Exterior Face
Outer stones
The exterior face is exposed to soil, rain, snow and changing grade conditions. Larger stones may be selected and bedded to create a reasonably stable outer plane.
Masonry Anatomy
An old stone foundation is usually much more than the stones you can see from the basement.
Traditional stone foundations are thick masonry assemblies that may contain two visible wall faces, smaller packing stones, mortar and a rubble core. Their strength comes from the way the entire wall works together, not from a thin surface layer.
This guide is based on more than 20 years of hands-on masonry experience from third-generation mason Dean MacArthur and focuses on the kinds of stone foundations found in older Prince Edward Island buildings.
Field context: Old stone walls can look irregular and still be stable. The important questions are how the wall is bonded, whether the shape is changing, where water is moving and whether the stones remain well seated.
Surface appearance tells you only part of the story. Wall depth, core condition, bearing and moisture matter too.
Stone Foundation Guide
Foundation Anatomy at a Glance
A traditional rubble stone wall often works as a thick three-dimensional masonry mass.
Construction varies from building to building, but the face-core-face model is a useful way to understand many older stone foundations.
Wall Core
Hearting, rubble and packing stones
Smaller stones and mortar can fill the space between the two wall faces. This material is not simply waste. It helps fill voids, distribute contact and make the thick wall behave as one assembly.
Interior Face
Basement-side stones
The interior face is the portion most homeowners see. It may later have been parged, plastered, painted or concealed, which can hide joints and movement behind the finish.
Conceptual Cross-Section
Think of the wall as a bonded mass, not two stone veneers with empty space between them.
Exterior soil / weather
Rain, snow melt, groundwater and grade conditions act on the outside face.
Stone wall core
Large face stones, smaller hearting stones, mortar and occasional stones extending deeper through the wall.
Basement interior
Interior face, parging or plaster, framing above and the visible signs of moisture or movement.
1. The Wall Faces
The outer and inner faces help define the wall, but neither face works independently.
Many old rubble foundations were built by establishing two faces and filling the space between them as the wall rose. Stones were selected, rotated and packed so they sat as securely as possible rather than simply being stacked like loose landscaping stone.
The faces are rarely perfectly straight. Irregular stone shapes naturally create variation in joint width and wall plane. That is why a rough-looking wall is not automatically a failing wall.
What matters is whether stones remain seated, whether the face is separating from the core, and whether the wall shape is changing over time.
What I look for in the field
✓Fresh gaps behind face stones
✓Stones that rock or move by hand
✓Bulging that is increasing over time
✓Loose small stones falling from deeper joints
✓Changes near openings or bearing locations
2. The Rubble Core & Hearting
Small stones inside the wall can be part of the structure, not debris to be pulled out.
In a thick rubble wall, the center may contain smaller stones, chips and mortar used to fill irregular spaces between larger face stones. This is often called hearting or packing.
Good hearting reduces large voids and helps transfer load through many points of contact. If the core washes out or becomes loose, the two faces can begin behaving more independently, which increases the risk of bulging or local collapse.
One common mistake is to remove loose-looking small stones from deep joints and replace only the visible surface with mortar. If those stones were providing packing or support, removing them can make the wall less stable.
Field example: Once the face is opened, the wall reads very differently from a finished basement surface. Large stones, smaller infill pieces, mortar and voids all become visible. This is why deep repair work should preserve useful packing and rebuild lost contact rather than simply covering the opening at the face.
3. Mortar in an Old Stone Foundation
Mortar fills, beds and bonds the stones, but it should work with the stone rather than overpower it.
Historic foundation mortar was commonly more lime-rich and more permeable than many modern high-cement repair mixes. The appropriate replacement mortar depends on the original material, stone type, exposure and wall condition.
Mortar helps distribute loads across irregular stone surfaces, fills joints and reduces water paths. In a rubble wall, however, stability also comes from stone shape, bearing, interlock, wall thickness and the condition of the core.
The National Park Service recommends replacement mortar that is physically compatible with historic masonry and warns that unnecessarily hard mortar can contribute to deterioration of softer masonry materials.
Repointing is not the same as filling the whole wall.
Repointing restores accessible joints. If the wall contains deep voids, loose hearting or separated faces, surface pointing alone may not reach the actual problem.
4. Through Stones, Bond & Wall Thickness
The better the two faces are tied together, the more the wall can act as one thick masonry unit.
