Flashings should always be completed in accordance with the manufacturer’s requirements and local building codes. (Photo courtesy of Johns Manville.)
Flashings are the most common source of roof problems. Leaks often occur at these areas due to improper termination design, improper workmanship or use of inappropriate materials. Oftentimes flashing design is omitted from project specifications and it falls upon the roofing contractor to provide guidance in these locations. Most of the manufacturers provide guidelines for flashings in typical situations. However, there are times when the manufacturer’s recommendations do not match “as-built” conditions. In these instances it is important that the contractor complete the flashing application in accordance with proper roofing procedures.
The importance of flashing is critical. Often overlooked in the design phase of roof construction, improper flashing is the single leading contributor to up to three-quarters of all roof failures. It is also the area where most roof leaks occur.
Contractors should be familiar with proper flashing design if they are required to perform this task on a project. Basic flashing design guidelines are as follows:
• Complete flashings in accordance with the manufacturer’s requirements and principles of NRCA, SMACNA and local building codes.
• Locate flashed joints at the roof’s high points. Avoid such locations such as points of ponding water or near drains.
• Flashing height should be a minimum of 8 inches above the completed roof surface. Maximum flashing height is 12 inches - any higher will require additional securement.
• Account for differential movement between base and cap flashings and at metal coverings.
• Anchor the flashings to the substrate.
• When possible, minimize the number of penetrations and group piping or roof mounted equipment together for fewer flashings.
• Locate roof penetrations a minimum of 2 feet away from walls or other vertical surfaces.
Application Methods
Hot-applied flashing applications require the use of proper materials and application methods. The use of steep asphalt is required in all vertical applications. This will prevent the flashing material from sagging or becoming displaced from the substrate. The proper application method used for hot mopping of flashings is by “back mopping” the flashing felt. In these cases the bitumen is applied directly to the back surface of the flashing material and not to the vertical substrate.
Make certain that the proper primers are applied over the substrate to promote full flashing adhesion. This is especially critical when applying base flashing materials over walls (concrete or masonry) or strip-ins over metal gravel stops.
It is essential that the flashing material is anchored to the substrate at the top of the sheet. This is particularly important in applications where modified bitumen is used as the top flashing material. If the material is not properly secured, slippage can occur due to the weight of the material in a vertical configuration. Securement should be completed using nails or fasteners appropriate to the substrate in a fastening pattern that meets the manufacturers requirements. Also, when using modified bitumen the corners can be reinforced with “butterfly caps” or a seal application of asphalt mastic and mesh.
In the past two decades there has been an influx of new materials to the roofing market. Most of these products rely on cold or self-adhered applications, as there has been a significant decrease in hot applications. The adhesives used in flashing applications typically fall into one of three categories: water based, solvent based or bonding adhesives used in single-ply applications. All of these materials have temperature constraints and must be applied within the ambient temperatures required by the manufacturer. It is the responsibility of the applicator to make certain that the materials are applied within the proper temperature range. The minimum temperature for plastic cement or flashing cement is 25 degrees Fahrenheit. Most solvent-based and water-based adhesives require temperatures above 40 degrees.
A best roofing practice is to apply all flashing and counterflashing components the same working day. This eliminates the potential for moisture intrusion. If the counterflashing cannot be applied as the work progresses than it is important to apply a waterproofing sealer at the top of the flashing. The proper sealer is a three-course stripping seal, which consists of one ply of flashing fabric between two trowel applications of flashing cement. The seal should be applied in an even application and it should cover the entire area. This protective measure prevents water from entering behind the flashing material.
Make sure to use materials that are compatible with the roof membrane. The applicator should only apply materials approved by the membrane manufacturer. Application should be in accordance with the manufacturer’s requirements and installation rates. Do not arbitrarily increase or decrease coverage rates of adhesives, as this may alter the performance of the material. The coverage rates provided by the manufacturer are the tolerance levels that they have determined - through testing - are required for sufficient performance of the product.
Proper flashings are also required at all roof penetrations and openings. Most roof openings are for mechanical equipment, such as HVAC units, and they require a curb for proper termination. The curb flashing is completed in similar fashion to vertical flashing with the mechanical equipment housing metal serving as a counterflashing.
Applicators should properly flash penetrations such as vents or tubular penetrations with targeted flashing that includes a roof flange and sleeve assembly and avoid simply “field wrapping” the penetration.
It has been estimated that nearly 90 percent of all roof leaks occur at flashings and penetrations. Many of these problem points can be eliminated through proper flashing design and application methods. From an applicator’s perspective, proper flashing installation can eliminate points of moisture intrusion and lead to a successful roof project and fewer callbacks.
