Stonework has several enemies: concrete/portland cement, 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, 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 as it is ground away especially at the surface contact points and where ever else water might get in.
Mortars made soley out of cement products are equally bad for stone, since cement was invented relatively late in the evolution of stone construction (the 1800’s) and was created to solve problems related to underwater footings. When cement-based mortars are used in stone buildings, after the natural shifts that occur over the decades, it is the stone and not the mortar that breaks, which means you’ve destroyed your building. Seek advice from heritage stone workers, who alone should be contacted for help or advice,.
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 spray foam-insulation on stone. The ONLY coating that can be 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.) Heritage mortar originally consisted of nothing but lime, sand and small stone ‘gravel’, which can be seen by looking at any old building that has been properly cared for. The mortar is white when it goes in and slowly darkens as it gets dirty and hardens back into stone. Grey mortars are cement mortars. Dark grey mortars are the worst mortars for stone.
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. Canadian 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, and no more than five feet of such archways are created at any given time.)
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.
As for insulating stone buildings because of their poor R-value, 2×2 stud walls with ‘bat’ or board insulation should be used instead of spray foams so that the walls continue “breathing.”
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 a community’s stone heritage.
For more information check your local listings for stone workers and masons who do heritage work.