A dry-stacked rock retaining wall is repaired by taking the damaged part down to sound stone and rebuilding it the way a dry wall should be built: each stone set with its length running into the bank, every joint covered by the stone above, the gaps packed tight by hand, and the face leaning back into the slope. Pushing fallen stones back in from the front, or smearing mortar into the joints, doesn't fix a dry wall.

This guide covers walls laid without mortar, from fieldstone walls to walls of large rock. If your wall has mortar in the joints, how to repair a stone retaining wall is the guide you want.

How a dry rock wall holds, and why sections fail

The Stone Trust, a Vermont dry stone walling school, defines a dry stone retaining wall in its design specification for dry stone retaining walls as a gravity wall built without mortar that relies on the mass of the stones and the friction between them to hold back soil. Water passes through it rather than building up behind it. That's why a well-built dry wall can move a little and survive, and also why it fails in recognizable ways:

  • Stones laid the wrong way. Stones set with their long side along the face look good but have little grip on the wall behind them. Stone waller Dan Snow describes rebuilding a failed retaining wall where he turned stones that had run along the face so they ran lengthwise into the wall.
  • Loose or missing hearting. Hearting is the packing of smaller stones in the voids inside the wall. When it's too small, washes out or was never placed, the face stones lose their support and start to tip.
  • Lost cap stones. Once the heavy top stones are knocked off, frost and water work down into the wall from above.
  • Roots and trees. A National Park Service account of a dry stone wall restoration in Ohio traces that wall's failure to large trees that had grown into it, then died and collapsed.
  • Heavy, wet soil. Clay soil holds water and pushes hard on the wall. Dan Snow's rebuild was in heavy clay.

Look at the damaged area. A bellied or slumped face with stones sliding forward is a section that needs rebuilding. A single fallen cap stone or one tipped stone near the top may be a small fix.

Small fixes: a fallen cap or a tipped top stone

Only the top of a dry wall can be repaired without taking anything down.

  • Resetting cap stones. The Stone Trust specification says cap stones should cover the full width of the wall, be heavy enough to be hard to dislodge, sit level from front to back, not tip or shift when weight goes on them, and bear on the wall stones rather than on small wedging stones at the face. If a cap rocks, pack the void beneath it with pieces of stone placed from above.
  • A tipped stone in the top course. Lift the cap off, take the stone out, repack the hearting behind and under it, and set it back with its length into the wall.

A stone that has fallen out from the middle of the face is different. The specification says wedging and hearting stones go in from the interior of the wall and must not be pushed in from the face. So a mid-wall hole can't be properly repaired from the front: the wall above it has to come down to that level and go back up.

Rebuilding a section, step by step

This follows the Stone Trust specification, written for contractors and designers, simplified for a low garden wall. It's heavy, slow work.

  1. Clear and take down. Remove plants and roots, then take the wall down past the damage to where the courses are tight and stable at both ends and below. Lay the stones out on the downhill side where you can see them. Dig the loose soil out from behind the damaged section.
  2. Sort the stone. Group the largest stones for the foundation, long stones for through stones, flat heavy stones for caps, wall and backing stones by height, and small pieces for pinning and hearting.
  3. Prepare a drained footing. The specification calls for clean angular crushed stone, typically 3/4 inch to 1 1/2 inch, compacted firm. Where the ground freezes, it adds, water must pass easily through the footing and have somewhere to drain well away from the wall.
  4. Lay the foundation. Use the largest stones, each with its length into the wall, level, tight to its neighbors, and bedded with no voids underneath. Set backing stones of the same height behind them. Some wallers, Dan Snow among them, tilt the lower courses so they slope down toward the bank, which resists stones sliding forward. Never let a stone tip toward the front.
  5. Build course by course. Every stone crosses the joint below it, with at least a third of its width on each side, sits on no more than two stones, and touches the course below at three or four points, two of them near the face. Keep courses level, with stones of similar height side by side.
  6. Heart as you go. Fill every void behind and between the face and backing stones by hand, using fewer, larger pieces rather than many small ones. The specification's test is simple: material you could shovel is too small for hearting.
  7. Add through stones. At the top of each lift, which the specification defines as up to 30 inches or two courses, set long stones that run from the face into the back of the wall, no more than 3 feet apart center to center.
  8. Keep the batter. Batter is the backward lean of the face. The specification's range is 1:12 to 1:6, meaning the face steps back 1 inch for every 12 to 6 inches of height, and a vertical face isn't acceptable. Use string lines or a batter frame so the rebuilt section matches the wall on either side.
  9. Backfill and cap. Fill behind the wall with clean angular stone of the same size range, separated from the soil by filter fabric. Compact within 3 feet of the wall only with light hand-operated equipment. Finish with cap stones.

Before you dig behind the wall, Massachusetts law requires notifying Dig Safe at least 72 hours ahead, not counting weekends and holidays.

A worked example: how thick a rebuilt section should be

The most common mistake in a rebuilt dry wall is making it too thin. The Stone Trust specification sizes the base with a simple formula: twice the height, times the batter, plus the width at the top. Here's an illustration of that formula, not a design for any particular wall:

Wall heightBatterTop widthBase width from the formula
36 inches1:618 inches2 × 36 × 1/6 + 18 = 30 inches
36 inches1:1218 inches2 × 36 × 1/12 + 18 = 24 inches

In other words, a 3-foot dry wall is roughly 2 to 2 1/2 feet thick at the bottom before you count the drainage stone behind it. If the section you're rebuilding was a single row of face stones leaning on soil, that's probably why it failed, and rebuilding it the same way will give the same result.

Large rock and boulder walls

Walls of big rock, where single stones weigh hundreds of pounds or more, follow the same principles: length into the bank, joints crossed, face leaning back, drained stone behind. The difference is that they're set by machine. Resetting a slumped boulder wall means lifting stones out with an excavator, regrading the base and setting them again, and a boulder that shifts while someone is beside or below it can crush them. This isn't hand work. If a boulder wall has moved, keep people and vehicles off the ground below it until it's been looked at.

When to stop and bring in a professional

Dry stone rebuilding is skilled work, and taller walls carry real risk. Call in help when:

  • The failed section is more than about two feet high. HUD's Residential Rehabilitation Inspection Guide (2000) recommends that failing retaining walls over two feet be inspected by a structural engineer.
  • The wall holds up a driveway, a building or a steep slope. The Stone Trust specification calls for every wall built to it to be designed and sealed by a licensed engineer, landscape architect or architect experienced in dry stone design.
  • The wall is over four feet, measured from the bottom of the footing to the top, or supports a surcharge such as a driveway. Only walls under that line are exempt from building permits under 780 CMR, the Massachusetts building code.
  • The wall is near a pond, stream, wetland or the shore. Work within 100 feet of a bank or bordering wetland needs a filing with your town's conservation commission first, as the main stone retaining wall repair guide explains.

For certified wallers, the Stone Trust specification accepts certification from the Dry Stone Walling Association of Great Britain and the Dry Stone Conservancy in Kentucky.

If a rock wall needs rebuilding or replacing, George Hardscaping builds retaining walls in stone, rock, brick and concrete for homes across Gloucester, Rockport, Essex and the rest of the North Shore, and our excavation work handles the digging and ground preparation. Request an estimate or call (781) 222-0805.