The existing pointing mortar entry covers mortar for new masonry. This entry addresses the decision most common on existing New England buildings: what mortar to use when repointing historic brick and stone. The answer matters more than most people realize. The wrong mortar — specifically, a mortar that is harder than the masonry it is joining — will damage the building. This is the most common mortar mistake in historic masonry repair.
Historic mortar in New England buildings is soft. It is soft by design. The mortar joint in a traditional masonry wall is the sacrificial element — it is intended to be softer than the brick or stone it joins so that movement caused by thermal expansion, settlement, and frost is accommodated at the joint, not at the face of the masonry unit.
Historic mortar in New England buildings from before 1900 is almost always a lime mortar — lime putty or hydraulic lime with sand, no Portland cement. Portland cement was not in common use until the late 19th century. Mortar types are classified by ASTM C270 into types M, S, N, O, and K, with M being the hardest and K the softest. Type N (1 part Portland, 1 part lime, 6 parts sand) is the softest Portland-containing mortar appropriate for repointing soft historic brick. Natural hydraulic lime mortars are correct for very soft historic brick and stone.
A mortar that is too hard for the masonry it joins causes spalling at the masonry face adjacent to the joint — the masonry, unable to crack the joint, cracks itself. This damage is irreversible. The evidence is visible on every old New England building that has been repointed with Portland cement mortar: the brick or stone faces adjacent to the repointing are spalled and damaged while the new mortar joint is intact.
The correct mortar for repointing historic masonry is the softest mortar that still provides structural integrity and weather resistance for the specific masonry and exposure. For most pre-1900 brick in New England, that is a lime-only mortar or a hydraulic lime mortar.
Natural hydraulic lime (NHL) is available from specialty masonry suppliers including Edison Coatings and Cathedral Stone Products. Type S mason's lime for lime-only mortars is available from building supply dealers. Sand must match the original in color and gradation — this is the element most commonly ignored and most visible when wrong.
Match mortar compressive strength to the masonry being repointed — softer than or equal to the masonry unit, never harder. For pre-1900 soft brick: natural hydraulic lime mortar or lime putty mortar. For post-1900 brick: Type N Portland-lime mortar at most. For granite and harder stone: Type N or Type S, depending on exposure. Sand color and texture must match the original. This is the specification that protects the masonry rather than the mortar.
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