Causes of Stone Deterioration
Limestone deterioration can be caused by multiple variables, but the most common cause is excessive exposure to moisture. Stone units that project out from the face of a structure, or cap stones with a mostly flat surface, are commonly referred to as “wash surfaces” because rainwater washes across these surfaces. This can cause surface erosion, but moisture absorption is the most detrimental effect. When surfaces become saturated and then freeze, the expansion from freezing can cause damage. In some cases, the damage is surface spalling and in more severe cases the damage is fracturing of entire stone units.
Other causes of saturation occur when leaking gutters or downspouts allow excessive moisture to cascade down the face of the building. Corrosion of embedded steel anchors and structural steel members can also cause significant stone damage. This is caused by the expansion of the corroding steel, often called “rust jacking”. The current use of stainless steel eliminates this issue. Open mortar joints can accelerate stone deterioration, as well, by allowing moisture to migrate into the wall, saturating the surrounding stone units.
Repair of deteriorated stones can often be accomplished with specialty stone patching mortars that can be carved, allowing decorative profiles to be recreated. When patching stones, square cutting the edges of the area to be patched and proper curing of the patching material are critical steps for the success of the application. In overhead patching applications, stainless steel pins or anchors should be used to mechanically fasten the patch to the structure.
Where severe stone deterioration has occurred, stone replacement is preferred. In this case, careful consideration should be given towards finding the correct stone material to match the original stone within the structure. Capturing precise dimensions and profiles is an extremely critical step in the process, allowing for accurate fabrication of replacement units. As always, mortar selection is an important process, as is the method for anchoring new units.