What is rammed earth (pisé)?

Rammed earth — French pisé de terre — is a building technique in which moist subsoil is placed in layers within formwork and compacted into a solid, load-bearing wall. No cement, no firing and virtually no transport are involved: in its classic form the soil comes from the site or nearby.

How it works

1. The mix. Suitable earth is a balanced blend of gravel, sand, silt and clay. The clay is the binder; too little and the wall dusts away, too much and it cracks as it shrinks. Overly clayey soil is therefore leaned with sand or gravel.
2. Moisture content. The earth must be damp: squeezed in the hand it forms a ball that breaks apart when dropped. Too wet will not compact, too dry will not bind.
3. Ramming. Earth is placed in layers of about 10 to 15 cm and compacted with a hand or pneumatic rammer to roughly two-thirds of the original layer thickness. The formwork only rises once a layer is finished.
4. Drying. The wall is visible as soon as it is struck, but goes on drying for weeks or months. Only then does it reach full strength.

Properties

• Compressive strength. Rammed earth typically achieves in the order of 1 to 4 N/mm² — enough for load-bearing walls in low-rise work, not for slender structures.
• Wall thickness. In practice usually 40 to 60 cm; thinner is rarely sensible structurally or thermally.
• Thermal mass, not insulation. Thermal conductivity is broadly 0.8 to 1.3 W/m·K. An earth wall buffers heat superbly but insulates poorly. In a climate with real winters an added insulation layer is unavoidable.
• Moisture buffering. Earth absorbs moisture from the air and releases it again, keeping the indoor climate steady.
• Fully reusable. Untreated earth can be rewetted and reworked at the end of its service life.

Applications and points to watch

Pisé is centuries old and used worldwide; in Europe particularly in France and Germany, where the technique saw a revival in the nineteenth century and is now used again in sustainable projects, internal walls and acoustic walls.

• Good boots and a good hat. The classic rule: a stone or concrete plinth lifting the wall above the splash zone, and a generous roof overhang to keep driving rain off.
• Never seal it with a vapour-tight finish. Cement render, plastic paint or a sealed membrane traps moisture in the wall and causes damage. Use lime render, earth plaster or limewash.
• Deal with openings structurally. Doors and windows need a lintel; earth takes no tension.
• Labour intensive. Ramming takes time and hands. Material cost is low, labour cost is not — and that is what drives the price.

Protecting a rammed earth wall

• Stone or concrete plinth: Lifts the wall above the splash zone.
• Generous roof overhang: Keeps driving rain off the wall.
• No vapour-tight finish: Cement render, plastic paint or a sealed membrane traps moisture and causes damage.
• Breathable finish: Use lime render, earth plaster or limewash.
• Lintel over openings: Doors and windows need one, because earth takes no tension.

Related terms

• Clay and earth
• Earth block
• Earth plaster
• Solid wall construction
• Thermal mass

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