While the concealed plaster always consists of mineral raw materials, there are a variety of options for the top plaster in terms of material and surface design. The properties of the top plaster, in particular water vapour permeability, dry bulk density and strength, must be matched to the properties of the base plaster.
Top plaster Top plaster is usually applied to a mineral base plaster. The properties of the top plaster, in particular water vapour permeability, dry bulk density and strength, must be matched to the properties of the base plaster. A distinction is made between thin-layer and thick-layer finishing plasters. Thin-layer top coats are applied in grain thickness, i.e. usually up to 5 mm. According to DIN 18350 (VOB C), the grain size for thin-layer top coats is at least 3 mm, if this is not the case, additional measures are required, e.g. fabric filling. For normal plasters, the average plaster thickness of the system consisting of bottom and top plaster must be a total of 20 mm, and in some places it is permissible to fall short of up to 5 mm. For single-layer plasters made of mortar, an average thickness of 15 mm applies. Here, too, a shortfall of 5 mm is possible in individual places.
DIN EN 13914-1 contains extensive information on the planning of exterior plasters in Section 6 (Planning recommendations), including with regard to the properties and suitability of the plaster substrate, the resistance of the plaster under various environmental conditions, the corrosion of metals, the occurrence of cracks, the protection provided by architectural design elements, the selection of plasters, the number/thickness/strength of plaster layers as well as the surface formation, colour and structure of exterior plasters.
plaster surfaces: see lexicon entry (right-hand column).
Aggregates and coatings: When applying a coating, the water vapour permeability of the coating must always be matched to that of the top plaster, and the coating must be more vapour-permeable than the top plaster underneath. Other properties can also be achieved either by admixtures in the top plaster itself or by a coating: Hydrophobic substances increase resistance to driving rain, algaecidal/fungicidal active ingredients improve the algae and fungus-inhibiting setting of plasters. However, the manufacturers point out that this is not a permanent effect, but only preventive/delaying measures. When using biocidal products, the product information and hazard warnings of the manufacturers must be observed. Further information on façade coatings can be found on page ► 304 │Facade paints .
Coating of non-pigmented top coats: In the case of non-pigmented top coats, a coating in accordance with DIN 18363 must be applied, which usually consists of primer, intermediate and final coatings in outdoor areas.
Coating of pigmented top plasters: In the case of mineral fine plasters and pasty plasters, a subsequent surface coating is not absolutely necessary. However, it is recommended by the manufacturers in many cases, as otherwise optical cloud formation can occur under certain conditions, e.g. due to calcium carbonate efflorescence, irregularities in application or weather influences (e.g. rain or fog). In this case, one speaks of an equalization coating. Further information on the coating of mineral fine plasters can also be found in the "Leaflet on levelling coatings on fine plasters - colour-levelling coating" (see standards and literature).
Single-layer plasters: If the VOB Part C, in this case DIN 18350 (plastering and stucco work), has been contractually agreed, exterior plasters must always be carried out in two layers in accordance with No. 3.2.4. However, there are also single-layer water-repellent plaster systems made of mortar with approval for outdoor use. These are also described in DIN EN 998-1 as exterior inlay plaster mortar, which must fulfil the same functions required of a multi-layer plaster system. However, smoothly grated single-layer exterior plasters do not generally comply with the recognised rules of technology.
Plinth plaster: Finishing plasters must also have increased resistance to mechanical impacts, moisture and frost in the (plinth) area that is prone to splashing water. For pasty top plasters, products approved by the manufacturers for use as base plaster must be used, as there is no uniform regulation for this in DIN EN 15824.
Plaster on basement walls: Exterior plaster on basement walls must be made of mortar with hydraulic binders. Specifications for the appropriate compressive strength category are contained in DIN EN 998-1. In the area in contact with the ground, waterproofing in accordance with DIN 18195 is also required.
Renovation plaster: In the case of masonry contaminated with salt and/or moisture, a special renovation plaster is applied as a base plaster, see also page ► 302 │Exterior plaster - Plaster. As far as suitable, either the same material as for the plaster can be used as a top plaster, or another suitable (mineral) top plaster that meets the high requirements for diffusion openness.
Reinforcement plaster with full-surface fabric insert: If the exterior plaster is subject to increased stress, it may be necessary to apply a reinforcing plaster with a full-surface fabric insert as an intermediate layer between the base and top plaster in order to prevent cracking. Increased stresses result
In any case, areas with material changes, e.g. insulated ceiling edge areas in a brick wall, must be provided with a reinforcing plaster with fabric insert, with at least 200 mm overlapping on the adjacent wall areas on the overleaf.
