Plaster is a wall covering that has been tried and tested for thousands of years, especially on masonry. As an exterior plaster, it usually consists of two layers: the base and the top plaster. The wide range of possibilities in terms of material, the combination of base and top plasters and the surface design enable a very versatile use of exterior plaster in new and old buildings.
choosing an exterior plaster, it is important to take into account the constraints resulting from the substrate (material, bulk density, salt/moisture load) and, if necessary, other requirements (e.g. monument protection, increased exposure to the weather). The properties of the concealed plaster, in particular its dry bulk density and strength, must be matched both to the properties of the respective substrate and to the intended top plaster.
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.
planning the substrate: The substrate must be firm, load-bearing, frost-free, free of efflorescence and adhesion-reducing residues (dirt and dust). The surface to be plastered must be evenly dried. Concrete should be rough-formed, otherwise special precautions for substrate pre-treatment are required (see notes on construction).
Top plaster: While the base plaster of a two-layer exterior plaster is always mineral, there is a choice between mineral (mineral plaster, mineral fine plaster) and pasty (synthetic resin plaster, silicone resin plaster, silicate plaster) products for top plaster. For more information, see page ►303 │ Exterior plaster – top plaster.
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.
Manufacturer systems: Many manufacturers offer complete systems consisting of concealed plaster, top plaster and other products, e.g. for substrate pre-treatment or reinforcement. As a rule, it is not absolutely necessary to purchase all products from the same manufacturer for exterior plaster, as they are not products with a common system approval, unlike 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.
Plinth plaster: Exterior plaster in the splash-prone (plinth) area must have increased resistance to mechanical impacts, moisture and frost. Depending on the bulk density of the substrate, normal base plaster or light base plaster is used. 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. The exact course of the transition line from the wall plaster to the plinth plaster must be determined before the plastering work begins, the plinth plaster should be passed at least 30 cm above the ground at each point (at least 15 cm in the case of additional measures such as the arrangement of a 30 cm wide coarse gravel strip or drainage channels).
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.
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 an overlap of at least 200 mm on the surrounding wall areas.
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.
Renovation plasters: Renovation plasters are characterized by high porosity, low capillary water conductivity and high water vapor conductivity and are therefore particularly well salt-tolerant. A renovation plaster does not block off the substrate, but enables the controlled storage of salts, which neither visibly crystallize on the surface nor destroy the plaster. The determination of the necessary renovation measures is to be made by the planner, taking into account the leaflet 2-9-04/D "Renovation plaster systems" from the WTA (Scientific-Technical Working Group for Building Preservation and Monument Preservation e.V.). When planning/determining the specific remediation measures, the cause of moisture must be clarified in advance, the water-soluble salts must be determined and the type and condition of the plaster substrate must be examined. In the case of very high salt load on the substrate, it may also be necessary to temporarily apply a sacrificial plaster for desalination, which absorbs the existing salts and is then replaced by the final plaster.
Barrier plaster: A barrier plaster is a highly water-repellent levelling and underplaster, especially for subsequent waterproofing measures in renovation. It has the opposite properties to a renovation plaster, which is open-pored and does not close off the substrate, but absorbs salts from the substrate.
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. Any release agents present on concrete surfaces, e.g. formwork oils, must be completely removed, as well as cement haze or glassy sintered skin. Open joints and slits must be closed, and larger unevenness must be compensated for. The need for additional substrate pre-treatment depends in particular on the absorbency of the substrate, which must be uniform and neither too strong nor too weak
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DIN 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 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 2-9-04/D Renovation Plaster Systems, Publisher: WTA (Scientific-Technical Working Group for Building Preservation and Monument Preservation e. V.)
Source: bauwion
