Tips for Counterflashing a Stone Chimney

Chimney’s come in a variety of styles, including wood, brick and stone.  Stone is probably the most common because it offers a natural look and appealing atmosphere to your home.  However, stone chimneys can also prove to be the most problematic because they are one of the most difficult metal flashing to install and often times they leak, causing substantial water damage to the roof and other areas of the home.

The most common leak source on stone chimneys is at the counterflashing because stone has an uneven surface with deep crevices that are very difficult to seal from water that runs down the chimney sides.

This article provides an overview of the design principle of counter flashing, the proper installation method, and some helpful tips for installing counterflashing on a stone chimney so that it is sealed properly and remains water tight.

 

Chimney Flashing

No matter what material the chimney is made from, flashing is needed where the chimney meets the roof.  A typical roof-to-chimney flashing will consist of two parts:

 

  1. The (lower) Step Flashing: Comprised of step tins, rubber, or some other type of membrane; and
  2. The (upper) Counterflashing:  A piece of metal that is sealed to the chimney surface, and covering the top of the step flashing to form a water tight barrier by directing water over and away from the step flashing.

 

Note:
Counterflashing Illustration_ChimneyMany roofers will surface mount chimney counter flashing by attaching a flat piece of metal to the chimney surface and apply a bead of caulk to the top edge of the flashing. This is not the correct way to install counterflashing because when the caulk fails water will simply run straight in behind the flashing and will leak.  Moreover, although the surface mount method may temporarily work on smooth brick, it is impossible to surface mount counterflashing on a stone chimney because stone is not smooth.  This is why proper counterflashing installation is critical. (See Illustration-right)

 

Proper Counterflashing Installation

The proper way to install all counterflashing is the “cut-in” method, or to cut a groove into the masonry surface. This is especially true for chimney counterflashing, and even more so for chimneys that are made from stone.

 

The “Cut-In” Counterflashing Method

The method of “cutting-in” means that a narrow groove is cut directly into the chimney masonry.  The groove is approximately 1/8” wide by 1/2″ deep, and is typically cut in with a small diamond blade angle grinder.  The counterflashing is made with a 1/2″ long ninety degree flange at the top which fits into the groove, thereby preventing water from running straight down behind the counterflashing.

 

Implementing “Cut-In” Counterflashing on Stone Chimneys

On brick or otherwise flat masonry surfaces, implementing the “cut-in” method is fairly easy because the surface is relatively flat and smooth.

 

On stone chimneys, however, it is difficult to cut the groove at an even, consistent depth due to the fact that the stone surface varies in depth and the grinder can only cut so deep. For example, where a stone protrudes out the cutting tool may be able to make a 1” deep cut, but in crevices the cut may only reach ¼”.  Typically, on stone chimneys you are left with a varied depth cut that makes it impossible to insert the straight, even edge of metal counterflashing.  Moreover, with an uneven groove there will be voids or holes between the flange and the groove.

The following illustrations demonstrate how to properly fit the counterflashing into the uneven, varied depth cut by marking and trimming it to custom fit the groove.  Although the illustrations depicted here are from a rubber roof, the same principle’s of the “cut-in” counter flashing method apply to situation where a step flashing is being used, such as on a shingle roof.

 

First: Preparation

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Unlike typical counterflashing with a 1/2″ flange, counterflashing for a stone chimney should be made with a 1-½”flange to account for necessary trimming.  Also, the groove cut in the masonry should made to the maximum depth allowed by the angle grinder (usually between 1” and 1-½”).  This way the cut reaches into the deepest crevices of the stone.

 

Second:

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After the groove is cut the flashing is inserted tightly into the groove and positioned so a line can be traced along the stone.  Trace a line along the top of the flange by following the contours of the stone with a permanent marker.

 

 

 

Third:

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Remove the counterflashing and systematically cut along the line, carefully following the contours traced on the flange.  In most cases it will take several attempts of inserting, tracing, and trimming before you get the counterflashing to fit properly.

 

 

Fourth:

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Once you get the flange to fit, align and bend the corners to properly lap so the corners are water tight.

 

 

 

 

Fifth:

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After all trimming and bending the flashing is installed and a bead of caulk is laid along the top of the flashing; with caulk being forced along the top of the flange and into the groove to create a water tight seal.  The recessed flange is an added water-tight feature to prevent any leaks should the caulk crack or otherwise fail.