5 Common Challenges When Building a Cob House (How to Tackle)

A cob wall is under construction. The latest course of cob mixture has been laid, giving the top of the wall a messy appearance. A stone house is in the background with a large wooden lintel over the door. The word "Challenge" is written in a cursive font at the top of the image, and the words, "Common Challenges When Building a Cob House (How to Tackle)" are written at the bottom.

Image courtesy of Leslie Cornell Building Restoration Ltd.

Building a cob house is no easy feat – it’s one of the most challenging DIY projects to take on.

From sourcing materials to dealing with difficult weather conditions, there are many common challenges when building a cob house that pop up to test your patience and perseverance.

Before you start building a cob house, you must know the common cob house problems to expect and how to tackle them. From plastering internal cob walls to cob house maintenance, having the right information and a solid plan is essential.

In the rest of this article, I’ll discuss the common challenges when building a cob house. I’ll also address how to tackle each challenge so you’ll be well-equipped with the knowledge to build that cob house of your dreams. Let’s get started!

A two-story cob house with orange-rendered walls, large windows, and a curved roof.
A cob house is made from mud and straw mixed together in specific proportions. They are full of character and can be designed to have modern or traditional looks. Image courtesy of Homedit.com.

What Is a Cob House?

A cob house is a house made from cob, a mixture of clay, sand, straw, silt, and gravel. For stability, these aggregates are mixed in the proportions of 25-30% sand, 30-40% gravel, 10-25% clay, and 10-20% silt.

After combining clay, sand, straw, gravel, and silt, the resulting material is easy to shape and can be molded into various forms. It’s also durable and eco-friendly.

This makes cob an excellent choice for building homes when timber is scarce or inaccessible. Cob houses are typically long-lasting and require minimal maintenance – perfect for sustainable living.

Cob House – How to Build

Building a cob house is a stepwise process that involves the following:

  1. Site selection: Cob houses are highly susceptible to water damage. Therefore, you must select a well-drained site or a slightly slanting one. This way, you’ll prevent rainwater from accumulating near the foundations of your cob house.
  2. Site preparation: Clear the site of any vegetation and debris and level it. For leveling, you can pour gravel and spread it out evenly.
  3. Laying the foundation: Since cob walls are usually 18 to 24 inches (45.72 to 60.96 cm) thick, they’re heavy and require a strong foundation. Therefore, you want a deep waterproof foundation with at least a 12-inch (30.48 cm) deep base. You should start by building a masonry plinth of at least 450mm (17.72 inches) to lift the cob off the ground. Concrete, stone, and rubble are some suitable foundation materials.
  4. Cob preparation: Mix clay, sand, straw, silt, and gravel in the right proportion with water to form cob. A rule of thumb is to have two parts sand, one part clay, and a fairly-rich straw addition. Aim for five parts dry ingredients to one part water. After achieving the right proportions, trample the mixture with your feet for a homogenous cob.
  5. Building the cob walls: Cob walls are built in courses from the ground up. The base course should be thickest (24 inches); keep reducing the thickness with height, ensuring the topmost layer is 18 inches (45.72 cm) thick. You should allow each layer to dry for two to three days before moving to the next until you reach your desired height. (Remember to leave spaces for windows and doors).
  6. Add doors and windows: Install your doors and windows after building the cob walls. You can fill the openings with wood, stone, or glass.
  7. Roofing: Cob houses require a waterproof roof. You can opt for thatched roofs or traditional hipped or gabled roofs with tiles, shingles, or metal sheets.
  8. Plastering: Plaster the walls for a finished look and protection against moisture. Lime and clay-based plasters are preferred because they’re breathable and prevent moisture accumulation within the walls.
A group of women and children mixing cob with their feet on a tarpaulin laid on the grass.
Cob is a very sustainable form of building but requires a lot of manual mixing, which is actually best done with your feet. Getting help from friends is recommended. Image courtesy of This Cob House LLC.

Now that you know the procedure, how long does it take to build a cob house?

It takes approximately fifteen months to build a cob house to completion. The drying requirement accounts significantly for the longer building duration.

Here is a how to build a cob house step by step pdf for reference.

Common Challenges When Building a Cob House

Now that you know what a cob house is and how to build one, let’s discuss the most common challenges you’ll likely encounter and how to tackle them.

