What can cause stack flashing to become misaligned?

expert EXPERT April 26, 2007

A vertical misalignment of the flashing is usually a result of movement between the plumbing stack and roof deck. This movement can occur either up or down. If the roof deck drops relative to the plumbing stack (when loaded the with snow, for example), the flange will tend to be pulled up off the roof. If the pipe or stack drops relative to the roof deck, the appropriate slope of the flange may be lost and a recessed low area may be created around the pipe. This can result inleakage in your roof.

When looking at pipes, stacks or metal chimneys, make sure that

• There is a flashing flange in place.
• You check the quality of connection around the pipe or stack.
• No more than the lower half of the flange is exposed.
• You check that the bottom edge of the flange directs water out onto the surface of the roofing material below.
• Exposed nails at the lower corners of the flange are sealed.
• You look for evidence of lifting of the flange relative to the roof deck, or shrinkage of the pipe and buckling of the flange, relative to the roof deck.