Some traditional stone walls contain long stones that extend deeply into the wall or across much of its thickness. These can help tie the two faces together. Even where true through-stones are limited, overlapping stones and well-packed hearting can provide internal bond.
Problems develop when one face separates from the core. A wall may then bulge outward even though many individual stones still look intact.
This is why wall thickness and internal construction matter during repair. The visible interior joint may be only a few inches deep while the actual wall is two feet thick or more.
Field example: Deep openings behind the visible face show why wall thickness matters. A joint that appears to need a few inches of mortar at the surface can open into a much larger internal space once loose material is removed.
A useful question during repair
“Are we repairing the face, or are we restoring the bond of the wall?”
Those can be very different scopes of work. The answer depends on how much of the wall remains sound internally.
5. The Base & Footing Condition
Older stone foundations do not always sit on a modern poured-concrete footing.
Depending on age and construction, a stone wall may widen at the bottom, bear on large footing stones, sit on compacted soil or use another traditional base condition. The exact arrangement should not be assumed without evidence.
The load path is still simple in principle: the weight of the building must travel through the wall and into soil capable of supporting it. Problems can develop where soil settles, water softens supporting ground, frost affects shallow conditions or later excavations disturb the base.
Underpinning or excavating below the existing foundation is structural work, not ordinary repointing. Where the bearing condition is uncertain, engineered sequencing may be required.
Field example: Excavation exposes the part of the wall that normally cannot be inspected from inside. The relationship between masonry, soil, lower courses and any widening at the base becomes visible, which is critical information before underpinning or deeper excavation.
6. Where the House Bears on the Foundation
The top of the stone wall transfers the building load into the foundation.
Floor framing, sill beams, masonry walls or other structural elements bear on the upper portion of the foundation. Deterioration near this zone can be more significant than the same loose joint in a lightly loaded area.
I pay particular attention to missing stones, crushed mortar, timber decay, large voids and movement directly below beams or concentrated loads.
A masonry repair can restore damaged stonework, but rotten framing or an altered load path may also need to be addressed before the wall can be considered complete.
Field example: Opening the masonry at the top of the wall exposes the transition between stonework and timber framing. Voids, decayed packing, altered infill or poor bearing at this location can matter more structurally than similar mortar loss lower in the wall.
Higher-priority signs near bearing
!Stone displacement immediately below a sill or beam
!Large voids at the top of the wall
!Wood framing sinking into or separating from masonry
!Movement visible in floors, doors or framing above
7. Openings, Lintels & Local Weak Points
Windows, doors, vents and service penetrations interrupt the natural load path of a stone wall.
Openings force loads around an interruption in the masonry. Older basement windows may use stone, brick, timber or metal lintels, and later alterations may have enlarged or partially filled original openings.
Cracking or loose masonry around an opening deserves closer attention because the issue may involve support above the opening rather than simply mortar deterioration around its edges.
New service penetrations should also be located and detailed carefully. Cutting through a thick rubble wall can disturb more internal stone than is visible from one side.
Look above openings
A crack that begins at the corner of a basement window or door can be more informative than a random crack in the middle of a wall because it identifies a location where loads have to change direction.
8. Grade, Drainage & Ground Moisture
Many old stone foundations were built before modern membranes, perimeter drains and damp-proofing systems.
That does not mean every old foundation should be excavated and coated. It means exterior water management deserves serious attention.
The National Park Service identifies below-grade ground moisture as a major source of unwanted moisture in older buildings and emphasizes management of roof runoff, downspouts and surface drainage around foundations.
National Research Council Canada research likewise identifies rain, condensation, capillary ground moisture and defective drains or flashings as important ways masonry becomes wet.
✓Keep roof water away from the foundation
✓Maintain positive surface drainage where practical
✓Watch for areas where snow is piled against masonry
✓Investigate recurring damp zones rather than only coating the inside
Field example: This downspout ends directly beside masonry at grade. Even when the stone itself is sound, repeatedly delivering roof water to the same location increases the moisture load on the wall and soil. Drainage corrections can therefore be part of the masonry repair strategy.
9. Parging, Plaster & Interior Coatings
A smooth basement wall can hide a rough or deteriorated stone wall behind it.
Old foundations are often parged or plastered on the interior. Parging can fill shallow surface irregularities, reduce dusting and provide a more uniform finish, but it does not automatically repair loose stones or rebuild the internal core.
Cracks in parging can sometimes act as useful indicators because movement behind the finish may telegraph through to the surface.
