Balk at Caulk - Guidelines for Repair of Joints in Historic Stone

Abstract

For structures unsheltered from rain and snow - such as monuments, bridges, tombstones, standing ruins - water in stone is a recurring, perhaps constant, condition. Therefore, rather than attempt to exclude water from historic masonry, the preservation goal should be to manage moisture movement, with emphasis on facilitating runoff and evaporation. Evaporation is an important mechanism for moisture egress, and the larger the area of the evaporative surface, the more efficient the removal of moisture. As joints between individual stone blocks function as micro-scale storm sewers, sealing open joints with elastomeric sealants may temporarily facilitate runoff, at the same time restricting evaporation. Impermeable joint materials in masonry structures without overhanging roofs or other water-diversionary tactics will trap the inevitable water within the masonry units, leading to biological decay and disaggregation of mineral matrices. In-kind replacement of mortar (25–50 years service) and lead (75–100 years service) are viable alternatives to sealants (5–10 years service) for joint repair in historic stone. The traditional materials are cost competitive with sealants, particularly if scaffolding costs are included in life-cycle estimates. There are very few, if any, cases of historic masonry where sealants are appropriate substitute materials for mortar or lead.