By A Web Design

Chimney Flashing
Written by Dennis Crookshanks   
Saturday, 24 September 2011 01:18

Chimney flashing examples, how to, and  step by step instructions for different chimney styles and  installations to prevent roof and chimney leaks.

Chimney flashing is a construction detail used to seal and protect the joints between a buildings roof and chimney from water penetration. The joints created by the intersection of the roof and chimney are among the most vulnerable areas of roofing systems. They constantly expand and contract in response to changes in humidity,temperature and age. The greater the number of such projections, the greater the potential for serious leaks. Flashing is used at these intersections to keep rainwater from leaking into the building. It makes joints at these junctions watertight, while at the same time allowing the natural expansion and contraction of materials to continue.


 Here is a short video of our recent chimney flashing installations.

Slate roof copper chimney flashing video.

New chimney flashing installation.

     Installing new copper apron, shingle step flashing and counter flashing on a stone chimney with slate roofing shingles.


The existing stone chimney with old steel flashing.

Old rusty chimney flashing to be replaced.

     This rusty old chimney flashing is about 60 years old. The stone chimney itself is in pretty good shape. There are some cracks and voids in some of the mortar joints but on the whole the structure is sound. The chimney flashing on the other hand is in pretty bad shape. Most of the joints are loose and open to the rain. 

The old metal, possibly galvanized or painted steel when first installed, is solid. But the mortar joints are loose. The original caulking is still in place but dried out, deteriorating and falling out of the joints. There were previous attempts at sealing the joints with new tar caulking but that quickly wore out and failed as well.


Close up photo of the front chimney apron flashing.



The above photo is a perfect example of an improper chimney apron flashing installation. The original roofers installed this apron flashing as a one piece unit, apron and counter flashing, instead of the proper two pieces of flashing. 

The wooden structure of the house has settled over time but the chimney, set on a firm foundation, keeps it's original position. 

The apron flashing, attached both to the roof and the chimney, has been pulled in both directions causing a gap between the slate shingles and the lower edge of the flashing and loosening the seal at the stone chimney joint. The first shingle step tin flashing has also lifted where it was bent over the apron flashing and has raised the slate shingles on the side of the chimney.


Close up photo of the apron/chimney flashing joint.

Old chimney apron flashing joint

The old apron flashing pulls easily away from the chimney. The old metal flashing was bent and inserted into the chimney masonry joint, held in place with 8penny galvanized nails, and then caulked. Over time the caulking failed, water entered the joint causing the mortar and the steel nails to deteriorate. Many chimney leaks will first show up at poorly installed flashing / caulk joints.


The slate shingles installed up to the chimney.

Slate shingles installed up to bottom of chimney.

The old slate roofing has been removed, two layers of underlayment installed and the new roofing slate installed up to the base and around the side of the chimney.


The new chimney apron flashing installed.

Old chimney apron flashing joint.

The new apron flashing, along with the first step tin flashing has been installed. For this installation, the apron flashing up leg, was cut and folded out , as opposed to folding around the corner of the chimney as is common practice in new flashing  installs. See photo below.

Then the step tin flashing is set next to the apron, marked and cut so as to be folded around and locked to the apron flashing. Done this way, there is a small corner, rather than a large flap of copper tin, that needs soldered for a leak free system.

Chimney apron and step tin flashing corner to be soldered.


Typical chimney apron flashing.

Shingle tin flashing run past the apron flashing and folded over to cover corner hole.

Typical chimney apron flashing.


Continue up the side of the chimney with shingle step flashings.

Second step tin flashing installed.

On a slate roof, the step tin flashings are cut long enough to cover the tin below by 3 inches, without showing from under the slate. And they must be long enough to enable nailing at the top corner above the slate. Typical flashing tins on asphalt roofing should be nailed near the bottom left corner to secure the asphalt shingle and keep the flashing tin from lifting.  On a slate roof the weight and rigidity of the slate shingle will hold the flashing tin in place.

