How Are New Hampshire Granite Foundations Repaired?

Water and frost do most of the damage, working together across decades. Mortar joints erode from the outside in as rain, snowmelt, and groundwater wash through them; each freeze-thaw cycle then wedges the weakened joints wider. Hydrostatic pressure builds behind walls where drainage is absent, pushing stones inward and bowing long wall runs. Frost heave lifts shallow footings and drops them unevenly each spring. The engineer maps these patterns wall by wall, because a bulging wall needs lateral restraint or rebuilding while a settled corner needs underpinning - and treating one like the other wastes the repair budget. Settlement gets special attention because it is the one mechanism that can be structural rather than just wet. The engineer distinguishes uniform historic settlement that stabilized generations ago from active differential movement: fresh cracking, sticking doors, sloping floors that are getting worse, and crack monitors that keep moving. Soil conditions matter here - New Hampshire's mix of ledge, till, and soft lakebed clays means two houses on the same street can have entirely different foundation behavior. Where active settlement is confirmed, the repair design addresses bearing capacity, not just the wall's appearance.

The guiding rule is compatibility: new work must move and breathe like the old wall. Repointing mortar is matched in strength and permeability to the original - a soft lime mortar for a 19th-century wall - so moisture escapes through the joints instead of being driven into the stone. Stones are reset with full bearing and chinking, and the wall's drainage plane is restored with washed stone and filter fabric behind rebuilt sections. Exterior excavation, where practical, allows a waterproofing membrane and footing drain that relieve hydrostatic pressure at its source; where excavation is impractical, an interior perimeter drain with a sealed sump manages water that gets through. Structural repairs go deeper. Underpinning extends the foundation to stable bearing in sequenced sections so the wall is never unsupported; helical piers or micropiles reach competent soil or ledge where shallow bearing is inadequate. Bulging walls may need partial rebuild with proper batter and drainage, or engineered lateral restraint. After repair, the engineer specifies monitoring - simple crack gauges or periodic survey - to confirm the movement has stopped. A granite foundation repaired with compatible materials and real drainage should outlast everyone involved in the project.