How to Repair Cracks in a Stone Retaining Wall
Most cracks in a stone retaining wall run through the mortar joints, and a crack that is narrow, no longer moving and has both sides still lined up is a maintenance job: rake out the joint and repoint it. A crack that keeps widening, runs along the wall with the upper part pushed out, or opens and closes with the seasons needs a different answer, because fresh mortar won't hold a wall that is still moving.
This guide covers only cracks: how to tell whether one is active, what each crack pattern usually means, and how to repoint a cracked joint properly. For everything else that goes wrong with a stone wall, including loose stones, bulges and rebuilding a section, see how to repair a stone retaining wall.
Is the crack still moving? Find out before you fill it
The single most useful thing you can learn about a crack is whether it's active. HUD's Residential Rehabilitation Inspection Guide (2000) gives two quick clues: a clean crack indicates recent movement, while a dirty or previously filled crack may be inactive. Then measure it over time:
- Draw a pencil line across each end of the crack and write the date beside it.
- Mark two or three points along the crack and measure its width at each with a ruler or a plastic crack-width card.
- Photograph it from the same spot, with the ruler in the frame.
- Check again after heavy rain, after the first hard freeze and in spring.
Give it time. HUD notes that seasonal movement can take six months or more to show up, and that width only means something alongside age: a half-inch crack in a new wall suggests rapid settlement, while the same crack in a 50-year-old wall may reflect very slow movement of about 1/100 of an inch a year. If the crack grows past your pencil marks or widens at your measuring points, treat it as active.
What each crack pattern usually means
The direction and shape of a crack tell you more than its width. These patterns come from the HUD guide and the National Park Service's Preservation Brief 2 on repointing.
| Crack pattern | Usual cause | How serious |
|---|---|---|
| Hairline cracks where mortar has pulled away from the stone | Normal movement relieved in the joint, which is what mortar is supposed to do | Low. Repoint when the joint starts letting water in |
| Stepped or diagonal cracks that taper, wide at one end and hairline at the other | One part of the base settling more than another | Moderate. Watch it; if it keeps growing, the base needs attention |
| Near-vertical cracks whose two sides no longer line up | Differential settlement, one side dropping or moving | Moderate to high, depending on whether it's still moving |
| A long horizontal crack with the wall above it pushed forward | The wall bending or rotating under soil and water pressure | High. This is structural |
| Cracks through the stones themselves, or stone faces flaking off | Often mortar harder than the stone, forcing movement into the stone | Moderate. The stones are being damaged; an earlier repair may be the cause |
| Cracks that open in winter and close in summer | Seasonal expansion and contraction | Low to moderate, but they need a flexible sealant rather than mortar |
Two of these deserve a closer look.
The horizontal crack with forward movement. HUD's guide shows exactly this failure and states that the outward movement of the upper part of a retaining wall can only be halted by structural reinforcement; patching the crack won't solve it. Don't repoint it and hope.
Cracks through the stone. Preservation Brief 2 explains that a mortar stronger than the masonry won't give, so stresses get relieved through the stones instead, causing cracking and spalling that is hard to repair. When stones next to a patched joint start cracking, look at what the last person used.
Cosmetic or structural: a quick field check
Before you buy mortar, run through these questions. Any "yes" moves the crack out of the cosmetic column.
- Has the crack grown past your marks, or does it look clean and fresh?
- Do the two sides of the crack sit out of line, with one face proud of the other?
- Is the wall leaning forward or bulging near the crack? Check with a 4-foot level.
- Does water or soil come out of the crack after rain, or is the ground behind the wall soggy?
- Is it a crack you, or someone before you, has already filled once?
If the answers are all no, repointing is the right repair. If any are yes, the crack is a symptom, and the cause is usually water behind the wall or a base that is moving. HUD's guide recommends that failing retaining walls more than two feet high be inspected by a structural engineer. The main stone wall repair guide covers drainage, rebuilding a section and the Massachusetts permit rules.
Mortar or sealant: choosing what goes in the crack
HUD's guide draws a clear line: active cracks should be sealed with a flexible sealant, and inactive cracks may be pointed. It explains why for seasonal cracks: filling them with mortar holds them open and causes more cracking. So a crack that moves with the seasons gets a flexible, exterior masonry sealant; a dormant crack in a joint gets new mortar.
When you repoint, the mortar has to suit the wall. Preservation Brief 2 says new mortar must be softer than the stones and no harder than the existing mortar, and that matching the sand is what makes the color and texture fall into place. In practice:
- Hard stone, modern cement mortar. Granite and similar stone set in cement mortar usually takes a Type S mix, which manufacturers such as Amerimix list for retaining walls (1,800 psi).
- Soft stone or old lime mortar. Match the softer existing mortar. A Type N mix (750 psi) or a lime-rich mix may be the right choice, and forcing a stiff Type S into an old lime wall is how stones end up cracking.
- Never concrete or a hard patching compound in a joint. It is the "too hard" problem at its worst.
Mix a small test batch with sand that matches the old joints and let it dry fully before judging the color. Wet mortar reads darker than it will end up.
How to repoint a cracked joint, step by step
This method follows Preservation Brief 2. It works on a joint that is cracked but whose stones are tight and the wall stable.
- Pick the weather. The brief gives wall temperatures of 40 to 95°F as the range that prevents freezing or overly fast drying, and prefers working in shade. In Massachusetts that means waiting until spring for most walls and avoiding late fall, when a hard frost could hit fresh mortar.
- Rake out the joint. Remove the cracked and loose mortar to at least 2 to 2.5 times the width of the joint, and further wherever it's still loose, leaving a square back to the cut. Hand chisels and a hammer are the safest tools; the brief warns that power grinders in unskilled hands damage the edges of the stones. Wear a respirator: mortar dust contains crystalline silica, a lung hazard OSHA regulates.
- Rinse and dampen. Flush out the dust with water. At the moment you fill it, the joint should be damp with no standing water.
- Mix carefully. Add about half the water, mix for roughly five minutes, then add the rest a little at a time until it holds together. Keep the water to a minimum: drier mortar packs tighter and shrinks less. Use it within about 30 minutes and don't add water to stiffening mortar.
- Fill deep joints in layers. Where you've cut back more than an inch, pack about 1/4 inch of mortar into the back of the joint, let it reach thumbprint hardness, then add the next layer. Keep going until the joint is full. Layering limits shrinkage cracks.
- Tool at thumbprint hard. When the final layer takes a thumbprint without sticking, shape the joint to match the rest of the wall. Too soft and it cracks and dries light; too hard and it streaks dark. Where the stone edges are worn and rounded, keep the mortar slightly back from the face so you don't leave a thin feather edge that breaks off.
- Brush and cure. Brush off crumbs with a natural or nylon brush, not a wire brush. Keep the work damp by misting for a day or two, and cover it with burlap, or plastic tented off the face, for the first three days.
Then keep your pencil marks. If the repaired crack reopens over the next season, the wall is still moving and the fix is structural, not more mortar.
When a crack means the wall needs more than mortar
Cracks that return, horizontal cracks with forward movement, and cracks paired with a bulge or lean all point to the same place: the soil and water behind the wall, or the base under it. Those are rebuild questions, and the stone retaining wall repair guide explains how to tell when a section has to come down.
If a section needs rebuilding, George Hardscaping builds retaining walls in stone, rock, brick and concrete for homes across Gloucester and the North Shore. Request an estimate or call (781) 222-0805.
