Plastering exterior walls has several advantages. The plastering creates a good base for further surface treatment with lime, felt or paint. The house brightens up and the facade gets more even, which creates a more calm expression. Furthermore, it can be easier to repair a plastered surface than a bare wall as damages on a bare wall often requires the bricks to be replaced or a re-pointing.
Preparing the façade
You’ll get the best result if you make sure to prepare the walls properly before the plastering.
1. The walls to be plastered with lime mortar must be absorbent, that is, having an open surface to which the mortar can adhere. If the façade has been painted with impermeable paint, these should be cleaned down to a solid base. The cleaning is done by low pressure washing, anywhere it’s possible.
2. In the case of damaged joints or bricks in the wall, these are repaired or hewn out and new bricks are set in.
3. Clean the masonry thoroughly to get rid of dirt and dust. For this you can either use soapsuds or groundwater that are purpose-made.
4. Joints are refilled if necessary, and all joints are compressed with a jointer.
Before you start plastering you should water down the base to ensure it gets an even absorbency. This applies to each layer of plaster you apply.
When plastering a façade the most important thing is to get an elastic surface in order to avoid cracks in the adherence zone. At the same time the mortar has to be strong enough to have a good adherence. You can achieve this combination of properties by plastering the façade in more layers, where you use the strongest mortar and apply the thickest layer on the inside, and use the weakest mortar and apply the thinnest layer on top of the inside layer. If a layer of plaster already has been applied to the wall, the layers you add on top can’t be stronger or thicker than the inside layer. Otherwise it won’t adhere probably.
If you want to plaster a rough wall, you start by rough-casting with a clean Jurassic mortar Kh 100/400, i.e. you apply a thick layer of mortar plaster, leaving it with a rough surface. Then you apply a weaker mixed mortar, e.g. KKh 20/80/475, to form a coarse-textured façade using a steel board. Finally, you use an even weaker mortar, such as KKh 35/65/500, to create the final finish with a thin layer of mortar.
The grain size of the sand determines how thick a layer of mortar you can apply. As a basic rule, the thickness of the layer is 3 x grain size. So as you apply thinner and thinner layers of plaster using progressively weaker mortar, the grain size in the mortar should be decreasing.
In exposed locations, such as when plastering footings and pointing and plastering chimneys, you should only use a very strong mortar, such as our clean Jurassic Mortar Kh 100/400. However, you should still apply several layers that become progressively thinner and, therefore.
Cornices and the like that needs a strong surface and to appear bright and sharp, requires a special type of mortar that is strong and can be applied in a thin layer. For this purpose, we have a final coat, which consists of Rødvig slaked lime, Scandinavian Jurassic lime and Kronhøj mortar sand (0-0,3 mm). You apply the final coat on the cornices in a very thin layer after they have been straightened as evenly as possible with the coarse Scandinavian Jurassic mortar KKh 20/80/475.
If you need to restore an older building where slaked lime previously has been used, or if you want a new house to get the flexibility and breathability that slaked lime can create, you can choose to use the slaked lime for the plastering. Due to it’s weaker strength, it’s important to plaster in many thin layers. It can be delivered as a dry mortar (K 100/400) or as a wet mortar – either Rødvig slaked lime mixed with mortar sand or Rødvig slaked mortar.