Cob Building Tutorial

Chapter 8: Plaster Finishes

Now that you’ve got all the cobbing done for your walls, you’ll want to apply a finish to the cob to weatherproof it and to add durability. There are several ways to plaster your home. Each has its advantages and disadvantages.
By no means should you use a cement-based plaster for your walls. While concrete and cement are durable, they don’t allow the walls to ‘breathe.’ If a cob wall gets any moisture in it, the moisture must have a way to escape. Concrete and cement are porous to a certain extent, but not enough to be used on cob. Not to worry…there are plenty of other plastering methods you can use.
One of the simplest methods is to forego plastering altogether and simply apply a coat of linseed oil directly to the cob. This will waterproof the cob while allowing the natural coloring to show through. This method works best in areas with low humidity and low winds.
The basic ingredient in most plasters is lime. Lime is a binder made from limestone or seashells. It is mixed with sand to create plasters. You can buy regular drywall plaster at any home building supply store. It is available in bags of about 20 to 25 pounds. To make your lime plaster go further, you can mix it with sand from your building site. This will also retain much of the color of the soil around your home, making it blend in better with the landscape. Experiment with the ratios to see how much and to add to how much plaster. A good place to start is about a 50/50 mix, then adjust from there.
A more natural and organic type of plaster is made with wheat paste. Wheat paste plaster is very durable and inexpensive. A good recipe for this plaster is to start with about 70% sand and 30% clay in a dry mix. Mix the clay and the sand thoroughly. Take about 18 shovels full of this mix, and add to it ¾ pounds of dry wallpaper paste (wheat paste). Add about four or five good handfuls of chopped straw to this dry mix. After mixing all of the ingredients thoroughly, add enough water to achieve the proper consistency. The plaster should be wet enough to spread easily, but dry enough to stick to the walls without oozing.

Another good organic plaster is, believe it or not, cow manure. A recipe for cow manure plaster is: one part clay, two parts cow manure, and three parts sand. Mix all the ingredients, then add enough water to make a workable consistency. Don’t worry about the smell. As the ammonia evaporates, it eventually goes away.
Maybe cow manure isn’t your cup of tea. There are still many other organic additives that have been used successfully in the past to make a binder for plasters. Some of these ingredients include: eggs, oils, cactus juice, cornstarch, other vegetable starches, tallow, sap, flour and molasses. As a rule of thumb, mix one part clay, three parts sand and two parts of your binding agent. With enough time and experience, you’ll find what works best for you.
If you just want to play it safe, use a mixture of one part clay, two parts lime plaster (drywall plaster), and three parts sand.
Cob takes plaster fairly easily. To apply plaster, get a trowel from a building supply store. Scoop the plaster onto the trowel, then apply it to the wall, smoothing as you go. If it looks a little rough, don’t worry. Plaster tends to smooth out as it dries. If it still looks rough after it dries, you can always apply another coat later, or sand the existing coat to the desired smoothness. Once the plaster is dry, you can paint it if you like or leave it in its natural color. If you don’t plan to paint, you may want to weatherproof it with a coat of linseed oil or some sort of weather sealant.
Be sure to plaster your walls both inside the house and outside. If you’re planning to hang pictures, you shouldn’t have any problems with the nails sticking into the cob. Just tap gently to keep the plaster from cracking off. One of the disadvantages of cob is that the plaster tends to fall off in small patches if you have an inexperienced plasterer applying the finish. Be prepared for this by remembering the recipe you used so you can repeat it if needed.
One final note on plastering your walls: you can get some interesting textures and effects by using different tools to apply the plaster. You can apply it with a damp sponge, or comb it after application with a rake, or stamp designs into it with almost anything you have around the house. Let your imagination run wild! Think outside of the box and see what happens. You can always add another layer of plaster later if things don’t work out.

About the pictures in this tutorial: Not all of the cob structures in the pictures shown here were built by the Culture Artist Cobbers. Many are from cobbers at various sites around the world. I have attempted to get permission for all the photos used on this site, and to properly credit them wherever possible. When the cobbers in question have a website, I have tried to link the picture back to their site, or list them in the Cob Builders' Directory, or both. I have made every effort to gain permission for all the pictures shown. If I inadvertently used one without permission, please contact me at chuck@cultureartist.org and I will either credit the source or remove it. Thanks!