7 Common Mistakes When Building a Dry Stone Wall (and How to Avoid Them)

Dry stone walling is forgiving in small ways and unforgiving in large ones. A slightly uneven face stone, a course that wanders a few millimetres, a stone that needed an extra tap with the hammer — none of that matters much. A soft footing, a vertical face or a ragged top does. Most walls that lean, bulge or come down within a few seasons fail because of one of a small number of repeated mistakes, and almost all of them are made either in the first hour of work or the last. Here are the ones that turn up again and again, and what to do instead.

Starting with a Footing That Isn't Fit to Build On

A wall is only as good as the ground under it. The footing is not a formality to be hurried through before the interesting part begins; it is the part that decides whether the wall is still standing when your grandchildren are arguing about who inherits the field.

The principle is simple: strip everything soft and organic until you reach ground that doesn't move when you stand on it, then build up from there with your largest, flattest stones. Topsoil has no place under a wall. It compresses, it holds water, and it lets frost lift the whole structure a few millimetres at a time.

1. Set out the line with a taut string and mark the width — the footing should be slightly wider than the base of the wall.
2. Strip turf and topsoil, and dig down to firm subsoil. On a slope, cut the footing in steps so each length sits level.
3. Level and firm the bottom. On heavy clay, a slight fall to one side helps water escape.
4. Lay the foundation course: the biggest, flattest stones you have, broad face down, no rocking. Tap each one until it sits dead.

Never build on old patio slabs, builder's rubble, tree roots or made-up ground and hope for the best. A free-standing dry stone wall generally works perfectly well on a well-prepared stone footing; a wall that retains ground is a different job altogether and needs proper design.

Choosing Stone by Eye Rather Than by Job

Almost every bad wall has the same ingredient list: rounded cobbles, thin slate, soft flaky sandstone, a few bricks, and a bag of concrete rubble. Rounded stones roll. Thin stones shatter. Bricks are too regular and too small. Rubble has no bed.

You need three grades of stone, and you need them sorted before you start laying.

• Foundation stones: large, flat, heavy, with at least one good bed face.
• Face stones: two roughly parallel faces, so each stone sits down properly and shows a clean face on both sides.
• Hearting: small chips and spalls for packing the middle. Never throw this away — it is the mortar of a dry wall.

Sort stone into piles close to the wall line as it arrives, not as you build. You will handle every stone several times otherwise, and the temptation to grab the nearest one is what produces a wall of mismatched lumps. Reclaimed stone with old mortar on it is fine once it is cleaned; mortar left on the faces stops stones bedding down tightly and never stops moving.

Losing the Batter

Batter is the deliberate inward lean of the wall face. It is what turns a stack of stones into a structure: the weight of the wall pushes down and inwards, and the wall settles tighter rather than spreading. A common guide is roughly 1 in 6 — about 50mm of lean for every 300mm of height — though the exact figure matters less than having some.

The classic error is to tilt each stone backwards. Don't. Keep the beds level and step the face back course by course, so the face of the wall forms a continuous slope. Tilting individual stones gives you a ragged, unstable face where water sits in the hollows.

Check the batter as you go with a line and a level, or make a simple batter frame: a straight edge with a small block at the foot. A bulge is easy to fix at course three and impossible at course thirteen.

Forgetting Through Stones

Through stones span the full width of the wall, from face to face, tying the two sides together. Without them you are effectively building two thin walls side by side and hoping they stay friends. One leans, the other doesn't, and the wall splits straight down the middle.

As a rule of thumb, put in a band of through stones roughly every metre along the wall, and at least one band in every metre of height, with a row just below the coping. They must genuinely go through. A stone that reaches halfway is just a face stone telling stories.

Then fill the middle. Every gap gets hearting, packed in with the edge of the hammer, no voids anywhere. A wall with hollow pockets is a wall waiting for a hard frost and a wet winter to work it loose.

Running Joints and Bad Bedding

Joints must be staggered. If a vertical joint runs three or four stones high, that line becomes a hinge, and the wall will fold along it. Keep the rule in your head as you work: one on two, two under one. Each stone should sit on at least two stones below it, and bridge the joint between them.

Other quiet killers:

• Balancing a stone on its narrow edge because it looks good from the front.
• Putting a small stone under a big one, which creates a pivot.
• Laying a stone with its rounded or weathered side down, so it never seats.
• Using one oversized lump where two smaller stones would bed far better.

Chipped corners and small spalls are not waste. They are how you level a stone without weakening it.

Getting the Coping Wrong

The coping is the roof and the clamp at the same time. It keeps water out of the heart of the wall and its weight presses the whole structure down. Getting it wrong undoes a lot of good work below.

Vertical coping stones should be set on end, crossways across the wall top, tightly packed, bedded on hearting so nothing wobbles, and large enough that they genuinely cover the width of the wall. Coverband coping — big flat stones laid across the top — works too, as long as the joints are staggered and the stones overlap generously.

What you must not do is finish the wall with whatever is left in the pile. Small, gappy, ragged tops let water straight in and add no weight. If you can lift a coping stone with two fingers, it is the wrong stone.

One more thing: don't mortar the coping onto a dry stone wall. The wall needs to move. Mortar stops it moving, and the mortar is what cracks first.

Finishing Well, and Knowing When to Stop

Build in short lifts and work along the wall rather than piling up towers in one spot. Check your line, level and batter every couple of metres, not every ten. Keep the top of the work reasonably flat as you go, so you are always adding to a stable surface rather than a peak.

When you are taking down an old wall, work from the top. Never pull stones out of the base of a standing wall to see what is underneath — that is how people get hurt.

Two sensible cautions. Walling is heavy, repetitive work, so lift with your legs, keep sessions short, and take breaks before your back tells you to. And if the wall will retain ground, stand over a metre high, or run beside a road or public footpath, take advice from a structural engineer or your local authority before you start. The rules vary by place, and a wall in the wrong spot is an expensive thing to move.

Beyond that, the recipe is slow and unglamorous: firm footing, sorted stone, steady batter, through stones, tight hearting, staggered joints and a heavy coping. Do those and the wall will still be doing its job long after you have stopped thinking about it.

Photo: sakulich / Pixabay