Why Do Flashings Fail?

Where 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
FAIA, CSI, FASTM

Partner, Henshell & Buccellato Consulting Architects

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.


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