Old Canaan Building Materials

Every old house in New England sits on a rubble stone foundation. The stones were pulled from the fields being cleared for the house site, stacked with lime mortar or dry, and expected to carry the building for as long as anyone could imagine. Many of them have done exactly that. Understanding what a rubble foundation is — and what it is not — is the beginning of maintaining one correctly.

The rubble foundation is irregular fieldstone stacked without a consistent course. No two are alike. The stones are whatever the site produced: granite, schist, gneiss, occasionally brownstone or sandstone in the Connecticut River valley. The mortar, where present, is lime — soft, sacrificial, repointed on a cycle measured in decades. The wall is thick: 18 to 24 inches is standard for a load-bearing foundation wall.

A rubble stone foundation uses undressed or minimally dressed fieldstone set in lime mortar or dry-stacked, in walls 18 to 30 inches thick, to transfer the load of the building to the bearing soil below. New England rubble foundations typically extend below frost depth — 42 inches in Connecticut — and are set on a footing of large flat stones or poured directly on ledge where available. They are not waterproofed in the modern sense. They drain through their mass and through the lime mortar joints.

A rubble foundation breathes. Moisture migrates through it in both directions — in during wet weather, out as conditions dry. This is correct behavior, not a failure. Waterproofing the interior face of a rubble foundation with a modern membrane or coating traps moisture in the wall and accelerates deterioration of the mortar and, eventually, the stones. The foundation is designed to be damp. Manage the water at the exterior — grade, gutters, downspouts — and the interior dampness resolves.

The rubble foundation is the correct foundation for an old New England building because it is what the building was designed around. Its flexibility — the ability to absorb minor settlement and frost movement through lime mortar joints that crack and can be repointed — is a feature, not a bug. Replacing a rubble foundation with poured concrete changes the building's structural behavior in ways that can damage the structure above.

Rubble stone foundations are repaired, not replaced, in traditional building practice. Find a mason with specific lime mortar experience. Repoint deteriorated joints with a mortar softer than the stone — NHL 2 or NHL 3.5 natural hydraulic lime. Never use Portland cement mortar in a rubble foundation. Manage exterior water before doing any interior work.