Clay plaster, also called earth plaster, is a plaster of loam, sand and usually a fibre such as straw, hemp or cellulose, applied to internal walls and ceilings. The coat does not set chemically but dries out; the binder is the clay naturally present in the loam.
That makes it fundamentally different from gypsum- or cement-based plaster. It is less hard, but it can be re-wetted, reworked, and at the end of its life reused in full. Factory-produced earth plaster mortar is covered by the German standard DIN 18947, which is widely referenced elsewhere.
Why it is used
- Moisture-buffering indoor climate. Loam takes up excess water vapour quickly and gives it back as soon as the air dries. In a clay-plastered house the relative humidity stays noticeably more stable, which reduces condensation on cold surfaces.
- Vapour open. The coat does not seal the wall; on earth block, calcium silicate or timber-frame construction the behaviour of the whole build-up is preserved.
- No solvent emissions and no binder that has to be fired — the environmental impact is very low.
- A matt, gently varied appearance in natural colours; clay plaster is available coloured through, so it need not be painted.
Coat build-up
- Bonding coat or primer. On smooth or weakly absorbent backgrounds, a clay bonding slurry or a quartz-filled primer.
- Base coat, typically in the order of 10 to 20 mm, with a coarser fibre; ruled flat. On large or crack-prone areas, bed a reinforcing mesh into this coat.
- Finish coat, in the order of 1 to 5 mm, with fine sand and optionally pigmented or with a decorative aggregate.
Stick to the manufacturer’s thicknesses: applying too thick in one pass gives shrinkage cracks and a coat that dries badly.
Suitable backgrounds
- Earth blocks, calcium silicate, brick, aerated concrete, reed and wood-wool cement board — these absorb and give good adhesion.
- Plasterboard: only with the correct primer, and not in wet rooms.
- Concrete and smooth cement render: only with a bonding coat or a roughened surface.
- Not suitable without preparation: existing emulsion paint, old grease layers, and any coat that is not itself soundly bonded. Remove or key it, or the clay will come away complete with the layer beneath.
Application
- Work above 5 °C and keep the air moving; loam only dries by carrying moisture away.
- Ventilate, but do not point a dehumidifier or heater at it. Drying the surface too fast causes shrinkage cracks and a weak top layer.
- Allow longer drying times than gypsum plaster, especially in winter and with thick coats.
- Work the damp surface with sponge, felt float or steel trowel; each treatment gives a different texture. Practise on a sample area — the final appearance is largely craft work.
- It can always be patched. Dampen a damaged spot, abrade it and refinish; the repair merges into the existing coat.
Points to watch
- Not in the splash zone of a shower or bath; that calls for tiling or a water-resistant finish.
- Limited scratch resistance. In a hallway, a child’s bedroom or along a staircase, a harder finish coat or a clay-based paint is the sensible choice.
- Alkaline paint and cement mortar do not belong on it; choose a vapour-open lime or clay paint if you do want to decorate.
- Raking light. Walls beside large windows show every irregularity; take extra care ruling the base coat flat.
Related terms
- Loam
- Adobe block
- Scraped render
- Facade render
- Polished concrete render
- Sustainable building
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