How to Install Chimney Flashing

In typical home construction, the house and chimney are set on different foundations. Over time, these foundations may move in different directions, causing a gap to form between the chimney and the roof. It is important to create a barrier between the chimney and roof that is waterproof and flexible to avoid leaking.

Chimney flashing is a thin metal barrier made from aluminum, stainless steel, copper or even lead. The thin, flexible sheets are wrapped around the chimney where it meets the roof shingles. As rain falls, it is directed over the flashing and away from the chimney/roof gap.

Chimney Flashing Parts
Chimney flashing has three main components:

  • Base flashing. Installed at the front (lowest) part of the chimney, base flashing is a single sheet of chimney flashing. Base flashing extends over the front of the chimney and wraps slightly around the sides.
  • Step flashing. Installed along the sloped sides of the chimney, step flashing forms the first layer of protection. Step flashing typically extends under the shingles along the chimney.
  • Cap flashing. Installed in the mortar joints above the step flashing, cap flashing extends down over the step flashing to provide protection from falling rain.

The back (upper) side of the chimney flashing provides a special case:

  • For chimneys that are wider than 24 inches or are constructed next to a steep roof, a saddle (sometimes called a "cricket") is installed. A saddle is like a tiny dormer that is installed behind the chimney to direct rain and snow around the back of the chimney.
  • For narrower chimneys or chimneys constructed near a shallow roof, a continuous piece of chimney flashing called "backer flashing" is installed.

Before You Start, A Word About Safety
Working on roofs can be dangerous. Make sure that you take proper safety precautions and consider the use of roof brackets to create a stable work platform. A personal fall arrest system (made up of a safety harness, rope and roof anchor) is a strong deterrent to dangerous falls.

Be sure to check the weather before you begin. Only begin a roofing project on days that are predicted to be dry and calm. Also, be aware of activity on the ground. Dropped material or tools can seriously injure anyone standing below.

On Sheet Metal
Chimney flashing is made from thin, flexible sheets of metal. If you've never worked with sheet metal before, consider having your chimney flashing parts professionally made. Once you've carefully measured for each of the components you'll need, look for a local sheet metal fabricator who can fashion the parts for you.
If you need to cut parts to fit, be careful around newly trimmed parts-the cut edges will be extremely sharp.

Installing Chimney Flashing

  • Apply masonry primer to the chimney where the base flashing will go.
  • Apply plastic roofing cement to the underside of the base flashing and attach flashing around the low side of the chimney, over shingles.
  • Secure the base flashing to the chimney with mortar nails.
  • Begin installing step flashing at the low end of each side of the chimney by applying plastic roofing cement to the chimney side of the step flashing.
  • Press the step flashing against the chimney and secure the lower portion with a roofing nail.
  • Apply roofing cement to the lower portion of the step flashing and cover it with a shingle.
  • Firmly seat the shingle over the step flashing and nail it in place.
  • Continue applying sections of step flashing, overlapping the previous section, until you reach the upslope end of each side.
  • If you need a saddle, construct one from two triangles of plywood so that the centerline of the saddle extends out from the roof to the center of the chimney side.
  • Wrap step flashing around corner and cover the point where the saddle meets the chimney.
  • If you're not using a saddle, apply the backer flashing, using plastic roofing cement and mortar nails.
  • Cover the upslope portion of the backer flashing with shingles, but leave a 2 inch gap at the base of the chimney to allow for water runoff.
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