How To: Build A Cob Wall

How To Build A Cob Wall
By Kirby Fry

After working with natural building materials for 25 years, the cob or “puddled adobe” wall system has become my favorite of all the adobe wall systems.  I define a natural building material as one that is locally available and non-manufactured.  This how to do it yourself guide will describe 4 different forms of adobe wall systems, explain why the cob wall system is my top pick, and tell you how to build a cob wall.

Types Of Adobe Wall Systems

Adobe is generally made from sand, clay and straw.  Adobe wall systems include handmade adobe blocks, compressed earth blocks (engineered adobe), earth bags (flex form adobe), and cob (puddled adobe).

The handmade adobe block is perhaps the most traditional system for building an adobe wall.  It is made from sand, clay and straw. Fresh, clean horse manure is often substituted for the straw component.  Adobe blocks are 10” wide by 14” long by 3 ¾” tall. A clay slip is applied between each course of adobes. The adobe block wall system can be either 10” thick or 14” thick. 

Compressed earth blocks (CEB’s) do not have to have straw in them, and are made by a machine that compresses moist sand and clay at a pressure of 1,200 to 1,600 pounds per square inch.  Often 6% of the mix is portland cement to keep the CEB’s from eroding during the construction process. CEB dimensions are the same as the handmade adobe block, 10” wide by 14” long by 3 ¾” tall.

Earthbag wall systems are also referred to as flex form adobe or superadobe.  This method includes mixing sand and clay together, getting it moist, and packing it into continuous bags.  Earthbag walls are about 16” thick. When this method was first developed by the Iranian-born architect Nader Khalili he was using a solid nylon or plastic bag, now earthbag builders have gone over to using the Raschel weave continuous onion bag that you commonly see being used to sell onions in at almost every grocery store.

Cob or puddled adobe wall systems are made from sand, clay, and straw that is mixed together and then dumped on top of the wall system.  Cob is an old English word that means “lump of dough” and because it is put into the wall system while still wet, the builders can only raise the walls up 4” or 8” per day before it begins to slump.  A cob wall may vary in width from 12” to 18” depending on how tall or long the wall is.

Why Cob Is My Favorite Adobe Wall System

After working with all of these adobe wall systems my favorite is now cob, and here’s why:

The cob wall system is monolithic and does not require a concrete bond beam at the top of it – a bond beam is made from concrete, steel, or wood and in adobe block and CEB wall systems a bond beam is necessary to hold the top of the wall system together.  

A concrete bond beam that we poured for a CEB wall system.

The cob wall system is ready to plaster when it is finished.  Earth bag, and CEB wall systems require more preparation for plasters like filling in between the bags, and or chipping up the face of the CEB’s.  

How we chipped the face of the CEB’s in order to prepare them for a lime plaster.

The cob wall system is more efficient for smaller job sites.  Making 5,000 to 10,000 adobes or CEB’s all at once on a job site is a big logistical chore and requires a lot of space.  Making adobes off site and hauling them to the job site is also a huge logistical chore. Making cob on site is slower, but requires less space, and makes less of a mess.  

The mixing machine, the CEB production machine and a skid steer loader on the job site.

Lastly, cob wall systems are a more ergonomic building material for the builders.  The cob is easily dumped out onto the wall system a half bucket at a time. Ladders and scaffolding keep the crew working between their knee and chest heights.  By contrast, filling earthbags means that you are standing on top of the wall system, holding a 5 gallon bucket with the continuous earthbag attached to it, walking backwards on top of wet adobe under your feet while 2 or 3 people dump the fresh adobe into your bucket.  

The crew making an earthbag wall at Earth Native Wilderness School.

How To Build A Cob Wall System

All buildings require a solid foundation, and the immense weight of an adobe wall system is no exception to that rule.  Traditionally for a 14” wide cob wall system an 18” wide by 24” deep rubble filled trench with a concrete or mortared-together masonry footer has been used.  Today, I wouldn’t hesitate to pour an engineered concrete slab for building a cob wall on top of.  

A rubble filled trench with a concrete grade beam set above it designed for a 12” wide cob wall.

I highly recommend that the roof for any natural building be a hip roof where there are protective eves on all four sides of the building.  A single gable roof leaves 2 sides of the walls exposed to catching more weather which can significantly damage lime and clay plasters.

Keep your cob wall system relatively low, somewhere between 8’ and 12’ tall.  A second story on top of a cab wall is possible, but above 8’ to 12’ in wall height, I recommend going over to a more conventional 2” x 4” stick frame.

Once the grade beam or concrete slab has been constructed, it’s time to have the materials delivered to the site.  Here is what you will need to begin; 10 yards of manufactured sand, 10 yards of clay, 10 yards of decomposed granite, and 12 straw bales.  These materials need to be close to the work site, and you need to be ready to have more truckloads of material brought in again and again in the proper order.