Continue installing the step flashing, alternating with the roof shingles, up the side of the chimney. When installing the flashing tins, depending on the roughness of the construction, it is sometimes helpful to bend the flashing not quite to 90 degrees. This will help to keep the flashing tins tight to the chimney. It may be neccesary to fasten the tins with two nails to keep them from pushing away from the chimney.

Second slate installed next to chimney.

Second slate installed.

Copper shingle flashing tins and slate installed up the side of the chimney.

Shingle flashing tins and slate installed next to chimney.

Installing the Counter Flashing

After the sub flashing has been installed it's time to remove the old counter flashing and install the new. Besides the tar caulking, there is the original caulking to remove and the existing mortar. The mortar should be removed to a depth of at least one inch making sure all the soft and loose mortar is removed. A good rule of thumb is to remove 2.5 times the width of the mortar joint.

On old stone or chimneys with soft brick and lime based mortar it is best to use a hammer and chisel to remove the mortar.  Newer brick chimneys with hard portland cement based mortars may have to be cut with an electric grinder and diamond blade. Care must be taken to cut only the mortar and not damage the brick.

 Old chimney flashing removed and joints cleaned.

Temporary fit of Counter Flashing

Cut the counter flashing to size and temporarily fit it to the chimney.  On this particular chimney, the roughness and different sizesof the stone will make this part a bit harder than on a standard brick chimney. Each piece needs to be measured to conform to the stone joint and angle of the roof. With a brick chimney, multiple counter flashing pieces can be cut  to match the rise and run of the brick courses, and then installed with minor adjustments to the exposure of the piece making up for any small difference in the brick joint spacing.

 When measuring, add enough excess to the top of the flashing so as to bend the top of the counter flashing into the chimney mortar joint as deep as the joint is cut and another quarter inch up the back of the joint. This will help to hold the flashing in place long term and prevent moisture that may wick in from between the top of the flashing and the mortar.

Temporary fit of counter flashing


Sealing the back end of the counter flashing.

As each piece of counter flashing is installed, the back end that will be covered by the next piece of flashing is sealed with mortar. This will help prevent water that may be blown into the vertical face of the flashing from leaking behind the lower flashing. This is especially important on stone chimneys as they tend to have more gaps between the flashing edge and the irregular stone surface.


Sealing the back end of the counter flashing.


The back side (ridge saddle flashing) of the chimney.

At the top, or back of the chimney, a copper saddle flashing is installed. A saddle flashing is usually one piece of flashing material that runs straight along the back side width of the chimney, as opposed to a chimney cricket flashing that is raised in the center to divert the rain water to either side of the chimney. First we've fastened copper sheet metal strips to the roof over the slate shingles with copper nails. At the right you will see the top shingle step flashing bent over the roof ridge. 

The back or chimney ridge saddle flashing.

Ridge saddle flashing installed.

The ridge saddle flashing is set and secured by bending the copper strips, after trimming to size, over the lower edge of the flashing.  The copper strips will then be soldered to the saddle flashing.  Notice that the saddle flashing is inter woven, the right side over and the vertical leg behind,  the chimney side step flashing.  Also notice how the flashing, slate course and stone edge are not quite parallel. Typical building construction and the effects of age.

Chimney saddle flashing installed

The chimney saddle/apron and top step flashing tin.

Closeup view of the connection between the chimney saddle and top step flashing installed to allow for soldering of the corner.

Chimney saddle and top step flashing.

Counter flashing installed.

The rear chimney counter flashing is installed over the apron/saddle flashing. The step flashing/saddle corner has been soldered together for a watertight permanent seal. The copper tabs securing the saddle to the roof are also soldered. All that is left is to point and seal the masonry joint with mortar.

Rear chimney counter flashing installation.

The finished chimney flashing.

Chimney flashing installation done and ridge vent on.

to be continued...

Copyright © 2015 Chimney Flashing. All Rights Reserved.
Joomla! is Free Software released under the GNU/GPL License.