Rendering of external walls

A historical overview

The use of render on external walls is an ancient craft based on traditional methods that have been handed down over the centuries and mainly make use of locally sourced construction materials, binders and additives. It wasn’t until 100 years ago or so that the first attempts at standardization were made and rendering became a subject of scientific research. This work was prompted by an increase in rendering failure that coincided with a radical change in building methods: walls were now being constructed using large-format blocks of lightweight concrete instead of small-format, solid bricks laid in courses consisting of alternate stretchers and headers, in patterns known as bonds.

The first research dates back to the 1940s, when surveys were conducted throughout the then German Reich to document the rendering techniques commonly used in different areas of the country (render structure/layers, binding materials, grade of sand). This was the first step toward the new field of plaster/render research. The results were published in 1950 in a book entitled “Außenputz für Massivwände” (author F. Kaufmann, Bauverlag Wiesbaden), which includes guidelines for applying render to solid masonry such as the following:

“The initial coat of render must be sufficiently damp and rough to provide a key when applying the final coat. The final coat must not be harder than the undercoat.”

This requirement was subsequently reformulated and incorporated in DIN standard 18550, published in 1955, as follows:

“As a basic rule, the initial coat of render must be at least as hard as the final coat.”

When a revised version of the standard was published 30 years later (DIN 18550-2, 1985), the sentence was reworded as follows:

“The hardness of the final coat should normally be lower than that of the undercoat, or the hardness of both layers should be equal.”

And the following requirement was added:

“This rule applies by analogy to the relationship between the hardness of the underlying masonry surface and that of the initial coat of render.”

These plastering rules (sometimes referred to as “soft on hard”), based on the experience of generations of craftsmen who only knew solid masonry, are still being followed to the letter by mortar manufacturers today, and any products that deviate from the rules are reluctantly approved as exceptions, even if they produce good results in practice. But since these rules were established, the criteria used to judge the performance of building systems for external walls have changed. Thermal insulation has become the most important consideration, rather than load-bearing capacity and stability. Modern building techniques call for large-format lightweight masonry blocks that provide a high level of thermal insulation with thin mortar joints that minimize thermal bridging effects. In a certain sense, this means that walls are now less stable and the render has to compensate for any deformation that might occur. To prevent cracking, the render must be capable of accommodating changes in the underlying surface. At the same time, priority is given to other aspects in new render systems that have been developed to provide specific functional properties. Such changes in the requirements and execution of rendering systems are described in the publication “Wandlungen in den Anforderungen und der Ausführung von Außenputzen” [1].

Development of render systems

The older types of render for conventional masonry, which mainly serve decorative or finishing purposes, can be classified as fair-faced render, by analogy with the term “fair-faced brickwork” for unrendered walls. The stipulations of DIN 18550 relate mainly to these types of render.

Water-repellent render

It is impossible to produce a sufficiently reliable hydrophobic render from hand-made mortar mixed at the building site using binding materials and sand. It wasn’t until factory-mixed mortars composed of precisely dosed ingredients and additives came into widespread use that it became possible to produce truly water-repellent render. After extensive studies to determine the necessary hygroscopic properties of render systems, two key characteristics were defined, namely the water absorption coefficient w and the diffusion resistance sd. These characteristics must not exceed specific values [2]. After many years of field tests and technical reviews, the requirements for water-repellent render were officially standardized in DIN 4108-3 (1981) and DIN 18550-1 (1985), which classify these and other special coating systems according to the required protection against rain. The example in Figure 1 compares the effect of a water-repellent render system and a lime cement render system.

Figure 1: Change over time of the moisture content of a west-facing porous concrete wall (building at the outdoor test site in Holzkirchen) coated with a conventional and a water-repellent render system, and the corresponding data on the intensity of driving rain [2].

Flexible, crack-resistant render systems

The afore-mentioned “instability” of large-format blockwork with thin joints was first observed in walls built of lightweight and porous concrete blocks. Initially it was thought that the damage was caused exclusively by shrinkage of the concrete blocks. But later, when similar cracks also started to appear in rendered walls made of pore-formed lightweight bricks, which was attributable to the bricks’ heterogeneity, it was recognized that the cause of cracking in all of these cases was differences in the standard compressive strength and the shear strength or transverse motion of the masonry [1]. The basic difference between the traditional stable base for rendering and today’s less stable base of lightweight blocks is shown in Figure 2. If the underlying structure is stable, the render is normally only exposed to external loads arising from weather factors, whereas if the underlying structure is less stable, the render may additionally be subject to tension stress caused by dimensional changes in the masonry, which can cause cracking. To avoid the appearance of visible cracks in the final coat of render, an intermediate layer of low-shear-strength material must be applied between the masonry surface and the top coat of render. Lightweight base renders compliant with DIN 18550-4 have proved suitable for this purpose. Existing studies show that the softer the base coat and the harder the final coat, the greater the anti-crack effect – in other words, it responds to the rule of “hard on soft”, as illustrated in Figure 3.

Figure 2: Adaptation of render systems to the stability of the underlying structure. The old “soft on hard” rule applies if the masonry is stable (left), but a flexible, anti-crack render system is required if the masonry is less stable (right).


Figure 3: Crack resistance of render systems consisting of a base coat of commercially available lightweight renders or thermal insulation renders with a final coat of varying hardnesses, measured as a function of the hardness of the final coat (the figures at the measurement points represent the drilling hardness – the higher this number, the harder the render). The softer the base coat and the harder the final coat, the greater the crack resistance.

