Repointing Stone Walls: Lime Mortar, Tools, and Technique

The fastest way to ruin an old stone wall is to repoint it with hard cement mortar. Stone moves, breathes and sheds moisture through its joints, and repointing stone walls the right way means using a mortar that is softer and more permeable than the stone itself, usually a lime-based mix. When the joint is the weakest part of the wall, it takes the wear and can be replaced every few decades. When the mortar is harder than the stone, the stone faces crack and spall instead.

Stone work also differs from brick in technique. Rubble and fieldstone walls have irregular joints of varying width and depth, so you rake out by hand, pack mortar in layers, and finish the joint to suit the stone rather than tooling a neat concave line. The work is slow but forgiving, and a patient homeowner can handle garden walls, foundation sections and low retaining walls with basic tools.

Why Repointing Stone Walls Calls for Lime Mortar

Portland cement mortars cure hard, rigid and relatively impermeable. On a stone wall, especially one originally built with lime, that traps moisture behind the joint. Water then escapes through the stone, carrying salts and freezing inside the face, which leads to crumbling and flaking stone around perfectly intact cement joints. Lime mortar stays slightly flexible, allows vapor to pass, and tends to heal hairline cracks as it slowly carbonates.

There are two main families of lime for stone masonry pointing:

• Natural hydraulic lime (NHL). Sets partly by reaction with water, so it cures more reliably in damp conditions. Graded NHL 2, NHL 3.5 and NHL 5 from softest to strongest. NHL 3.5 suits most exterior walls; NHL 2 suits soft sandstone or sheltered work; NHL 5 is reserved for very exposed or wet locations with hard stone.
• Non-hydraulic lime putty or hot-mixed lime. Sets only by absorbing carbon dioxide from the air. Very soft and breathable, traditional on historic buildings, but slower to cure and more sensitive to frost and rain during curing.

For most homeowners, a bagged natural hydraulic lime mortar or a premixed lime mortar is the practical choice. Match the mortar to the stone: softer stones like sandstone and some limestones need weaker mixes, while granite and hard fieldstone tolerate stronger ones.

Tools and Materials

• Plugging chisel and a lump or club hammer for raking out old mortar
• Assorted pointing irons or narrow tuck-pointing trowels in several widths
• Hawk or small board to hold mortar at the wall
• Stiff natural-bristle brush and a soft brush
• Garden sprayer or mister for dampening joints
• Natural hydraulic lime, or a premixed lime mortar
• Well-graded sharp sand, washed, chosen to match the color and texture of the original mortar
• Mixing tub, gauging bucket and a drill with a paddle mixer
• Burlap or hessian sacking and plastic sheeting for curing
• Safety glasses, gloves, and an N95 or P100 respirator

Lime is caustic when wet. Wear gloves and eye protection when mixing and working, and rinse any splashes from skin or eyes right away with clean water.

Step 1: Assess the Wall and Test the Old Mortar

Walk the wall and mark joints that are recessed more than about 1/2 inch, crumbling, cracked or missing. You do not have to repoint every joint; spot repointing sound walls is common and preserves original material. Scrape a sample of the old mortar. If it crumbles easily and fizzes when a drop of vinegar hits it, it is lime based and you should stick with lime. Hard gray mortar that resists a screwdriver is likely cement, and removing it takes care so you do not break stone edges.

Look for bulges, leaning sections, loose stones that rock, or cracks that run through stones. Those are structural signs that repointing alone will not fix.

Step 2: Rake Out the Joints

Remove loose and decayed mortar to a depth of at least twice the joint width, typically 3/4 to 1 inch, and deeper on wide rubble joints. Rake back to sound material with a square-backed cavity rather than a V shape so the new mortar has something to key into.

On stone walls, hand tools are usually the right choice. Angle grinders are fast but easily cut into stone edges, widen joints and leave permanent scars, and they throw off heavy silica dust. If you must use power tools to remove hard cement mortar, use a grinder only to make a relief cut down the center of the joint, then finish with chisels. Mist the work, wear a P100 respirator and eye protection, and clean up with a HEPA vacuum or by wetting and sweeping rather than dry sweeping.