Clay, manufactured sand, and decomposed granite on the job site.
Clay, manufactured sand, and decomposed granite on the job site.
Clay, manufactured sand, and decomposed granite on the job site.

On smaller job sites our crews mix cob on a 4’ by 4’ plywood mixing board with a mortar hoe.  Initially we were mixing the cob on blue tarps but after just 2 weeks the tarps fell apart and everyone on the crew preferred the mixing boards anyway. On a bigger job site a large gas powered mortar mixer is the only mixer that will turn over the heavy cob.  Some crews will also use a skid steer loader with a mixer attachment but I have never used one of those before.

A crew member mixing cob on a 4’ x 4’ plywood mixing board.

So let’s start our first batch of cob with a ratio of 1 to 1 to 1.  That’s 1 half bucket of clay, one half bucket of manufactured sand, and 1 half bucket of decomposed granite.  The straw is chopped up into 6” to 10” lengths with a machete on a board and 4 large handfuls will be added into each mix.  First dry mix the clay, manufactured sand, and decomposed granite. When all of those ingredients are thoroughly mixed together in dry form, then add about 4” to 6” of water in a 5 gallon bucket to the dry mix.  Next, wet mix those ingredients. When all of those ingredients are thoroughly mixed together add 2 large handfuls of chopped up straw. When all of those ingredients are thoroughly mixed together add 2 more large handfuls of chopped straw (total of 4 large handfuls) to get the final batch of finished cob.

[Images 10 through 18 show how the cob ingredients are added to and mixed on a 4’ x 4’ mixing board]
How the cob ingredients are added to and mixed on a 4’ x 4’ mixing board.
How the cob ingredients are added to and mixed on a 4’ x 4’ mixing board.
Clay has been added.
Manufactured sand has been added.
Decomposed granite has been added.
Mix all of those ingredients thoroughly together in dry form.
Then, prepare the mix to add about 4” to 6” of water in a 5 gallon bucket.
Next, wet mix those ingredients.

Load the cob into 5 gallon buckets filled only half way.  Never fill a bucket up all the way as it becomes too heavy.  Always try to carry a half filled bucket in each hand so as not to strain one arm by carrying just one bucket at a time.  Dump the half full buckets of cob directly on to the wall system and begin to evenly work the cob out onto the wall system.  Press the fresh wet cob down onto the foundation or onto the dried cob beneath it by using a cobber’s thumb, which is usually just a 1” diameter by 10” long branch or old tool handle.  The aggregate in the cob is deliberately sharp and will quickly tear up your hands and fingers if a cobber’s thumb is not used.

If the cob is dry enough, one should be able to build 4” to 8” in height of cob wall per day.  When the cob begins to slump, it’s time to stop adding material to it.  The next day, yesterday’s slumped cob will need to be trimmed off and reused.  Every morning, without exception, the cob wall will need to be trimmed, and the wall system kept square and plumb.

Some of the tools we use to trim the walls every morning.

Wooden door and window frames called “bucks” will need to be framed and set flush to the exterior of the cob wall.  A 3’ 0” wide by 6’ 8” tall standard exterior door should have a door buck with dimensions of 3’ 2” wide by 6’ 10” tall by 4 ⅝” deep.  A 2’ 6” wide by 5’ 0” tall standard window should have a window buck with dimensions of 2’ 6 ½” wide by 5’ ½” tall by 4 ⅝” deep. The window bucks are typically set at a height of 6’ 8” and down in order to match door heights, and are held in place with wooden cleats or pegs attached to their sides called “dead men.”  The doors and windows themselves will be set into the bucks after all of the cobbing has been finished.  

The door and window bucks set into the cob walls after the walls are built up to the proper height.

A structural header or piece of timber framing is set over the door and window bucks to carry the load of the cob wall above it.

The timber header after installation.
A window after the window sill and trim were set.

When the cob wall reaches its final height a simple top plate, not a concrete bond beam, is embedded into the wall using more wooden cleats or “dead men.”  The roof’s rafters will sit on this top plate.

The top plate right after it was set.
The rafters after they were set on the top plate.

The electrical wiring is notched into the wall system after the cob wall is built.  This cob building was permitted and inspected by the City of Austin and the city required this method of wiring because it did not want the wiring to be buried behind the cob allowing them to visually inspect it.

How we notched the cob walls and set flexible conduit for the electrical wiring into it.

The exterior walls were plastered with a lime plaster and the interior walls were plastered with a clay plaster.

The 2 different types of plaster that were used. A lime plaster was used for the exterior cob walls…
…and a clay plaster was used for the interior cob walls.

My advice to anyone building with cob for the first time is to start on a small scale.  Building with natural building materials requires just as much, if not more, skill and caution than building with conventional building materials.  It will also require more time than building with conventional building materials, and cost about 30% more.  

Do not be daunted by these suggestions and warnings though, the rewards of living in a home made from natural building materials and or that you have built yourself are infinite and your life will change for the better because of it!

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