February, 2016
Inspection News and Views from the American Society of Home Inspectors

Pipe and Stack Flashings


Most homes have round pipes penetrating the roof, such as these examples:

  • plumbing stacks
  • electrical masts
  • exhaust vents from fans or combustion appliances

These roof penetrations may be plastic, cast iron, steel, aluminum or copper. The flashing materials may be steel, rubber (neoprene), lead, copper, aluminum or a combination of materials. In some areas, these flashings are called roof jacks. The flashing details are similar for any of these penetrations, although the approaches are different for sloped and flat roofs (or steep and low-slope roofs). In this article, we’ll focus on sloped roofs.

Sloped-Roof Installation
Look for these details:

  • The roof is shingled up to the height of the stack.
  • With asphalt shingles, a shingle is typically cut and slid over
  • the pipe.
  • The flashing flange then is placed over the stack and sealed or nailed in place.
    • The flange is a flat rectangular surface that sits on the roof.
    • The flange typically has a tapered collar that is approximately the same diameter as the pipe.
    • Some flanges have an integral sealant or gasket that creates a seal when the flange is slid over the pipe.
    • In other cases, a separate storm collar is provided.
    • Some flanges are friction-fit only.
    • Some are gasketed.
    • Some are sealed with caulking or other sealants, a draw band (band clamp) or a combination of these.
    • Some flanges have a sleeve soldered to the flange, with a return on the top of the sleeve that fits over the top of the pipe.
    • Others have a sleeve and separate cap that fits over the pipe.
  • Once the flange is in place and sealed, the shingling is continued so that at least half and, in some cases, almost all of the horizontal section of the flashing flange is covered with roofing materials.
  • The shingles (if asphalt) that are on top of the flange often are set in a continuous layer of asphalt cement.
  • Some roofing authorities recommend that nails should not be driven through the flange.

Adverse conditions to watch for on these flashings:

  • rust
  • damage
  • vertical misalignment
  • installation problems
In all of these conditions, the implication is the possibility of leakage.

Rusted flashings are caused by the following:

  • age
  • failure to maintain or paint
  • incompatible materials
  • tar over the flashings

This stack has rusted through and roofing cement has been used to make a temporary repair.

This electrical mast flashing is rusted and the rubber collar has failed.

Damaged flashings may be caused by the following:

  • snow and ice accumulation on the roof
  • animal activity
  • foot traffic
  • careless roof work nearby
  • replacement of a pipe or stack without replacing the flashing
  • deterioration of the flashing due to age

Stack flashing and shingles damaged by vermin.

This is a common failure of the rubber collar. Collars often fail first on the uphill side of the vent.

Both of these rubber collars on the ABS piping are torn.

Roofing cement was used as the flashing material for this roof—a very poor approach—and over time, it has been damaged by UV exposure.

Vertically Misaligned Flashing
Vertical misalignment of the flashing is usually the result of movement between the plumbing stack and roof deck. This movement can occur either up or down. If the roof deck moves relative to the plumbing stack (when loaded with snow, for example), the flange will 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. Again, leakage is the implication.

Things to look for when looking at pipes or stacks:

  • There is a flashing flange in place.
  • The connection around the pipe or stack is weather-tight.
  • No more than the lower half of the flange is exposed.
  • The bottom edge of the flange directs water out onto the surface of the roof below.
  • Exposed nails at the lower corners of the flange are sealed.
  • There is no 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.

Improper support of this heavy cast iron stack has allowed it to drop. The flashing is not weather-tight.

Buckling of the flashing can result in entry of vermin.

Common Installation Problems
  • Missing flashing
  • Improper flashing material (for example, asphalt cement on asphalt shingles or rubber flashing flanges on curved concrete tiles)
  • Top half of flange exposed above roofing material or bottom edge of flange concealed below roofing material
  • Flashing located in a valley
  • Exposed fasteners not sealed
  • Missing fasteners

A poorly installed flashing may leak, which will reduce its life expectancy.

There is no flashing here.

Watch for what isn’t there, but should be. Missing components are a very common flashing problem. Most of the installation problems we’ve listed can be readily seen if you remember to check for them.

In this article, we’ve introduced pipe and stack flashings and explained common problems associated with them. In the ASHI@HOME Training Program, we explain various other types of roof flashing materials including chimney flashings, valley flashings, and roof and sidewall flashings.

Thanks to Roger Hankey and Kevin O’Hornett for their many valuable contributions to this article. Special thanks to Roger for providing several great photos!