Masonry Magazine October 2006 Page. 22
MASONRY CHIMNEYS
On many chimneys, the top of the wall is capped with a cast iron or steel cap. Over the years, either the segmental cap sections or the connecting bolts that join them will tend to corrode, rendering it a hazard. In many instances, the use of dissimilar metals, such as mild steel in contact with stainless steel, causes an enhanced form of degradation known as Galvanic corrosion. This factor causes the material, which is more prone to corrosion, to be deteriorated at a much faster rate than it would normally experience, and is often observed in chimneys that have been repaired by personnel unaware of the concept.
Where chimneys have been cracked by various causes, it is common to see a series of external tension bands retrofitted to the exterior of the chimney to control the degree of vertical crack width present after their initial formation. Without such tension bands, each of the chimneys that have them would continue to grow radially until the cracks became so wide as to render the structure unstable. These bands are generally retrofitted to chimneys starting at the top, where the unrestrained edge of the masonry is weakest, but they are often in existence over the entire structure, when vertical cracks are present along the entire wall. These bands are attached to the chimney exterior and are post-tensioned, usually by the use of bolts that join the band segments together. It is important to note that the tighter the bands are, the more restraint against further cracking exists. Loose bands are virtually worthless against crack.
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20 Masonry
October 2006
Brick Brief:
FLASHING CHIMNEYS
# Introduction
Flashing is an important element in brick masonry's line of defense against water penetration. Flashing may either direct moisture out of a cavity or prevent moisture from entering. Without flashing, any intersection or Interruption of materials becomes an avenue for moisture to enter the construction. Several specific areas of potential moisture penetration must be addressed in the flashing of chimneys. This Brick Brief addresses these areas of concern and provides details for optimum performance. The Brick Brief "Proper Chimney Crowns may also be of interest.
# Chimney Construction
The use of flashing in chimneys presents some unique construction details. The three primary areas of concern are: the base of the chimney, the intersection of the chimney with the roof and the chimney crown.
# The Base
The base of the chimney is constructed in much the same manner as a brick cavity wall. Flashing must be used at the joint between the foundation and the brickwork. The flashing should extend through the brick wythe and turn up behind the exterior face of the brick, ending in a mortar joint in the interior wythe. Weeps are placed immediately above the flashing.
# Intersection with the Roof
The chimney penetration of the roof must be properly flashed to prevent water from entering the interior of the structure. Base and counter flashing must be installed at the interface. Base flashing is placed first, starting with the lower ends of the faces of the chimney perpendicular to the ridgeline, with tabs at each corner. The base flashing should extend a minimum of four inches (102 mm) along the roof and four inches (102 mm) up the chimney face as shown in Figure 1. Counter flashing is then lapped over the base flashing a minimum of three inches (76 mm), extended through the chimney wall, and turned up at an angle into the air space to collect water draining down the back of the brickwork as shown in Figure 2. All joints and laps in the base and counter flashing must be completely sealed. Weeps are not typically included.
# The Chimney Crown
The area around the chimney crown is often overlooked as a source of moisture penetration. Flashing and sealant are important here to prevent water from entering the space between the chimney wall and the flue liner. The first piece of flashing should be placed directly below the chimney crown, extend through the brick wythe, and turn up against the outer face of the flue liner. This flashing should form a drip above the outside chimney face. A second piece of flashing should be folded over the top of the flue.
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