Once you have mixed your deliciously smooth finish earthen plaster, it’s time to apply it to make your cob walls shine. It’s a seemingly simple process, but there is a certain hidden finesse that will make the job easier with time and practice. I’m no expert, but here’s how I went about plastering the interior of my cob house.

(This entry is a continuation from Finish Earthen Plaster: Part 1: Materials and Recipe.)

Earth plaster tools

Your hands and a quality trowel are the two most important tools you’ll need to apply earthen plaster. I have tried using a typical American pool trowel and a Japanese trowel, and I definitely prefer the Japanese trowel, hands down.

A Japanese trowel is thinner than a pool trowel, and it has a bit of flex. The shape is also quite different. The one pictured above is the style that I used — note that the point is very useful for getting into tight corners. Below is a picture of an American pool trowel for comparison:

Pretty different. It’s nice to have a smaller sized trowel and a larger size, whether you borrow or buy ‘em.

Applying earthen plaster

Wet down your walls with a pressure sprayer until they are sufficiently saturated. (I usually spray until some of the cob drips down the face of the wall.) It’s important to get a good bond between the plaster and your wall. Some folks spray a bit in advance and then come back to plaster, but I spray and plaster right away. I don’t know if it makes a difference. 

Next, grab handfuls of plaster and smear them against the wall. You might choose to go pretty heavy with your material, or more lightly. I started my interior with a lighter amount, but later I went heavier. You may risk increased cracks if you apply your plaster too heavily. Do some experiments.

The initial smearing of the plaster.

In tight spaces where it is difficult to smear plaster, I would grab little handfuls and throw the material into a corner, etc. Most typically I did this in the corners of windows and around my rafters.

Now for troweling… The bigger your strokes, the more smoothly the process will go. As you glide along the wall with your trowel, you may find that you want to remove excess plaster that builds up on the steel, so slop the extra mud off into a bucket. Keep your trowel clean. Apply steady pressure to the wall, and try not to overwork the plaster. It can be tempting to try and remove every single stroke mark, but it’s not really necessary. (You can work those out with a burnisher later.) Troweling is the real art in plaster application. It’s the job that takes the most finesse and practice to master. Take your time and be conscious of your strokes, and don’t spend a lot of time going over the same area. 

Burnishing the plaster

The last step in plastering your walls is burnishing the surface. Burnishing the walls will result in an even smoother surface, and will allow you to remove any marks left from troweling. Ianto Evans and company recommend waiting for the wall to reach a dryness level of “leathery hardness” in The Hand-Sculpted House, but it’s best to monitor your walls as they dry and burnish as soon as the plaster becomes dry enough to rub without disrupting it too much. 

I have used both scraps of EPDM from my pond liner (which is flexible) and scrap pieces of another black plastic liner (which is much stiffer) and found that the stiffer black plastic liner works best as burnisher material for most situations. I have very briefly tried using burlap as a burnisher, but didn’t like the feel of it. If you can, try to score some scraps of black plastic liner, or be creative and try out some other suitable material.

With burnisher in hand, go over the wall in circular motions or gentle strokes to remove any lines or marks in the plaster. It takes a fair amount of time to do, so be sure to leave enough time in the day to do the job before the walls have too much opportunity to dry out. 

Other earthen plaster tips

If you are doing a large plaster job that you know won’t be complete within a day, it’s important to think about how you will deal with seams. My plaster job took four days (working by myself for one, and with a friend the next three days), so at the end of each day, I sprayed the fresh plaster at the seam between it and the unplastered wall. I made sure it was wet enough so that the next day, I could continue from where I left off, and there would be no distinguishable seam.

Plastering is an art that takes time to really feel out. My experience has been pretty limited before doing my own house, so take these suggestions for what they are worth. In a future post, I will address issues of plaster cracking as it dries, which seems nearly inevitable. I have seen pretty big cracks in some earthen plaster, and in different houses, very small ones. (Thus far, I have not had large cracks in my plaster, thankfully.) 

Happy plastering!

(This post is a continuation from Finish Earthen Plaster: Part 1: Materials and Recipe.)

Finish earthen plaster recipe

First and foremost, it’s important to know what you’ll need for your earthen plaster mix. The following recipe includes the exact ratios that I used. However, as is the case with natural building, you may find slightly different quantities to perform better, depending on your materials. This should be a good guide, though:

1. 3 (5 gallon) buckets of 1/16 inch sifted sand
2. 1 bucket of clay
3. 1 bucket fresh cow manure (not dried)
4. 8 cups wheat waste
5. fluff of 3-4 cattails

Preparing your earthen plaster material

Unlike cob, which can be made pretty sloppily without much worry, earthen plaster requires a bit more delicacy.

