hearting

Hearting is the bulk material packed into the cavity between two outer wythes of masonry, typically stone or brick, in thick solid walls or cavity walls. It consists of brick fragments, stone chips, mortar scraps, and other rubble from the building site, compacted to fill voids and reduce material costs. The outer wythes carry the structural load; the hearting provides mass, thermal resistance, and lateral bracing between them.

In traditional solid stone walls, particularly common in Scottish and Northern European construction from the 18th through early 20th centuries, hearting occupies 40 to 60 percent of wall thickness. A 24-inch stone wall might have 8 inches of dressed ashlar on the outside, 8 inches on the inside, and 8 inches of random hearting between them. The quality and compaction of hearting directly affects wall performance: poorly compacted hearting allows water penetration and reduces the wall's ability to act as a unified element.

Failures and degradation

Movement of hearting is a major problem in old masonry. Differential settlement, frost heave, and mortar erosion create voids as hearting subsides or loosens. Rain penetrates through failed pointing on the outer wythe, saturates the hearting, and migrates through to the interior surface, causing damp patches that may not appear until years after construction. Repointing alone cannot stop this; the inner wythe must be also repointed to create a moisture barrier. In cavity walls built from the 1920s onward, hearting was gradually replaced by air cavities or partial fill, which eliminated this failure mode.

The term reflects its function: hearting is the heart or core material between the two strong outer skins. It was economical because builders could use site waste and off-cuts rather than quarried material. Hearting quality was seldom specified in detail, making it a variable and often weak point in restoration work today. When opening historic walls for repairs, surveyors expect to find anything from well-compacted clay mortar and brick to loose stone and soil, depending on the builder's care and the materials available.