Technical Details: Roofing Details That Just Don’t Work

I typically inspect millions of square feet of roofing each year. This includes analysis of existing systems and inspection of roof construction. Over the years I have observed workmanship defects in the field and have identified material concerns in our laboratory. Most of these types of problems are concentrated in the membrane application. I have also witnessed problems that were not attributed to poor workmanship or inadequate material, but rather due to improper design. These types of problems directly relate to flashings, penetrations and accessories. I see the same problems pop up again and again. Following are some of the detail concerns that I have encountered and some design recommendations to avert these problems in the future.

Flashing Problems

More than 75 percent of all reported roof leaks occur at flashing locations. Flashing problems occur when the material is not properly secured to the substrate. Loss of attachment and buckling provide openings for moisture intrusion. This can be corrected with proper design. The roof designer must be certain that the flashing detail for the walls, parapets and other vertical surfaces provides for differential movement of the roofing system in all areas.

In areas of structurally independent building elements, such as walls that do not support the structural deck, base flashing should not be directly connected to the walls. A wood nailer should be horizontally secured to the structural deck with appropriate fasteners every 24 inches on center. A vertical wood nailer should then be installed behind the cant strip to form a blocking baker for the base flashing. The vertical backer must also be installed to the structural deck and not the wall.

On buildings where the structural decking is supported by the wall, the need for vertical wood nailer is eliminated. The horizontal wood nailer should still be secured over the structural deck; however, the base flashing can be applied directly to the wall.

 Defective flashing conditions in the location of mechanical units frequently result from improper height of unit supports, making it difficult to construct the flashings properly.