity of any roof flashings.
It is very important to survey roofs that are to be replaced so you are aware of the existing conditions. Once you have that information or the conditions encountered in a new roof design, you must put your mind to designing the proper detail. If you leave it to the roofer in the field, the contractors become designers by default. Since this is a critical component of a roof and the building overall, and is ultimately the design professional’s responsibility, you must maintain control from start to finish. This article is intended to help you appreciate and implement appropriate flashing designs, keeping in mind that when a flashing design is less-than-adequate due to unanticipated circumstances on the roof, an inexperienced contractor or poor materials, the liability reverts back to the designer.
Finally, the critical points necessary for successful flashings reviewed in this article have underscored the vital importance of inspection and on-site monitoring. Because it is difficult to inspect the overall quality of a flashing detail after it has been installed, having a roofing professional monitor the install to ensure the materials are being installed according to the manufacturer’s specification and NRCA guidelines is critical to long-term performance. Supervision of the work in progress will help make certain that any potential problems are found and remedied quickly, avoiding costly water penetration issues down the road.
Further, since flashings often age more quickly than the roof because of exposure, gravity and abuse, ongoing proactive flashing maintenance as recommended by the manufacturer is essential to long-term performance.
Flashing failure is a primary cause of water penetration into commercial buildings. Analysis-driven design is a critical factor in assuring that desired performance outcomes are met. When good design is combined with application-appropriate material specification, quality workmanship, and proper oversight — you can be certain that the waterproofing objectives of every flashing detail will be met.
Rick Catley is a six-year veteran of the construction industry with a keen interest in concrete applications. In his current role as product manager for sealants and accessories for The Garland Company, Inc. – a Cleveland, Ohio-based manufacturer and distributor of high-performance solutions for the total building envelope – Catley provides technical support to field representatives across the U.S., Canada, and the U.K. He is also an active participant in a number of construction and sealant industry organizations.