1. Laborious Building Process

Let’s call a spade a spade; cob building is labor-intensive. Creating the structure requires sufficient elbow grease and time.

Since cob walls must be monolithic without mortar joints, you must blend and shape the cob until a uniform consistency is achieved.

Moreover, you’ll be building the wall in courses – allowing sufficient drying time between each course.

So, if you plan to build a cob home, ensure you have enough time and labor for the project.

A rule of thumb is to partner with others and break the task into smaller chunks.

2. Sourcing Suitable Materials

Although cob is made from natural materials, the suitability of the soil and its composition are crucial.

Remember, you want to have the right clay-to-sand ratio.

Moreover, the soil must be free of organic matter and microorganisms that can cause structural challenges.

That said, sourcing the right soil can be challenging, depending on your location. You might have to travel long distances or import them from other areas.

Consequently, this can affect your budget and timeline.

To make things easier, opt for subsoil instead of topsoil. The former has an appropriate clay and sand composition suitable for making cob.

Consider using a mason jar test to more accurately determine the soil’s composition.

3. Weather Hindrances

A closeup photograph of a cob wall with pieces of straw on the surface of the mud. Overlain is a cartoon image of a cloud with rain falling.
Rain is one of the main elements you must protect cob buildings from. This can be achieved by having large overhanging roofs and applying lime plaster. Rain will also hinder the building process by not allowing the new layers of cob to dry.

Weather is one of the common problems when building a cob house.

Rain (high humidity) is your enemy, as it prevents the cob from drying out properly. This can lead to excessive dampness in the walls, making them weak and prone to collapse.

On the other hand, if it’s too dry, the cob won’t be firm enough to hold up. Additionally, extreme temperatures tend to cause cracking and structural damage to cob walls.

Therefore, if possible, plan to start building during dry months; avoid rainy seasons and scorching heat.

You should also consider your area’s microclimate before embarking on a cob building project.

That said, check your local weather forecast before beginning the construction and schedule your work routine accordingly. This will ensure the cob dries out without any hindrances.

If you’re in the U.S., the best state to build a cob house should have a temperate climate with hot and cold seasonal swings to utilize the cob’s thick thermal mass. Such states include California, Arizona, New Mexico, Nevada, and Hawaii.

4. Building Codes and Regulations

Building codes and regulations vary, depending on where you live.

Unfortunately, most countries are yet to approve cob as a building material. This means it can be challenging to get cob building permits.

It was not until 2021 that a Cob Construction Appendix was approved for inclusion in the International Residential for building codes in the United States.

Therefore, familiarize yourself with local building regulations to create a legal and safe structure.

Consult with the relevant authorities and obtain permits before starting the project. This will prevent any complications during or after the completion of the cob house.

5. Maintenance

A cob wall has been damaged by masonry bees and woodpeckers who have drilled into it, peppering it with holes and harming its structure, causing parts to crumble.
Masonry bees and woodpeckers have caused this damage to a cob wall. Image courtesy of BRIAN FRANKLIN & SON LTD.

Like any other earthen building material, cob houses require sufficient maintenance to remain in top shape for longevity. Basically, cob house maintenance entails protection against water.

Pests such as masonry bees can cause significant damage to cob walls, but water is the main culprit that can cause cob walls to erode over time. This means you must render the walls with a permeable plaster for longevity.

While impermeable and cementitious renders and plasters can be used, they do not promote breathability. Therefore, they’ll eventually cause cracks in the wall.

Instead, opt for lime plaster, as it maintains the natural beauty of cob houses while allowing moisture to pass through.

Furthermore, carry out periodic inspections of your cob house. Look for any signs of wear and tear or damage and repair them promptly.

What’s more, you should ensure the roof has two to three feet (60.96 to 91.44 cm) of overhang to protect the walls from driving rain.

Besides the overhang, build the roof so it’s connected with the wall’s top without any gaps. This way, you’ll prevent water from entering the cob house.

Final Thoughts

As with any sustainable construction, there are common problems to expect when building a cob house.

These issues can be addressed with proper planning and research, from weather hindrances to building codes and maintenance. However, once you’re done with all the challenges, a cob house should give you a lifetime of good service.

Now that you know the common challenges when building a cob house, here is everything you ever wanted to know about straw bale building to give you more inspiration on eco-friendly building.

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