External plasters through the ages (render on external walls)

Temporal progression of the moisture content of a west-facing exterior wall made of cellular concrete
© Fraunhofer IBP
Figure 1: Change over time in 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]
Assignment of render base and render system
© Fraunhofer IBP
Figure 2: Assignment of plaster base and plaster system. For stable plaster base the rule "soft on hard" applies, for less stable base a "decoupling plaster system" is required.
Anti-crack dimensions of commercially available lightweight renders and some insulation renders
© Fraunhofer IBP
Figure 3: Decoupling dimensions of commercially available light plasters and some insulation plasters with differently hard finishing plasters depending on the hardness of the finishing plaster (numbers at the measuring points; the hardness was indicated by the drilling hardness; the larger the number, the harder the plaster). The softer the light plaster and the harder the finishing plaster, the better the decoupling.

Plastering exterior walls is a traditional craft that has developed over centuries based on experience with locally-sourced wall building materials, binders and additives. It was only about 100 years ago that rendering became a subject of scientific research.  This was prompted by an increase in plastering problems that coincided with a radical change in building methods: walls were now being constructed using large blocks of lightweight concrete instead of small solid bricks laid in courses consisting of alternate stretchers and headers.

The first research dates back to the 1940s, when surveys were conducted throughout what was then the German Reich to document the plastering techniques commonly used in different areas of the country (structure/layers, binding materials, grade of sand). This was the first step toward the start of plaster/render research. The results were published in 1950 in a book entitled "Außenputz für Massivwände" (author F. Kaufmann, Bauverlag Wiesbaden). In the guidelines derived from this, it is stated that:

"The initial coat of render must be sufficiently damp and rough to provide a key when applying the final coat. The final coat may 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.”

The following requirement was also 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”) are based on the experience of generations of craftsmen who only knew solid masonry. These rules are still being followed to the letter by mortar manufacturers to this day, and any products that deviate from them are reluctantly approved as “exceptions” if they stand the test. In the meantime, however, the criteria used to judge the performance of building systems for external walls have changed. The focus is no longer on load-bearing capacity and stability, but on thermal insulation. 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.  To a certain extent, this makes walls less stable and the plaster has to compensate for any deformation that might occur. To prevent cracking, it 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. This development is described in the publication "Changes in the requirements and execution of exterior plasters" [1].


Development of render systems


Fair-faced render

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 non-plastered walls.   The stipulations in 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 made with binding materials and sand at the building site.  It was only when factory-mixed mortars composed of precisely dosed ingredients and additives came into widespread use that it became possible to make truly water-repellent plasters.  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 may 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 with a lime cement render. 


Flexible, crack-resistant render

The aforementioned “instability” of walls made from large-format blocks with thinner layers of mortar was first observed in walls built with lightweight and porous concrete blocks. Initially it was thought that the damage was due exclusively to the concrete blocks shrinking. The above-mentioned 'instability' in masonry made of larger blocks with 'sparing' grouting initially occurred in particular in walls made of lightweight concrete and aerated concrete blocks, with shrinkage of the blocks being assumed to be the sole cause of the damage. But later, when similar cracks also started to appear in plastered walls made of porous lightweight bricks, which was attributable to the bricks' heterogeneity, it was recognized that the cause of cracking in all of these cases was due to differences in the standard compressive strength and 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 stress due to the weather. However, if the underlying structure is less stable, the render may also be subjected to tension stress caused by dimensional changes in the masonry, which may lead to cracking.  To prevent visible cracks from forming in the final coat of plaster, an intermediate layer of low-shear-strength material must be applied between the surface of the masonry and the topcoat of plaster.  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 equates to the rule of “hard on soft”, as illustrated in Figure 3.

Correlation between render systems and the stability of the underlying masonry
© Fraunhofer IBP
Figure 4: Diagram showing the correlation between render systems and the stability of the underlying masonry.
Diagram illustrating the effects of remedial rendering
© Fraunhofer IBP
Figure 5: Diagram illustrating the effects of remedial rendering. Remedial render has large pores, is highly permeable to water vapor and slightly hydrophobic. Only a small proportion of the salt-laden moisture in the masonry is transferred 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.

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, their structure differs to that prescribed by DIN 18550:  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 solution for repairing cracks caused by dimensional changes in the masonry.  Cracks due to 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 is more resistant 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 only effective 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 illustrated in the graph in Figure 4. The render or thermal insulation system 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], provided they are regularly maintained and repaired.


Remedial rendering

When renovating 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 adjective “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 of 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 moisture “rising” but rather 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. is hydrophilic), the affected areas of the wall are damper, which may 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, a large part of these salt deposits can be removed by simply stripping off the old plaster.  The subsequent application of a remedial render, 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 outer surface of the wall by capillary action. Figure 5 shows how remedial rendering prevents the accumulation of soluble salts.


Summary and Conclusion

Aus der ursprünglichen Aufgabe eines Putzes als äußere Bekleidung eines massiven Mauerwerks sind im Laufe der weiteren Entwicklung – wie dargelegt - zusätzliche Aufgaben entstanden, welche bei der Zusammensetzung und Herstellung der Putze berücksichtig werden müssen. Dies kann nicht mehr durch eine einzige »Grundregel« erfolgen, deren Sinnhaftigkeit bezweifelt und widerlegt werden kann [9]. Die verschiedenen Funktionen eines Putzes sind wichtig, um entwicklungsbedingte Änderungen im Mauerwerksbau auszugleichen bzw. um Schäden zu vermeiden. Hierzu sind Normen und Richtlinien erforderlich, die den Stand des Wissens berücksichtigen.


[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

Summarized descriptions of the subject area covered in the following publications:

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