Flashing: It’s Vital Role in Preventing Water Infiltration Part II – Roof Flashing
By: Sean McPartland
In a December 2017 newsletter article, we discussed the importance of flashing and its vital role in preventing water infiltration. History has proven that most roof leaks occur due to a failure in the flashing system. There are several different types of roof systems available in today’s market. They are generally categorized as being either low-slope or steep-slope systems. There are five generic classifications of low-slope roof systems: built-up roof membranes (BUR), polymer modified bitumen membranes, single-ply membranes, metal panel, and spray polyurethane foam-based membranes. There are six generic classifications of steep slope roof systems that include: asphalt shingles, clay or concrete tile, slate, wood shake or shingles, metal and synthetic roof systems. One of the biggest challenges that designers and manufacturers face is selecting and developing a system that can effectively protect the roof assembly from water infiltration at vulnerable flashing locations.
Roof flashing, more commonly referred to as base flashing, is designed to protect the roof assembly at points where the roof deck and vertical surfaces meet. Its primary functions are to prevent water infiltration at terminations in the roof membrane and to divert water away from these vulnerable locations that typically join two different materials. Base flashing is a continuation of the roof membrane that is turned up onto a vertical surface. These vertical surfaces include parapet walls, chimneys, bulkheads, vent pipes, drains, towers or any other roof mounted structure or projection.
Seasonal climate changes result in thermal expansion & contraction movement in masonry structures above the roof assembly. In a built-up roof system, a nonmetallic flashing allows for the roof membrane and base flashing to have the same thermal movement and work together as a unit. However, the parapet wall and the roof assembly experience thermal movements at different rates. Therefore,the stresses in the base flashing are typically greater than the stresses in the roof membrane. Due to the increased stress, a design professional should always specify base flashing that has a greater life expectancy than the roof membrane. Very often, these differential movements result in failures of the flashing membrane.
Proper design of the base flashing for any roof assembly is an essential component in preventing water infiltration. Firstly, the flashing and roof membrane should always be from the same manufacturer. Using a different manufacturer’s materials may result in warranty voidance or long-term problems for the roof system. Several design considerations can help optimize performance of the flashing in a built-up roof system. As its name implies, a built-up roof system is multi-layered and provides redundancy in the system. In the event that a layer is compromised, additional layers of protection prevent water from entering the roof assembly and the interior space below. Flashing should be designed and installed so that a minimum of 8” extends above the roof assembly to keep the top edge of the flashing out of the water line. A cant strip allows for a smooth transition between the vertical wall surface and the horizontal roof deck, and pushes water away from the edges of the roof assembly. A termination bar secures the base flashing to the masonry wall to eliminate displacement of the base flashing.
Proper design and installation of flashing components that work in conjunction with the roof membrane are vital in preventing water infiltration. Many variables should be considered when replacing a roof assembly, re-roofing, or making routine roof maintenance repairs. Design professionals have the experience and knowledge to assist property managers and owners in determining the system best suited for their application.