Lehmunterlage

Lehmunterlage refers to a layer of clay used as an underlay or bedding material in construction and historical building conservation. Typically composed of finely ground natural clay mixed with small amounts of sand, lime, or organic stabilisers, the material is applied to a substrate before a primary finish such as stone, brick, or plaster is laid. The clay layer serves several purposes: it creates a moisture barrier, reduces differential thermal movement, and improves adhesion of the topcoat to the support wall. In many European vernacular buildings, Lehmunterlage was traditionally applied over lime-rendered walls to prepare the surface for the installation of floor tiles or stone walls, especially in regions where the climate demands a moisture-resistant barrier to protect the structural fabric.

In modern applications, Lehmunterlage is used in both restoration projects and new construction where historic authenticity is required. Its low porosity compared with pure clay makes it suitable for moisture buffering, while its plasticity allows it to accommodate minor structural shifts. When mixed correctly, it sets into a hard, non-porous layer that resists delamination of the cover material. However, improper mixing ratios can lead to cracking or weak adhesion, and the material typically must cure for several days before the next layer can be applied. As such, the use of Lehmunterlage demands careful judgment of environmental conditions, substrate type, and the properties of the overlying material.