Brush out dust and debris thoroughly when you finish each section.

Step 3: Mix the Lime Mortar

A typical mix for natural hydraulic lime is 1 part lime to 2.5 or 3 parts sand by volume, but follow the maker’s guidance and match any existing mortar. Measure by the bucket, not the shovel, so batches stay consistent. Add water gradually and mix for several minutes; lime mortar should be firm and fairly dry, holding its shape on the trowel without slumping. Wet mortar shrinks and cracks as it dries. Mix only what you can use within the working time on the bag, and never re-temper stiffened hydraulic lime with more water.

Step 4: Dampen and Pack the Joints

1. Dampen the joint. Mist the raked joint and surrounding stone an hour or so before and again just before pointing. The surface should be damp but not shiny wet. Dry stone sucks water out of lime and weakens it.
2. Fill deep areas first. In joints deeper than about 1 inch, pack mortar in layers no thicker than 3/8 to 1/2 inch, pressing each firmly with the pointing iron. Tuck small stone chips (pinning stones) into very wide voids to reduce the mortar volume, as the original builders did.
3. Work from the top down. This keeps droppings from falling onto finished joints.
4. Pack, do not smear. Press mortar off the hawk into the joint with a narrow iron, compressing it against the back and sides. Keep mortar off the stone faces.
5. Fill flush or slightly recessed. On rubble and fieldstone, stop the mortar slightly back from the stone edges so the stone reads clearly and water sheds off the face. Avoid feathering thin mortar over stone edges, which cracks and looks smeared.

Step 5: Finish and Cure

Wait until the mortar is thumbprint hard, stiff enough that a firm press barely marks it. This can be an hour or several, depending on weather. Then beat or brush the joint with a stiff bristle brush to compact the surface and expose the sand grains. The textured finish looks traditional and helps the lime carbonate.

Curing is where lime work succeeds or fails. Protect fresh pointing from direct sun, drying wind, rain and frost for at least a few days, ideally longer, by hanging damp burlap with plastic sheeting over it, kept an inch or two off the wall. Mist the burlap to keep the joints slowly damp. Avoid working when temperatures are expected to fall near freezing within a week or so, or above roughly 85 degrees F in full sun. Spring and early fall are the best seasons.

Working Safely at Height

Many stone walls and chimneys go well above head height. Use a sturdy ladder on firm, level ground with the top secured, keep three points of contact, and avoid carrying heavy buckets up the ladder. For long runs, a scaffold tower with guardrails is safer and much faster than moving a ladder repeatedly. Leave two-story walls, gable ends and chimneys to a masonry contractor with proper scaffolding.

Troubleshooting

• Cracks along the joint edges. The mix was too wet, the joint dried too fast, or the stone was not dampened. Rake out and redo with a stiffer mix and better curing.
• Powdery surface. The mortar dried before it could set or froze while curing. Rake out the soft layer and repoint.
• Mortar stains on the stone. Let smears dry, then brush them off. Avoid acid cleaners, which can etch limestone and marble.
• Color too dark or too light. Make small test patches with different sands and let them cure a week before committing.

Cost and When to Hire a Mason

DIY materials are inexpensive: a bag of natural hydraulic lime and several bags of sand cover a surprising area, typically well under a few hundred dollars for a garden wall. Professional repointing of stone walls commonly runs from the high single digits to $25 or more per sq ft, depending on joint width, access, height and the difficulty of removing old cement mortar. Historic buildings with specialty mortar matching sit at the top of that range.

Call a masonry contractor experienced with lime when a wall bulges, leans, has loose stones or cracks through the stone, or when it retains soil more than about 3 to 4 feet high. Those are structural issues. Hire out for work above one story and for historic or listed buildings, where local rules may govern materials and methods.