Stone Chimney Flashing - Replacing a poorly installed fieldstone chimney flashing with new sheet lead.

Being asked to install flashing on a fieldstone chimney is the nightmare of every average roofer. Most roofers are used to simple flat sided brick and block chimneys and the uneven round stones of a fieldstone chimney will send chills up their spine.

Your typical roofer is equipped with standard roll of aluminum sheet metal and a standard eight to ten foot portable sheet metal brake. For fasteners he normally uses hardened concrete nails, expanding nail drive pins, or worse a powder actuated(pistol style) fastener driver. And he seals the flashing to the chimney with a cut rate bead of caulk.

This first picture shows a leaking fieldstone chimney with a thin piece of aluminum sheet metal used as the counter flashing. The aluminum is held in place by hardened concrete nails. And the flashing is sealed with a cheap asphalt based caulk that has cracked loose from the chimney stone and is leaking.
The roofing is wood cedar shakes. They are about 35 years old and just about ready to be replaced. The flashing on the chimneys has already been replaced because of poor installation. It now needs to be replaced again because of poor installation.

 

Opening up the flashing reveals that the original roofing contractor didn't do much better. They actually did worse by using steel sheet metal that was easily corroded by the chemicals in the chimney mortar. Their technique still included a straight piece of flashing held in place by concrete nails and sealed with caulking.

 

To flash this stone chimney properly, first we remove all the old counter flashing, the fasteners, and the tar and caulking.

The next step is to cut a groove in the mortar joint above the concrete masonry block and below the fieldstone. The cut is about an inch deep and follows the contour of the stone. This is a tedious, dirty, and time consuming task.
We will leave in the existing sub flashing, or shingle tins. They will be replaced in the near future when the wood shakes are replaced.

 

Next, sheet lead is cut into  short sections. Short sections make it easier to work the flashing around the stone. Short sections of flashing also prevent the metal from buckling due to expansion and contraction of the metal during heating and cooling cycles.
The top is bent at about a 90 degree angle about an inch into the masonry counter flashing groove. 4lb. sheet lead, approximately 1/16 inch thick, is used for the counter flashing.
The lead sheet counter flashing is held in place by oversize lead wedges pounded into the groove pinching the flashing in place.
Masonry mortar, mixed to match the existing mortar, is then pushed into the groove with a trowel. Then tooled, or worked, to match the style of the existing mortar.
The lead is soft enough to mold around the curves of the fieldstone without buckling or needing to be cut.