If your chimney is leaking, cracking, or losing bricks, the problem may not be the bricks at all—it may be the top of the chimney. Many masonry chimneys are built with a simple mortar wash instead of a properly constructed reinforced concrete crown. While a mortar wash costs less initially, it is one of the most common causes of water damage and deterioration that we see during chimney inspections.
Almost all masonry chimneys are built originally with a mortar wash, or splay, instead of a proper crown. An argument can be made that this is what causes most problems with masonry chimneys. A mortar wash is typically made of hand formed mortar with no reinforcement. There is usually no bond break between the wash and the brick chimney and no expansion joint between the wash and the flue tiles. Often, a mortar wash is supported by plywood or other combustibles and it is not unusual to find these combustibles burnt and laying on top of the smoke chamber.
So, what does all this mean?
- Mortar doesn’t finish like concrete and so is often much more porous allowing water to soak through and damage the wash during freeze/thaw cycles
- A mortar wash doesn’t overhang the chimney which allows all of the rain water to run down the face of the brick causing much greater chances of brick spalling and mortar deterioration
- A mortar wash is almost always bonded to the brick chimney below it meaning that during expansion something is going to crack. This may be the wash or a mortar joint in the chimney or both
- There is no expansion joint around flue tiles. When the flue tiles get hot during use, they expand and the flue grows taller. Without an expansion joint the flue can literally pick the mortar wash up off the chimney and then set it back down once it cools and contracts. Since there is also no bond break between the wash and the brick it will sometimes pick some bricks up with it. This results in a crack one or two courses down from the top. This can also result in a large gap between flue tiles if proper contraction wasn’t a possibility due to lack of an expansion joint. Without an expansion joint, something is always going to crack.
A proper crown is built of reinforced concrete, it overhangs the chimney and has a drip edge to prevent water from streaming down the face of the brick. This is comparable to putting an umbrella over the chimney. Another similar option is a full coverage cap, however with a proper crown you also get a bond break and expansion joint.