Dense paints or waterproof coatings should also be used carefully. If moisture continues entering from outside or below, reducing interior drying can shift the visible damage rather than solve it.
Field example: The smooth parged surface hides irregular stone and deeper joints behind it. When parging cracks or releases, the exposed area can reveal whether the issue is only the finish or whether the masonry beneath also needs repair.
Before covering an old stone wall
✓Confirm the masonry is stable
✓Repair deep loose joints first
✓Understand where the wall currently dries
✓Do not hide active movement behind a cosmetic finish
10. Excavating Beside an Old Foundation
Exterior waterproofing can be useful in the right situation, but excavation changes the conditions around the wall.
A weak rubble wall can rely partly on its own mass, internal bond and the surrounding soil conditions. Excavating a full-height trench beside a deteriorated wall exposes the exterior face and can remove lateral support.
For that reason, badly deteriorated or moving walls may need staged excavation, temporary support, selective rebuilding or engineering rather than simply opening the entire foundation at once.
The waterproofing material is only one part of the work. Drainage, backfill, grade, downspouts, foundation condition and safe sequencing all have to be considered together.
When engineering review becomes more valuable
!Major bulging or leaning
!Underpinning below the existing wall
!Large sections of full-height excavation
!Loss of bearing at the top or bottom of the wall
!Alterations that change structural load paths
What Different Failures Usually Mean
Match the visible condition to the part of the wall that may actually be failing.
Recessed mortar, stable stones
Often a joint-maintenance issue. Repointing may be appropriate if the wall remains bonded and stable.
Small stones falling from deep joints
Can indicate loss of packing or hearting deeper in the wall, not only surface mortar deterioration.
One face bulging outward
Can indicate separation of the face from the wall core or broader movement.
Persistent dampness at the base
Points toward exterior grade, roof runoff, groundwater or capillary moisture as part of the diagnosis.
Cracking around an opening
May involve a lintel, altered load path, settlement or movement concentrated around the opening.
Fresh parging repeatedly cracking
Can indicate that the finish is moving with an unresolved condition in the masonry behind it.
Technical & Conservation References
Recognized guidance supports the same basic principle: control moisture, preserve sound masonry and repair only as aggressively as the condition requires.
National Park Service Preservation Brief 39 emphasizes managing roof runoff, downspouts and surface drainage because below-grade moisture is a major cause of deterioration in older buildings.
Preservation Brief 2 recommends compatible repointing materials and careful removal of failed mortar rather than unnecessarily replacing sound historic work.
National Research Council Canada has also documented the different ways masonry becomes wet, including rain, condensation, capillary moisture and defective drainage details.
National Park Service: Controlling Unwanted Moisture in Historic Buildings →
National Park Service: Repointing Historic Masonry →
National Research Council Canada: Moisture Degradation of Masonry Walls →
Parks Canada: Standards and Guidelines for Historic Places →
About This Guide
Prepared by Dean MacArthur, DinoMac Masonry Ltd.
Dean is a third-generation mason with more than 20 years of hands-on experience working with brick, natural stone, chimneys, foundations, repointing and masonry restoration in Prince Edward Island.
Last reviewed: August 25, 2026
Old Stone Foundation FAQ
Common questions about how traditional foundation walls are built.
Are all old stone foundations built the same way?
No. Construction varies by era, available stone, workmanship and building type. Some walls are regularly coursed, while others are irregular rubble or fieldstone.
What is inside a rubble stone wall?
Many contain two wall faces with smaller stones, mortar and hearting or packing material filling the core between them.
Does repointing repair the core?
Not necessarily. Repointing repairs accessible joints. Deep voids, loose packing or separated wall faces may require additional work or rebuilding.
Why do these foundations get damp?
Roof runoff, downspouts, exterior grade, groundwater, capillary moisture, open joints and drainage conditions can all contribute.
Should loose little stones be removed?
Not casually. Small stones can be part of the wall's packing or hearting. A mason should determine whether they are debris or structural packing before removing them.
When should an engineer be involved?
Major movement, underpinning, uncertain bearing, substantial excavation, altered openings or changes to structural load paths can justify engineering review.
Related DinoMac Guides
Continue from foundation anatomy into diagnosis and repair.
Have an Older Stone Foundation?
Wide photographs, close-ups and exterior drainage photos can reveal very different parts of the problem.
Send the property location, approximate building age if known, interior and exterior photographs, and any information about water entry or recent movement.