Sifted sand

For a truly smooth finish, it is important to sift your sand to remove small stones and larger particles. I used a 1/16 inch window screen to sift all of my sand. This is a pretty tedious task, so it’s nice to have company around while you push sand through a tight screen. Also, it is a huge help to have completely dry sand, as it will go much, much quicker.

Clay

Many builders choose to sieve their clay to remove large particles, but I actually skipped this step. I was pretty confident that my clay did not include too much particulate matter (and it didn’t, for the most part), so I gladly passed up sieving my clay. If you choose to screen your clay, prepare for a similar process to sifting your sand, except that you will use a larger screen.

Wheat paste (and how to make it)

To make wheat paste, mix 1 cup of wheat flour (I used white flour, but I guess you can use whole wheat…? I doubt it matters that much) in 2 cups of cold water, while you bring another 6 cups of water to a boil. Once the water is boiling, whisk the flour and water in, and mix. Lower the heat, mix for another minute, and it should become slightly translucent and sticky.

Be sure to use the wheat paste right away! It’s generally recommended that you don’t let earthen plaster with wheat paste sit for too long – it can get stinky. (I think the smell is reminiscent of rotten mashed potatoes.) The wheat paste makes for a stickier, gluey plaster. (Although I’m sorta convinced that it’s not totally important – in fact, I think some of the mold spots in my plaster are from the wheat paste, although this is just a guess for now…)

Cow manure!

I am a strong adherent of cow manure. If you are squeamish, just get over it. Seriously. Manure has been used in plaster for eons – it is totally safe and beneficial, and yes, the smell will go away. Manure lends strength, durability, and texture to earthen plaster. The fibers make for great tensile strength, and I think that overall, manure helps make the plaster softer (at least to work with).

Try to get manure from cows that are strictly grass-fed. I am a little leery of manure with grain in it, although that is what I had to resort to using in my interior plaster. I don’t think that it is a huge deal, but 100% grass-fed is what to aim for. Fill up some 5 gallon buckets and keep them covered to prevent the poop from drying out. After a day or two, the manure will ferment a bit and expand and become quite airy. That’s the best time to use it.

Cattail fluff

Cattail is a common plant found around marshes, ponds, lakes, and wetland areas. In the fall, pick the mature punks and keep them in a dry place to use in your earthen plaster. The punks explode upon twisting them, resulting in a cloud of beautiful, soft fiber that is a perfect addition to finish plaster, adding tensile strength in place of straw. I typically use about three or four punks in each batch of plaster. I like to be able to see the fibers in each handful of material. (Split your handful in half to see the fibers sticking out.)

Some folks break the cattails ahead of time into a large barrel and then fluff it with a weed whacker, but I find this to be unnecessary. You can break the punks open right into your mix.

Mixing finish earthen plaster

When mixing finish earthen plaster, it helps to have a relatively clean tarp (or a tarp free of straw and large chunks of old material, at least). Dump your sand and make a well, add your clay and manure, and stomp away. Once you have a nice even mix, throw in your cattail fluff, and try to avoid getting chunks of fiber (or “cattail bombs”, as we say) in there. You can either add the wheat paste now, or to individual buckets of material. Your final product should be consistent, smooth, wet, and sticky! Add water during the stomping process if you have to, but it may already be wet enough between the clay and manure.

Next up I will describe the finish earthen plaster application process…

The last week has been quite damp there, but damn… it’s already been two weeks since the floor has been finished. (It’s 1.5″ of new material, so it’s not that much stuff that has to dry out.)

I’m a little concerned about what looks like definite mold… but should I be worried? Anyone experience a situation like this? I won’t be able to pluck the grass and mushrooms and wipe the mold until the floor dries…

Hmm.

You can help spread awareness about cob building by helping The Year of Mud to continue to share cob building stories and information.

Cob building is a time-tested, sustainable natural building technique that can thrive in these times of increasing unease about the future. Cob houses can help promote a lower impact lifestyle and increased ecological awareness, and contribute to local community and culture.

Help The Year of Mud demonstrate sustainable building techniques and simple living by supporting this website’s efforts to share continuing examples of natural building.

Many have already been inspired, and many more can be motivated by examples of cob building:

Your blog is great! Of course, it’s making me incredibly impatient to get started on my own cob adventure. Thank you so much for sharing all of your experiences! – Robin

Can’t wait for more. I found this blog today and have just completed reading it. Whew! Thank you for the extreme detail. Very enjoyable. – Matt

You are living my dream! I have been wanting to build a cob house for five years, but one thing or another has always held me back. But reading your blog has been almost like doing it myself. – Knobbly

Your donation will help promote cob building and support The Year of Mud to continue to share cob building stories, how-to tutorials, documentation, and photos for my first cob house and future natural building projects.

