How to Apply Clay Plaster to Straw Walls

Natural clay plaster is applied to straw-bale walls in several layers. The correct plastering technique helps create a durable, solid, reliable and attractive wall finish.

How many layers there are, what they are made of and how to apply them — we describe in this article.

Clay plastering of straw walls is carried out in 4 stages:

1. Applying the slip coat.
2. Applying the base coat.
3. Applying the fill coat.
4. Covering with the finish coat.

What each layer is made of, what it is used for and how to apply it, we will look at next.

Slip coat

This layer is applied directly onto the straw-bale walls. It is used to create a good base for the other layers.

How to prepare the mix?

Only clay and water are needed to make this layer. If you take clay straight from the ground, take a portion of clay and mix it with water. The sand and other particles will sink to the bottom, while the clay and silt stay on top. This clay will form the base of the slip coat.

How to apply it?

The best way to apply the slip coat is with a sprayer or a plastering machine. Its job is simply to cover the straw with a thin layer of clay, to which the plaster will bond. It should cover the entire surface.

Can you skip the slip coat?

You can, but it takes a lot more effort. The plaster may not bond to the straw, and cracks and hollows may form. So we recommend applying this layer anyway.

Base coat

This layer earned its name for a reason. Its main task is to cover the straw walls with the first layer of plaster and even out all the irregularities.

The base coat has a high clay content, which improves adhesion.

It should be applied in a layer 0.5–1.5 cm thick. It covers the straw, already coated with clay, and creates a good foundation for the following plaster layers.

How to prepare it?

There is no exact recipe for this plaster, since clay, soil and climate all vary. So you need to experiment. As a starting point, you can try the following recipe:

10 parts clay

10 parts sand

1 part straw

When starting the mix, first add a little water, then add half of the sand. Mix well. Then add half of the clay. Mix again. Then add the second part of the sand and repeat the whole procedure. Finally, once the batch is ready, add straw or another fibre.

However, it should be said that there is a very fine line between plaster that is too dry and plaster that is too watery. So be very careful with water.

Make a few trial batches. The plaster should be sticky, smooth and thick.

How to apply it?

This layer is applied only once the previous one has fully dried.

First, the walls should be moistened. Water activates the binding properties of the clay. However, you should not use a strong jet of water, as it can wash away all the clay.

The base coat is usually applied by hand. However, if the sand or straw turns out to be quite prickly, rubber gloves can be used.

It is also a good idea to keep a bucket of water nearby to wash your hands from time to time.

When applying the plaster to the wall, focus on making it adhere well to the straw walls.

Fill coat

The job of the fill coat is to fill in all the irregularities. That is, to fill all the hollows and make the surface relatively even. This layer of plaster is applied on average 2–5 cm thick, though in certain places it can be as much as 8–10 cm (where there are very large gaps).

How to prepare it?

If the previous base-coat proportions worked for you, then the fill coat should be made in the following proportions:

10 parts clay

15 parts sand

3 parts straw

To prepare the mix, also add a little water first, then half of the sand. Mix it in. Add half of the clay. Mix it in. Repeat the whole procedure from the start, then add the straw.

How to apply it?

Clay plaster should be applied only after the previous layer has dried.

However, before applying the fill coat of clay plaster, the plastered surface should be moistened with water. This improves the binding properties.

This layer is usually applied by hand, but a trowel can also be used.

In addition, if you are a creative person, this is exactly the layer with which you can create relief walls and decorative wall elements.

Finish coat

The finish coat performs several functions: it is responsible for colour, texture, and it also determines the durability of the whole plaster. In the finish coat, clay plaster has a high clay content and much less sand. Straw can be left out of this layer entirely. The thickness of this plaster is 0.3–0.6 cm.

This layer should not produce dust; it should be smooth and strong.

Read also: giving texture to clay plasters.

How to prepare it?

The finish coat consists of clay and sand, as well as natural additives, pigments, and possibly wheat paste.

How to apply it?

It is best applied with a trowel to create a smooth surface. If some ridges or grooves form after using the trowel, you can remove them with a sponge while the plaster is still damp.