June 10, 2008
Flashing Design and Roof Drainage SystemsStanding water is more detrimental to flashings than it is to the membrane because of the increased possibility of leakage at the seams. There is also greater possibility of relative movement. Flashings must be designed to divert the flow of water away from them. To accomplish this, a tapered cant strip must always be installed at all flashing areas between the roof and the vertical surface. In addition to allowing for the positive drainage of water away from the flashings, tapered cant strips also provide structural strength and serve as a gradual angular transition from the horizontal surface to the vertical surface. Flashings should be positioned at a maximum 45-degree bend to reduce the risk of cracking that could occur if they were positioned at a right angle.
To further ensure water-tightness at the flashing areas and reduce the risk of leakage at the joints, flashings should be located at the high roof areas. When preparing details on a remedial roofing project, the layout of the existing roof plan may present a challenge. In instances where roof sumps are located at the building’s perimeter edge in areas where no vertical surfaces exist or directly next to equipment penetrations, the most feasible way is to elevate the flashings above the water level resulting from buildup around the sumps. This can be accomplished by installing a cricket around the roof sump and elevating the perimeter edge or equipment base with wood nailers. It is important that the seams are adequately coated with roof mastic and membrane fabric in these locations.
The base flashings at the vertical surfaces should be a nominal height of no less than 8 inches and no higher than 12 inches above the finished roof surface. The flashing should extend above the surface 8 inches to protect the flashing from the effects of weathering, such as rain and snow. A vertical dimension of 8 inches is also necessary to provide sufficient working room for the application procedure. The maximum height limit of 12 inches provides a safeguard against the flashing sagging. When the vertical height exceeds 12 inches, the asphalt has a tendency to flow downwards when a high range of roof temperatures are experienced. On vertical surfaces that extend beyond 12 inches in height, such as walls, proper measures must be taken to adequately waterproof the remaining vertical surface area.