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Why Do Flashings Fail?
Have you spotted diagonal wrinkles or tearing in your roof base flashings? This vital waterproofing component is responsible for roughly 95% of all roof leaks, but little attention is paid to the reasons why.
The Limitations of Roofing Resources
Reams have been written about flashing materials, especially with the new MB and single ply systems available now. However, you may not have the original construction details, which can throw a wrench into the problem-solving process. These details were probably provided by licensed architects and engineers originally, but they may not be available when it’s time to re-roof or re-cover. Those two tasks account for about two-thirds of low-slope roofing activity, so without the original details, you’ll need to deduce the problem and solution with other resources.
For example, material manufacturers and roofing trade associations such as the National Roofing Contractors Association (NRCA) will furnish generic details for roof base flashings, but retrofitting vertical walls, parapets, and copings is usually beyond their scope of work.
Building owners often rely upon the roofing contractor to furnish drawings of the roof-to-wall configurations that will be discovered under the scope of work, but contractors generally do not have the license to prove to a code official that their drawings will meet the current codes.
What Causes Flashings to Wrinkle
Climate is a major player in flashing issues. Significantly more attention is focused on energy considerations now than it was just a few years ago. This means that thicker thermal insulations may be specified, as well as air barriers to reduce air leakage and consequential condensation problems.
Those same seasonal climatic fluctuations are also responsible for temperature-related differential movement in the parapet wall above the roof, which is alternately subjected to freezing and baking temperatures on both sides that cause dimensional changes. The wall above the roof, too, not being a load-bearing wall, is usually not as substantial as the walls below the roof.
PageBreakWhere the roof deck is supported by the parapet walls, differential movement is minimized. However, diagonal wrinkles occur in the base flashing when the wall moves differently from the roof membrane. NRCA now includes illustrations in which a curb is fastened to the deck and base flashings are applied to that curb. This eliminates the differential problem. However, a counterflashing, usually metal, must be fixed to the wall to cover the flashed curb.
Justin Henshell, a fellow of the American Institute of Architects (AIA) and partner at Shrewsbury, NJ-based Henshell & Buccellato Consulting Architects, shines some additional light below on treatments for vertical surfaces. These principles can be extremely useful when the original construction details no longer apply.
In addition, while roofing contractors are knowledgeable on roof decks, thermal insulation, and roof membranes, they may not have access to job-specific details on masonry, cast-in-place, tilt-wall construction, insulated wall panel constructions, and other components and systems.
Every roofing consultant should be aware of the issues Henshell addresses here:
Wall and Coping Treatments [PDF]
How to deal with parapet headaches.
Don’t Seal the Parapet [PDF]
Causes and treatments for masonry leaks.
Block That Parapet [PDF]
Built-up base flashing cannot be made strong enough or flexible enough to remain watertight when it must bridge between surfaces moving at different rates.