Stonework has several enemies: mistaken use of concrete/portland cement as mortar, paint, spray foam, uncontrolled water/ice because of faulty eavestroughs/downspouts and grading; roots/vines and neglect.
Stone buildings breathe: heritage mortar is softer than stone, so that over the decades and centuries as the building ages, the shiftings and settlings that occur, grind the mortar into dust and powder, preserving the stones, so that, all things being equal, nothing but repointing/restoration with new heritage mortar should ever be required.
Concrete is made out of Portland cement, and doesn’t breathe. Concrete moves as a mass when it moves; if it is poured against stone or on top of stone, the concrete mass destroys both the stone and the mortar. Stone however, can sit on top of concrete pads and footings, because there is a mortar bed between the stone and the concrete, so if concrete has to be added against a stone wall there should be a thick mortar buffer between the two, and the buffer will have to be renewed over a generation.
Mortars made solely out of cement products are bad for stone. Seek advice from heritage stone workers, not handymen.
Concrete/cement also seals stone in such a way that it’s natural breathing/moisture exchange characteristics are blocked, leading to mold issues. The same problem occurs when you paint stone, or when you some brands of spray foam-insulation on stone. Make sure the brand breathes. The ONLY coating that can always be safely applied to stone, is lime wash.
Heritage mortars originally consisted of nothing but double-baked/water-slaked lime (limestone heated in kilns and turned to paste by adding water. to which sand and small sandless stones are added. The look can be seen in the joints of any old building that has been properly cared for. The little white bits, are lime. The mortar is white when it goes in and slowly darkens as it gets dirty and hardens back into stone. Mortars that dry dark grey are cement, and not mortar at all.
The other enemy of stone is water and ice. More stone buildings have been damaged by faulty downspouts/eavestroughs/drainage than any other cause. Downspouts MUST have extensions that carry water away from stone footings: water that is allowed to collect at the base of walls erodes the ground, destroys grading and thus allows more water to collect. Northern winters are especially hard on wet stone buildings because water wicks inward and upward as well as sinks, so that once the cold comes, the stone is vulnerable to ice, which can break stone and destroy subgrade mortar, which then gets replaced with dirt and roots; further compromising the building.
The effects of gravity and the downslope movement of buildings over time can be eased by controlling water, because erosion will take its toll on neglected buildings, hence the importance of always grading away from the building, of using swales or terracing to direct water away from higher ground.
For the reasons stated above, the solution to subgrade damage can NEVER be concrete. The solution is to dig a ditch, remove the compromised mortar, dirt and roots, clean the stone, replace the heritage mortar, and tamp the earth as the ditch is refilled, before grading the ground so that NO water is EVER allow to stand. (Stone can be removed from walls and cleaned, as long as archway patterns of weight distribution are maintained in the stones above those removed, with no more than five feet of such archways created at any given moment.)
Gardens around stone buildings, especially ones with trees and bushes and retaining walls that trap water near buildings should be redesigned; all large root plants should be removed, including varieties of vines that dig into mortar: ask a garden shop about non-destructive vine variety advice.
Subgrade window wells should covered with see-through plastic caps to prevent water from settling below grade.
Foundation corners are especially critical. if the corner goes, everything above it will start to give, if it’s brick they’ll start to crack and widen allowing in more water. If bricks have white film on them, it’s phosphorous, and means water is getting behind the brick and drying outward drawing salts and calscium with it, coating the surface.
Cracks appear running diagonally down and away from lintels and sills, which also lead to splits in the stone work that open up, fill with ice and relatively quickly turn a century old building into a ruin, especially if the roof goes. If you have a stone building with a bad roof, fix the roof, cover it/seal it, whatever you can, because you’ll create a ruin in no time.
If treated with care and the application of proper materials, stone buildings will last for centuries. Which means that owners of such buildings are essential to the survival of every community’s stone heritage
If you don’t want the bother of fixing a heritage home as soon as a problem occurs, sell it to someone who cares what your community looks like.
For more information, check your local listings for stone workers and masons who do heritage work.