DIY Plastering – Preparation

For effective DIY plastering of a straw bale house, the DIY homebuilder must gather the proper tools and materials and then take the time to get the straw bale wall ready for plaster.

Before we picked up our trowels, we put in a number of hours prepping walls and it paid off. Adobe clay is one of the most pleasant materials available for building, but it does require some preparation.

Why Use Adobe?

Adobe is the name applied to a mixture of sand, clay, water and some sort of binder such as chopped straw, aged horse manure, etc. See our book store for access to a great book, Natural Plasters.

Adobe is forgiving. Make an error or have a problem and you can start over by wetting it down and peeling it off or retroweling. For newbies like us, this was really important.

We love the fact that adobe is made from natural material with only the embodied energy involved in transport, unless you count the energy expended by the builders. Big breakfasts are the ‘fuel’ costs for adobe plastering.

Adobe is economical. Our 320 sq ft adobe cottage, the Bear Cave, has adobe block walls, adobe plaster, and adobe floors. Total cost for the clay was less than $500 (USD) in 2008.

The cost was more for transport (40 miles) than for the cost of clay. The greatest cost of clay is transport. As clay can commonly be found in most habitable areas, transport costs should not be prohibitive.

We used the same clay, with different screenings, for every application except the finish coat on the interior. For that, we bought some Kaolin clay mined and bagged in northern AZ and mixed it with 60 grit bagged sand.

We love the outcome of DIY plastering - our home has a nice balance of finished wall with a flavor of rusticity.

Plaster Preparation

Once our straw bale walls were up, windows and doors installed, and interior build out finished, it was time to plaster.

Chopping straw

The first step was to chop straw. We had a few bales left from the house walls and bought a couple more from the local feed store.

At first, we chopped using a weed whacker and a thirty gallon heavy “rubber” garbage barrel. The straw chopped with this method was fine for filling seams, but when we began work with the trowels after this preparation process, we wanted more uniform chopping and borrowed a neighbor’s mulcher. 

For some finish applications, we ran the straw through the mulcher twice. Using a mulcher or weed whacker is seriously dirty work, but part of DIY plastering.

Eye protection and a mask for mouth and nose is imperative. I used a damp bandana and goggles.

Screening Clay

Our adobe clay, fresh off the truck, had been ostensibly screened to 3/8“ minus. However, we found rocks up to 3/4” or larger. We set up two screens outside, one with 1/4" hardware cloth and one with 1/8” hardware cloth. For brown coat and scratch coat trowel work, we screened with 1/4" and for finer work used the 1/8”.

Haircut for Straw Bales

Straw bales stacked to form a wall are irregular. Lumps and straw ends stick out and a wall plastered without trimming will be anything but smooth.

Using a string trimmer aka weed whacker, we gave our walls a haircut. Because the strings are on the inside of the wall, there is no danger of breaking the bales.

With the trimmer held against the wall, take off excess straw until your wall is straight and reasonably smooth. A great surface for DIY plastering.

Mudding seams

For filling in the seams and holes left in stacking our straw bale walls, we mixed clay screened through 1/4" mesh with chopped straw. The mixture was heavy in straw and the clay was simply a binder.

Straw, clay, and water were put into our old mixer until the right consistency was achieved. Firm enough to hold when slapping it into the seams and wet enough to bond to the bale.

For major holes and seams such as around window bucks and above the doors, we would push in as much as would stay, let it dry over night, and make one or even two more filling applications. This process was the first step in getting a straight and evenly finished wall.

Slip Coat vs. Chicken Wire

It must be noted that there are probably as many different ways to plaster a straw bale wall as there are plasterers. There are many who will swear that there is only one method for plastering straw, theirs, that should be followed. Probably not.

One of the methods often promoted is the use of poultry net or chicken wire as well as metal lath and other such materials as the base for plaster.

I have to come right out and say that I have a strong aversion to plastering with adobe over chicken wire. Granted, it grabs the mud, but often leaves air space between straw and plaster that compromises the integrity of the wall. I know that if other plasterers read this, many will strongly disagree. So it goes.

For DIY plastering, the extra time for our method of plastering was well spent.

I do agree that it is important to provide a surface with “tooth” that will allow the brown coat to adhere to the straw. To that end, we did a final screening of our adobe through a kitchen colander, getting a very fine and uniform clay.

Mixing water and this fine adobe gave us an aliz or fine plaster about the consistency of heavy paint.

Spraying Slip Coat aka Aliz

With a sprayer designed for applying joint compound, we sprayed all the surfaces we intended to plaster.

Please note, there will be over-spray, so mask all windows and doors as well as any outlets or fixtures already installed that you don’t want coated in a fine layer of adobe clay.

As we wanted consistency on all our walls and our interior build out was done with dimension lumber and sheet rock, we taped the sheet rock and then sprayed it with the aliz as well.

We did not put either a brown coat or a scratch coat over the sheet rock and the finish coat of plaster adhered beautifully.

Ready for the trowel!!!

The pictures you see here were taken while preparing the interior walls for DIY plastering. The exterior wall preparation was identical. The finish plastering process was different and will be addressed in another page.

I suggest a practice wall to experiment with various suggested techniques.

Stack and support three straw bales, strings inside, and cover the practice surface with coarse burlap. Apply different plaster/straw mixes until you find one that works for you. When you have finished with one mix and evaluated it, simple peel the burlap off and try another.

It would be pointless for me to try to give proportions of straw to clay as all clays are different. I would be happy to answer specific questions.Simply use the comment page to ask.

So, read, think, plan, experiment, and, above all, enjoy the process. It’s worth it!

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