No building material suffers from a prejudice so disproportionate to its actual behaviour as raw earth. The idea that it's "not very durable" almost always arises from observing raw-earth buildings poorly protected from water, not from any intrinsic limit of the material: a rammed-earth or adobe wall correctly designed and kept dry doesn't degrade chemically, doesn't oxidise, doesn't carbonate, doesn't undergo alkali-aggregate reactions — in ideal conditions, it is essentially eternal. The problem is never the structural load itself. It's almost always water, in one of its three pathogenic forms: direct driving rain, capillary rise from the ground, or prolonged saturation that wipes out its residual strength.
Erosion from driving rain: the most visible enemy
Rain that hits an unprotected raw-earth surface directly progressively washes away its surface clay matrix, exposing the coarser aggregate and creating a characteristic eroded profile — a phenomenon visible to the naked eye on any earthen wall left without maintenance. The erosion rate depends on rainfall intensity and frequency, on the particle-size distribution of the earth (an earth with more clay is generally more cohesive but also more prone to shrinkage cracking, which in turn accelerates water ingress), and, decisively, on whether architectural protection is present. An insufficient roof overhang is the most common and most easily preventable cause of accelerated decay: the English and Breton cob-building tradition sums it up in a saying used by local craftsmen — a raw-earth wall needs "a good hat and good boots", that is, a roof with a generous overhang (a minimum of around 40 cm is recommended in current practice) and a draining foundation that isolates it from ground water.
Capillary rise: the invisible enemy
Less visible but equally destructive is capillary rise of water from the ground through the base of the wall, a phenomenon that, absent a physical barrier (a waterproof plinth course, a draining slab, a raised foundation plinth), can climb tens of centimetres into the masonry, progressively saturating the earth from the bottom up regardless of rainfall. The combination of capillary rise and freeze-thaw cycles in temperate climates — absent from the arid climates of origin of many traditional techniques such as Andean or North African adobe — is one of the reasons why adapting raw earth to the European context requires a level of construction detail at the base of the wall far more thorough than the original tradition ever needed in its own climate.
Residual strength under saturation
The most critical technical fact for a designer is how far raw earth's mechanical strength collapses under saturation: unstabilised raw earth can lose up to 70–80% of its compressive strength when saturated with water, dropping to values in the range of 0.1–0.5 N/mm² against dry values as much as ten times higher. It's a behaviour no structural design can afford to ignore: the guiding principle shouldn't be "how much residual strength remains under saturation", but "how to stop the material ever reaching saturation" — shifting attention from the material's mechanical strength to the construction detail that protects it.
Stabilisation as a complementary strategy, not a substitute
Stabilising earth — with lime, cement or more recent additives — reduces its vulnerability to water but doesn't eliminate the need for protective construction details: a CEB stabilised with 5–8% cement achieves higher mechanical strength and better resistance to surface erosion, but remains a porous material that benefits from protection against direct rain. The mistake to avoid is treating chemical stabilisation as a substitute for good detail design — overhang, plinth course, compatible plaster — when in fact the two strategies are complementary: stabilisation reduces the material's sensitivity to whatever water still reaches it, construction details reduce the amount of water that reaches it in the first place. A well-conceived raw-earth project invests in both fronts, not just one.