Stone Foundation Repair: Repointing Old Fieldstone Walls

Stone foundation repair on an older house usually means three things: repointing the mortar joints with a soft, lime-rich mortar that matches the old one, resetting or replacing stones that have come loose, and stopping the water outside that washed the mortar out in the first place. When a section of wall has bulged or shifted, that section is taken down and rebuilt.

Many homes around Woburn and the nearby towns sit on fieldstone or rubble stone foundations, the "rock basement" walls that owners often wonder about. This post explains how those walls are built, what the warning signs mean, how old stone foundation repair is done, and the mortar mistake that causes the most damage.

Types of stone foundations

• Fieldstone. Rounded or split stones gathered from fields and laid in mortar. The most common type under older New England houses.
• Rubble stone. Rough, irregular quarried stone, laid much the same way.
• Cut granite. Squared granite blocks, often used for the visible top courses, sills and corners.

Most of these walls were laid with lime-based mortar. Lime mortar is softer than modern cement and lets moisture pass through and dry out. That softness is a feature: the mortar is meant to wear before the stone does, and to be renewed over time.

What your rock basement wall is telling you

• Sand or crumbled mortar on the floor at the base of the wall. Water moving through the wall has dissolved and washed out the lime binder. This is the most common sign, and it means the joints need repointing.
• Loose or missing stones. Once enough mortar is gone, stones lose their support. Reset them before more follow.
• Damp patches or water at the bottom of the wall. Water is getting in through open joints, usually because of drainage outside.
• White, powdery deposits. Minerals left behind as water dries on the surface. Our post on efflorescence vs spalling explains what those deposits mean.
• A bulge or lean. A section pushing inward needs a professional look soon, before the next freeze-thaw season.
• Hard gray patches cracking around the edges. Earlier repairs with hard cement mortar, which often cause more harm than good (see below).

How a stone foundation is repointed

1. Rake out the loose mortar by hand to sound material, deep enough for the new mortar to hold. Power tools can chip the stone edges, so most of the work is done with hand tools.
2. Clean the joints of dust and loose grit.
3. Dampen the stone so it doesn't suck the water out of the new mortar too fast.
4. Pack the mortar in layers, filling deep joints a little at a time instead of all at once.
5. Tool the joints to match the look of the original wall.
6. Protect the work while it cures, especially from frost. Our post on cold-weather masonry repair explains when it is too cold to work.

Our repointing guide covers the same process on brick.

The mortar mistake to avoid

A common source of damage on old stone foundations is earlier repairs made with hard, modern cement mortar or hydraulic cement smeared over the joints. Hard mortar doesn't flex or breathe like lime mortar. Water that used to dry out through the joints gets trapped, and the stone or the surrounding old mortar breaks down instead.

The National Park Service's guidance on repointing says new mortar should match the historic mortar and be softer and more permeable than the masonry it holds (Preservation Brief 2). For a fieldstone foundation, that means a lime-rich mortar matched to the existing one, not the strongest bag of mortar at the store.

When a section has to be rebuilt

Repointing works when the stones are still in place. When a section has bulged inward, stones have shifted out of line, or the wall has partly collapsed, the section is supported, taken down stone by stone, and rebuilt on sound footing with matching mortar. Work like this may also call for a structural engineer's opinion, particularly if the house above shows signs of movement.

Should you parge, seal or paint a stone basement wall?

Owners often ask whether to coat the inside of a rock basement wall. A few cautions:

• Parging (a thin coat of mortar over the wall) hides the joints, so you can no longer see mortar failing. On the inside of a damp stone wall it can trap moisture and fall off in sheets.
• Sealers and waterproof paints don't stop water that is coming through the wall; it pushes the coating off. They treat the symptom.
• Repointing the joints and fixing the water outside is the lasting approach.

Lasting foundation repair starts with the masonry and the drainage outside. If the basement may also need an interior waterproofing system, ask us whether that is part of your project.

Fix the water outside, or the mortar washes out again

Mortar in a stone foundation fails where water moves through it. Downspouts that dump against the wall, soil that slopes toward the house, and wet side yards all feed the same joints year after year. Repointing without fixing the drainage buys time, not a cure. See our guide to drainage around the house foundation, and if water is already coming in, foundation leak repair.

What drives stone foundation repair cost

There is no honest per-foot number for old stone. The cost follows how much wall needs repointing, how deep the mortar has failed, whether any section needs rebuilding, access (a finished basement, a furnace or an oil tank in the way), and whether outside work like regrading or drainage is part of the fix. Our foundation repair cost guide explains each factor.

When to call a mason

Call when you find sand at the base of the wall, loose stones, a bulge, or water coming through the joints. Gerrior repoints and repairs fieldstone, rubble and block foundations and fixes the grading and drainage that cause the damage. Owner Joseph Gerrior oversees the work. See our foundation repair page. For freestanding garden or yard stone walls, see repairing a mortared rock wall, and for low foundations under crawl spaces, crawl space foundation repair.

Common questions

Is a stone foundation a problem when buying an old house?

Not by itself. Stone foundations have held up houses here for well over a century. What matters is the condition of the joints, any movement, and how water is handled outside.

Can I repoint a stone basement wall myself?

Small areas, with the right lime mortar and patience, yes. Large areas, loose stones or any bulging should be left to a mason.

Why is there sand on my basement floor?

It is almost always mortar washing out of the joints of a stone or brick wall. It means the wall needs repointing and the water source needs attention.

Should I use Type S mortar on my stone foundation?

Usually not. Type S is a strong cement mortar. Old stone foundations need a softer, lime-rich mortar matched to the original.

If your stone foundation is shedding sand or taking water, we can look at the joints and the drainage outside and explain what the wall needs. Gerrior Masonry & Landscape Construction Corp. is a family-owned company in Woburn, MA, with over 30 years in masonry and drainage work. We are fully insured, our work is guaranteed, and estimates are free. Call (781) 933-4157 or request a free estimate.