Thermal insulation render and external thermal insulation composite systems (ETICS)

As well as providing additional, external thermal insulation, these products also have an anti-crack effect. However, the configurations prescribed by DIN 18550 are contradictory: In thermal insulation render systems, the final coat is harder than the base coat, whereas in composite systems the render is harder than the substrate. Nonetheless, both types of system are well suited as a long-term repair solution to the problem of cracking due to dimensional changes in the masonry. Cracks caused by movement of the masonry are observed far less frequently in the finishing render of such thermal insulation systems than in conventionally rendered walls [4]. The layer of thermal insulation or the thermal insulation render provides greater resistance to cracking due to dimensional changes in the masonry than lightweight render systems. It should be noted that the use of mesh to reinforce the structure is effective only if it is in direct contact with a “soft” layer or coating [5]. The stability or instability of the underlying masonry is defined on a sliding scale, as the graphic in Figure 4 shows. The render or thermal insulation systems should be chosen according to the relevant point on this scale. The long-term performance of composite systems has been the subject of numerous in-situ tests, including tests where they were used to renovate buildings up to forty years old. The results show that the lifespan of such systems is comparable to that of conventionally rendered façades [6], subject to the condition in both cases that they are regularly maintained and repaired when necessary.

Figure 4: Diagram showing the correlation between render systems and the stability of the masonry.

Remedial rendering

In the renovation of old buildings, efforts to combat the phenomenon known as “rising damp” have often led to unsatisfactory results. Part of the reason for this is that the word “rising” is inaccurate. Rather than describing the source of the problem, in many cases it simply relates to a set of symptoms that can be explained by a variety of causes, which are dealt with in more detail in [7]. One of the main causes for the appearance of damp patches on walls is a high concentration of soluble salts in the masonry. However, contrary to popular belief, this is not caused by the upward movement of moisture (aka “rising damp”) but by water that has penetrated the wall from the outside as a result of poor ground and surface drainage or plumbing leaks. Because salt attracts water (i.e. has hydrophilic properties), the affected areas of the wall are damper, which can give rise to salt blooms (efflorescence) and cause damage to the plaster or the wall itself. In such cases, the problem can be solved by remedial rendering [8]. If the walls are already rendered, stripping off the old render also removes a large part of these salt deposits. The subsequent application of remedial rendering, using special products that are slightly hydrophobic and highly permeable to water vapor, allows the wall to dry out principally by means of water vapor diffusion, thus preventing salts from being transported to the outside surface of the wall by capillary action. Figure 5 shows how remedial rendering prevents the accumulation of soluble salts.

Figure 5: Schematic diagram showing how remedial rendering prevents the accumulation of soluble salts. The rendering material has large pores, is slightly hydrophobic and highly permeable to water vapor. Only a small proportion of the salt-laden moisture in the masonry is carried by capillary action into the render, where it is transported to the outer surface of the wall by diffusion. The salts form deposits in the pores of the material, where they can do no damage.

Summary and conclusion

Stucco and other forms of rendering were originally conceived as a means of concealing raw masonry behind a smooth layer of plaster. Rendering has since taken on other functions, as described above, which need to be taken into account in the composition and manufacture of render products and systems. It is no longer possible to apply a single “rule of thumb” that has rightly been called into doubt, as the evidence proves [9]. The different functions of render systems are an important means of compensating for changes in construction methods and preventing damage to buildings. This calls for new standards and regulations based on state-of-the-art knowledge.


[1] Künzel. H.: Wandlungen in den Anforderungen und der Ausführung von Außenputzen.
Das Mauerwerk 4(2000), Nr. 3, S. 95-102
[2] Künzel, H.: Der Regenschutz von Außenwänden.
Mauerwerk-Kalender 1986, S. 735-751
[3] Künzel, H.: Außenputze – früher und heute.
Baumarkt 1994, H. 9, S. 26-28
[4] Künzel, H.: Warum sich WDV-Systeme durchgesetzt haben.
Bauphysik 20 (1998), H.1, S. 2-8
[5] Künzel, H.: Wie funktionieren Armierungsputze.
Der Bausachverständige 10 (2014), H. 4, S. 18-21
[6] Künzel, H.M., Künzel, H., Sedlbauer, K.: Hygrothermische Beanspruchung und Lebensdauer von WDV-Systemen.
Bauphysik 28 (2006), H. 3, S. 153-163
[7] Künzel, H.: Problembereich aufsteigende Feuchte.
Bausubstanz 2014, H. 4, S. 34-40
[8] WTA-Merkblatt 2-9-04D Sanierputzsysteme.
Oktober 2005
[9] Künzel, H.: Putznormung – quo vadis? Zur europaweiten Vereinheitlichung der Normung von Außenputzen.
Der Bausachverständige 2014, H. 6. S. 21-25

Combined publications on the topic:

Künzel, H.:       
Außenputze – früher und heute.
Stuttgart: Fraunhofer IRB Verlag, 2014
Künzel, H.: Bautraditionen auf dem Prüfstand. Die Entwicklung der Bauphysik im Spannungsfeld zwischen Tradition und Forschung.
Stuttgart: Fraunhofer IRB Verlag, 2014
Künzel, H.: Schäden an Fassadenputzen, 3. Auflage.
IRB-Verlag Stuttgart 2011
Künzel, H.: Bauphysik und Denkmalpflege, 2. erweiterte Auflage.
Fraunhofer IRB-Verlag 2009


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Prof. Dr. Hartwig Künzel

Fraunhofer Institute for Building Physics IBP
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