A dry stone wall stands without mortar by gravity and friction: two battered faces of interlocked stones with rubble packed between them, tied together by long through stones and capped with flat stones. The parent rule that the base should be at least a third of the height is too thin if the wall retains soil: a 1.5 m wall 0.5 m thick has safety factors of only 1.06 against sliding and overturning, while a wall 1.0 m thick has 2.1 and 4.2.
Dry stone walling is the laying of unmortared natural stone into walls and enclosures, held together by the weight of the stone and the way it is placed.
How A Dry Stone Wall Holds Together
The parent article gives the principles. Two parallel faces are built with rubble fill between them and each face leans slightly inward (the batter). Long through stones span the full width every 1 to 1.5 m vertically and horizontally, tying the faces together. Joints are broken: no vertical joint sits above another. The wall is capped with flat stones. It needs no mortar, but it needs time: Fence Types gives about 3 to 4 feet of 4-foot wall a day for a skilled waller.
Method: Face, Fill And Tie
Step 1: Dig to firm ground. Strip the topsoil and set the first course on firm, level, drained ground; the foundation principles of Stone Foundation apply (every stone bridging the joint below, a slight inward batter). Put the largest stones at the base.
Step 2: Lay the faces. Set face stones with their longest dimension running into the wall, not along it, so each is held by the weight above it. Tilt each slightly inward. Use a string line between two upright markers for the line, and a template of the batter to check the lean.
Step 3: Pack the heart. Fill the space between the faces with small stones, packed tightly by hand, not tipped. Loose heart is what lets a face bulge.
Step 4: Set through stones. Place them every 1 to 1.5 m, as the parent article specifies, across the full width. They are the stones that stop the faces separating.
Step 5: Break the joints and cap. Offset every course. Finish with flat stones set on edge or laid heavily, to shed water and bind the top. Overhanging the cap makes climbing harder (parent article) but also makes the cap easier to pull down; do not overhang by more than a few centimetres.
Worked Example: Is A Third Of The Height Enough?
The parent article says the base should be at least a third of the height, giving a 1.5 m wall a base of 50 cm. Walls — Stone and Earth describes a wall 1 m thick and 1.5 m high. The two differ by a factor of two. For a freestanding wall with no sideways load, either stands; the real stresses are internal (a bulging face, a loose heart). For a wall that retains soil, use the method in Retaining Wall Design with its assumed inputs: H = 1.5 m, wall unit weight 18 kN/m³ (dry stone with voids), retained soil 17 kN/m³ with Ka = 0.333, base friction 0.5, drained backfill.
- Thrust P = ½ x 0.333 x 17 x 1.5² = 6.38 kN per metre. Overturning moment = 6.38 x 0.5 = 3.19 kN·m.
- t = 0.5 m: W = 0.5 x 1.5 x 18 = 13.5 kN; Mr = 13.5 x 0.25 = 3.38 kN·m. FS(overturning) = 3.38 / 3.19 = 1.06; FS(sliding) = 0.5 x 13.5 / 6.38 = 1.06.
- t = 1.0 m: W = 1.0 x 1.5 x 18 = 27 kN; Mr = 27 x 0.5 = 13.5 kN·m. FS(overturning) = 13.5 / 3.19 = 4.2; FS(sliding) = 0.5 x 27 / 6.38 = 2.1.
The textbook minimums quoted in that article are about 1.5 for sliding and 2 for overturning. The one-third wall fails both as a retaining wall; the 1 m wall passes both. These are textbook formulas applied to assumed values and were not tested on stone walls.
What The Wall Does Not Do
- It does not resist being pulled apart. Loose stones can be lifted off the top by hand, and a dry stone wall can be dismantled stone by stone. A mortared wall is harder to take down; see Mortared Stone.
- It gives missiles to the other side. Spilled stone from a damaged wall is throwable.
- Its stopping power is untested. The parent article table gives “Good (60cm+)” for dry stone with no threat named, while the settlement ballistic table lists stone at 20 to 30 cm; the two disagree. Neither names a cartridge or range, and this corpus holds no test data. See Defensive Architecture.
Common Mistakes
- Face stones laid lengthways. They slide out. Lay them into the wall.
- Hearting tipped in. Voids let the faces bulge. Pack it by hand.
- No through stones. The faces peel apart.
- Building on topsoil. It settles unevenly and the wall splits.
- Using the one-third rule for a retaining wall. See the arithmetic above.
Walls fall and stones crush
A dry stone wall can shed a face without warning when a through stone is missing or the footing has moved. Do not stand on or lean against a freshly built or damaged wall, never put hands under a stone being set, and keep children and animals away while it is built. Wear eye protection when knapping.
What’s Next
- Learn next: Mortared Stone — bonding the stones with mortar
- Combine with: Gabion Baskets — a faster stone wall