A Beginner's Guide to Repointing a Stone Wall

A stone wall built a hundred and fifty years ago is still standing because it was designed to move, breathe and get wet. The mortar between the stones is the part that gives. When it starts to crumble and drop out, water sits in the joints, frost gets to work, and the wall begins to fail from the inside out. Repointing is the fix — but it is also one of the easiest jobs to get badly wrong. Done properly, it adds decades to a wall. Done with a bag of cement and enthusiasm, it can wreck the faces of stones that have survived two centuries of weather.

Why the mortar matters more than the stone

Traditional stone walls are not meant to be waterproof. They absorb rain, then dry out again through the joints. That is why the mortar must be softer, weaker and more permeable than the stones either side of it. It is the sacrificial layer.

Cement-rich mortar is harder than many sandstones and limestones. It also traps moisture behind it. When it eventually cracks, it tends to take the face of the stone with it, and there is no repairing that. A stiff 1:2 cement mix might look neat for a few years, but it is a slow-motion demolition job.

The right material for almost all traditional walls is lime. Non-hydraulic lime putty mixed with sharp sand gives the softest, most breathable result and is the closest match to historic work. Natural hydraulic lime — usually NHL 2 or NHL 3.5 — sets faster and tolerates damp conditions better, which helps on an exposed gable or a wall that never quite dries.

If the wall is listed, or sits in a conservation area, check with your local authority before you touch it. Listed building consent is sometimes required for repointing, and the conservation officer will usually want to agree the mix.

What you will need

You do not need much, but you do need the right things. A pointing trowel, a jointing iron or two (a piece of copper pipe flattened at one end works surprisingly well), a churn brush with stiff bristles, a soft hand brush, a bucket, a mortar board or spot board, a plugging chisel and a lump hammer. Add a spray bottle, dust sheets and a hose.

For raking out and for safety, you will also want eye protection, gloves, a dust mask and long sleeves. Lime is caustic. Wet lime burns are slow, sneaky and unpleasant, and they keep burning until the lime is washed off. If it gets in your eye, rinse thoroughly and get medical advice.

Raking out the joints properly

This is the stage most people rush, and it is the stage that decides whether the job lasts. Aim to remove mortar to roughly twice the width of the joint — usually somewhere between 15mm and 25mm — leaving a square-bottomed channel rather than a shallow scrape.

1. Work in small areas, a couple of square metres at a time. It keeps the work manageable and stops you leaving joints open for weeks.
2. Cut out old mortar with a plugging chisel, working along the joint rather than digging into the stones. A grinder is faster but removes too much, cuts the arrises and throws out silica dust, so think hard before reaching for one.
3. Clear every trace of dust and loose material. A stiff brush, then a hose or spray bottle, gets the joint back to sound mortar.
4. Deal with any loose, cracked or drummy stones before pointing. If you find movement running through a wall, stop and take advice — that is not a repointing problem.
5. Damp the joints before filling. Dry stone sucks the water straight out of fresh mortar, which weakens the bond and leaves you with a crumbly, sandy finish.

Mixing lime mortar

For lime putty, a good general starting point is around one part putty to two and a half parts sharp sand by volume. For hydraulic lime, follow the manufacturer's figures, which are typically around one part NHL to two and a half parts sand. Buy washed, well-graded sharp sand, not builder's sand or a soft, silty material.

Mix the lime and sand dry first, then add water gradually. The finished mortar should hold its shape on the trowel and ball together in your hand without oozing. If it slumps, it is too wet.

Things to leave out

• No cement, in any quantity, for any reason.
• No washing-up liquid, detergent or plasticiser.
• No salt, no frostproofer, no accelerator.
• No mixing more than you can use in an hour or two.

Lime putty mortar actually improves if it is left to stand for a while, and it will keep for days under a damp cloth. Hydraulic lime is less forgiving, so mix it in smaller batches and use it before it stiffens.

Packing the joints

Fill deep joints in two passes. Press the mortar firmly into the joint with the pointing trowel, working from the bottom of the joint upwards and from one side to the other, so there are no voids behind the surface. Take care to keep mortar off the face of the stones — smears set hard and look dreadful.

Let the first layer firm up, then top it up so it sits just slightly proud of the finished level. When the mortar has stiffened to the point where a thumbprint only just shows — leather-hard, in the old language — it is ready to be tooled. On a warm day that might be twenty minutes. On a cold, damp one, considerably longer.

Finishing and curing

Finish with a jointing iron, then tidy up with a churn brush once the mortar has firmed. The churn brush compresses the surface and pulls back the lime, exposing the sand grains and leaving a texture that matches old work far better than a smooth, shiny surface.

Common joint finishes

• Flush: struck level with the wall face, then brushed back. Traditional, understated, and usually the safest choice for rubble stonework.
• Weatherstruck: sloped from the top of the joint out to the stone face, so water runs off. Useful on very exposed walls facing the prevailing weather.
• Bucket handle: a concave curve formed with a shaped iron. Familiar from brickwork, and less often appropriate on irregular stone.
• Beaded: a raised, rounded ridge. It holds water and rarely suits traditional walls, so avoid it unless you are matching existing work.

Whatever you choose, match the rest of the wall, and keep it consistent. Then leave it alone. Lime cures slowly and needs to stay damp, not soaked. In hot, dry or windy weather, mist the joints daily for the first few days and hang damp hessian over exposed elevations. Do not point in frost, direct sun, or rain, and cover the work with sheeting if the forecast turns. Spring and early autumn give the best conditions.

Where to stop and call someone

Do a test patch first — a metre or so in a hidden corner — and leave it a few weeks to see how it cures and how it looks against the rest of the wall. Work in small sections, keep your joints damped, and resist the urge to tidy up with cement.

There are times to hand over, though. Listed buildings and conservation areas need consent and a considered mix. Bulging walls, cracked elevations or long runs of movement need a structural opinion, not a trowel. And high, long elevations are a scaffolding job, and doing them off a leaning ladder is how people get hurt. Get a specialist in for those, and save your energy for the garden wall.

Photo: Michael_Laut / Pixabay