Chimney reconstruction.

Q The chimney on an older building needs to be rebuilt from just below the roofline due to excessive deterioration. The appearance of the chimney must be retained because the building has some historic significance.

A consultant recommended that the owners replace the chimney with

reinforced concrete cast against a thin brick veneer shell (made of 2-inch deep units) to increase the durability of the assembly, instead of merely rebuilding the masonry chimney. This new concrete/thin brick chimney will be cast directly on top of the original masonry chimney and flue.

Is there a problem with material compatibility using this approach? What about the bond between the old chimney and the new one of reinforced concrete above it?

A A properly built masonry chimney can be durable. However, casting concrete or grout against the interior face of a brick chimney can potentially reduce the durability of this masonry.

Moisture penetrating the masonry in this system may be held within the veneer since concrete prevents drying out to the interior. This scenario can lead to moisture problems such as efflorescence and deterioration. Although the concrete likely bonds to the original masonry as well as the new mortar in a traditional rebuilding approach, differential movement resulting from drying shrinkage of the concrete may result in some bond failures.

Also, in traditional masonry chimneys the clay tile flue is typically isolated somewhat from the outer portion of the masonry chimney to reduce heat transfer and allow some independent movement when it is heated. Casting a concrete flue against the top of the masonry chimney and flue may generate problems due to the restraint of this flue.

The durability of a chimney is often dependent on the integrity of the liner and the cap. If the liner is reasonably tight, it can prevent most of the moisture generated by combustion gases from penetrating the chimney, which can result in efflorescence or freeze/thaw damage. If the liner is deteriorated, consider using a stainless steel version within the clay tile liner if possible.

Similarly, the cap over the chimney must be watertight and should have an overhang and drip on the underside to facilitate shedding water away from the face of the chimney. This approach protects the chimney from rainwater penetration at the top as well as from snow and ice that can collect on the chimney and melt when it is in use or is heated by the sun.

In many cases, the primary cause of the chimney failure is water penetration along the top due to deficiencies in the cap.