Please take a moment now to support The Year of Mud in promoting and inspiring cob building methods and techniques. Your donation will go towards maintaining The Year of Mud website and future cob building projects.

No matter the amount, every contribution helps!

Yours truly,

Ziggy

You can also contribute by sending a check to:

Ziggy’s The Year of Mud
1 Dancing Rabbit Lane
Rutledge, MO 63563
USA

Thank you!

Here it is…. the first food from the sod roof of my cob house. Mm mm.

So now I must simply wait.

Nearly all of the work is done. All that remains is installing the skylight and plastering the cob bed and bench (which are still not dry), but neither of those things has to happen before I move in.

It’s a strange feeling to be nearly ready to move into this house that’s been the focus of my every day since last spring. It’s nice to be able to think about what I’ll do once I’m settled in. I’m excited.

Perhaps I’ll take the opportunity to relax while I wait on the floor. (Perhaps I’ll finally get to writing more about my rocket stove, earthen plaster, and now the earthen floor, too.)

The past three days I have been busy applying finish earthen plaster to the interior of my cob house. I expect it will be done tomorrow. Here are a few photos of the plaster work in progress, along with some of the mosaics that are being installed underneath the rafters of the reciprocal roof.

Expect a writeup in the near future about my finish earthen plaster recipe and application! It’s been a very successful job so far.

The plaster work will probably take the longest length of time among these three things, since I want to do a bunch of tile mosaics inside as I plaster. Those mosaics can take a while…

My skylight (a 60″x60″ piece of 1/4 in. clear polycarbonate) should be arriving at the end of the week, and I’ll probably start mixing my finish plaster tomorrow, as soon as I get some more cow manure.

Of course I’m tempted to estimate when I’ll move in… I figure I’ll give it a month. The floor and walls will need some time to dry before I can move my stuff in, so even if I finished in a week and a half, I can’t move in immediately.

At long last, I have a living roof! I’ve been anticipating this day for over a year now and it’s finally done. On Monday, a group of about twelve folks assembled to help me lift sod onto the top of my roof, and in two and a half hours, it was effectively covered.

Soil retaining ladders

A few days prior to Monday, I made several soil “retaining ladders” using rope and some thin wood poles to help keep the soil in place. These poles were spaced a couple of feet apart to provide a brake for the soil from sliding down the roof. At some point I ditched the idea of using any loose soil on the roof in favor of entirely using sod, so these ladders became less important to me, but I left them up anyway.

Building the sod roof

Since I already wrote about cutting sod for my roof, I will descrive how the sod was loaded. Once the perimeter of the roof was covered with sod, we covered the rest of the roof. Doing the edge was the trickiest part, since I had the fold the EPDM underneath itself and leave a little bit exposed to protect the edge logs.

Once that was done, it was only a matter of lifting the material onto the roof. People worked at ground level to pass the turf up to folks on scaffolding and ladders, who then passed the material to others on the roof. The turf squares were packed closely together so that there were no gaps. If any awkward gaps could not be filled with sod squares, loose soil was loaded into cracks.

It was all fairly simple. Heavy heavy, but simple. I still have to load some sod up at the very top of the roof to completely cover the tractor tire skylight frame, but the roof is effectively done.

Planting the living roof

On the east side of the roof, I dumped several buckets of loose soil so that I could put in several strawberry plants. I mulched the strawberries with straw to help conserve moisture.

Eventually, I will plant wildflowers, grasses, and edible perennials on the roof, focusing on plants that are tolerant of drier soils and full sun. Since the sod is only about four inches thick, it is likely that the roof will dry out during high summer.

Although… yesterday was an incredible first test of the integrity of my living roof. At 6:30 p.m., we were warned of a tornado headed straight for Rutledge. Within 30 minutes, an inch and a half of rain had fallen, but thankfully the tornado turned at what felt like the last minute, and we were spared. It missed us by a mere 10 miles. I was happy to find that the soil remained intact on the roof, and none of the sod slipped. I didn’t see any signs of wear on the rafters, either. Success!

It will be interesting to see how the living roof ecology develops and changes over time. It will be especially nice once the individual sods root together and get totally established.

My living reciprocal roof formula

Here is my living reciprocal roof formula, in layers:

1. 28 round pole rafters (black locust and pin oak, 14 primary, 14 secondary)
2. black walnut wood decking (mostly 1-1.5x)
3. muslin fabric
4. double layer of cardboard
5. EPDM
6. four inch thick sod squares

And finally… a few more photos from the sod roof party: