Q: I was looking at the interior of a bell tower of a church recently. The floor of the bell tower is covered with the spalled faces of the brick. What is causing this problem and how can it be corrected?
A: The interior brick wythe is likely spalling due to moisture in the brick that is either freezing within brick units near the interior face or causing salts within the brick to migrate and re-crystallize beneath the interior surface, which will also result in spalling. This is a common problem in unheated bell towers that are exposed to significant wetting, as shown below.
The masonry in unheated bell towers will be cooler than the masonry in the heated areas and, as a result, will have more freeze-thaw cycles. The cooler temperatures will also slow drying of the masonry. Because moisture evaporates slowly, there is a greater potential for salt migration in the masonry, which can re-crystallize just beneath the surface within the pores of the brick units (sometimes referred to as cryptofluorescence). Forces generated by the crystallization can fracture the brick surface resulting in spalling.
The interior wythes of older masonry walls are generally poorer quality common brick units that are more likely to deteriorate from freeze-thaw action. They are also more likely to contain soluble salts which can deteriorate when salts migrate to the interior surface and re-crystallize.
Many bell towers have significant water penetration during rains. Wind-driven rains can also blow through the louvers at the top of the bell tower. Water collecting on the floor can wet the masonry on the interior face in addition to the exterior face. There are many ledges in bell towers that may allow water penetration, especially when they are deteriorated. Water can also penetrate cracks in deteriorated masonry.
To repair bell towers with interior spalling, I typically recommend replacing the spalled interior brick with Grade SW brick units and performing exterior repairs to minimize water penetration. The exterior repairs may include covering ledges or other features to shed water and eliminate entry points for water penetration, replacing or repointing deteriorated or cracked masonry, repairing or replacing louvers or windows that may allow water penetration, replacing failed sealant joints, repairing or replacing any damaged roofs, and considering the addition of gutters on the roof at the top of the bell tower where practical.
You may also consider adding heat to the bell tower to assist in drying out the masonry during cold weather thus reducing the potential for the masonry to freeze.