It should be noted that cracking in the plaster layer due to component deformations of the plaster substrate (e.g. as a result of deflection, settlement, temperature deformation) cannot be prevented by reinforcement measures . Therefore, e.g. movement joints of the building must also be included in the plaster layer.
Manufacturer systems: Many manufacturers offer complete systems consisting of concealed plaster, top plaster and other products, e.g. for substrate pre-treatment or reinforcement. In general, it is not absolutely necessary to purchase all products from the same manufacturer for exterior plaster, as they are usually not products with a common system approval, in contrast to external plaster, for example. Nevertheless, the use of products from the same manufacturer is highly recommended, as in this case the products are optimally matched to each other and no additional warranty interfaces are created.
Surface colour, light reference value: A dark colouring is only possible to a limited extent with an exterior plaster. The decisive factor here is the so-called light reference value. If this is below 30%, sunlight leads to increased thermal effects on the façade surface and thus to increased stresses, which can lead to cracking. In addition to the light reference value, the degree of sun exposure and the bulk density of plaster and substrate also play a decisive role. The lower the bulk density, the more difficult it is to dissipate the heat to the subsoil. If the exterior plaster is to have a dark surface (light reference value <30%), the basic feasibility must therefore be clarified in consultation with the manufacturer and, if necessary, what additional measures (e.g. multiple coat of paint, additional reinforcement measures) must be taken. Light reference values below 20% can only be achieved to a very limited extent with an exterior plaster.
Fire protection: On facades or façade parts that may only be clad with non-combustible building materials, e.g. on fire walls, the exterior plaster must also be non-combustible. It is therefore important to pay attention to the corresponding building material class, which can be found in the technical data sheet of the selected top plaster.
on the construction site, preparation and execution: DIN EN 13914-1 contains extensive information on the execution of exterior plasters in Section 7 (Work on the construction site, preparation and execution of exterior plasters), including with regard to the storage of materials, scaffolding anchors, the protection of adjacent surfaces, the pre-treatment of the plaster primer, the dosing and mixing of the raw materials on the construction site, the formation of architectural design elements and the application of the various plaster layers.
Substrate pretreatment: The substrate must always be dry, clean and load-bearing. As a rule, the top plaster is applied as the second layer of an exterior plaster system by the same contractor. In this case, the plaster must be checked in particular for sufficient drying and the weather conditions must be observed. If the top plaster is applied to an existing base plaster or in a single layer to the load-bearing substrate, this must also be checked with regard to absorbency and load-bearing capacity, see also ► 302 │Exterior plaster - Concealed plaster under "Substrate pretreatment".
Weather conditions: The manufacturer's specifications must be taken into account. As a rule, plastering work may only be carried out at temperatures above 5°C without special protective precautions. In the case of paste-like plasters, manufacturers often specify higher minimum temperatures, e.g. 8° C. Not only the air temperature is affected, but also the surface temperature of the substrate and the processing temperature of the plaster mortar. Frost-free must be ensured until the applied plaster dries out.
Drying time: Before applying the top plaster, a minimum service life of one day per mm of plaster thickness must be observed for the base plaster. If the plaster substrate is soaked or at low temperatures, the service life is further extended. When carrying out a reinforcing plaster, an additional service life of at least 7 days must be taken into account.
Mortar corrosion: Fresh mortar causes mortar corrosion on metal. In the case of corresponding work above sheet metal, suitable protective measures must therefore be taken.
18350, VOB Part C, General Technical Terms and Conditions of Contract for Construction Services (ATV) Plastering and Stucco Work
DIN 18550-1, Planning, Preparation and Execution of Interior and Exterior Plasters - Part 1: Supplementary Specifications to DIN EN 13914-1 for Exterior Plasters
DIN 18558, Synthetic Resin Plasters; Terms, requirements, execution
DIN EN 998-1, Specifications for mortars in masonry construction - Part 1: Plaster mortars
DIN EN 13914-1 Planning, preparation and execution of exterior and interior plasters - Part 1: Exterior plasters
DIN EN 15824, specifications for exterior and interior plasters with organic binders
► Guideline for the Plastering of Masonry and Concrete – Basics for Planning, Design and Execution, Publisher: Industrieverband WerkMörtel e.V., Nov. 2014
► Leaflet Equalization Coatings on Fine Plasters – Colour-Leveling Coating, Publisher: Industrieverband WerkMörtel e.V., April 2009
Leaflet 2-9-04/D Renovation plaster systems, publisher: WTA (Scientific-Technical Working Group for Building Preservation and Monument Preservation e. V.
Source: bauwion
