Final Clay Plaster, Part 1

In January of 2020 we made a huge push to complete the middle floor of the addition, which are the bedrooms, hallway, and the connecting passage from the existing house. After finishing the ceiling and floors, we needed to complete the final clay plaster on the walls before we could build the closets and shelves and move in. In retrospect, moving into long awaited personal space a month before we were all ordered to stay home was a serious blessing. Our kids were now in grades 9 and 12 and completing their school year at home, so providing some study and private space was an immense step in handling the pandemic.

I have already described the details of mixing and applying clay plaster back in Sept 2011, when we completed the first half of the straw bale house~ read through Final Plastering for all those details. We used the same recipe, did the same procedure for colour sampling, and plastered alongside our friend and mentor Tracy Calvert once again.

We mixed up the estimated amount of plaster needed in 5 gallon buckets. The next day we started with the hallway, which we wanted to leave uncoloured and as bright as possible since it runs down the north side of the bedrooms. Working around all the swoopy sculpture definitely takes up more time! Tracy and I did the plastering, while Colin looked after keeping the plaster mixed and refilled into our work buckets, and doing the sponging and burnishing as the plaster dried slowly.

It took us one whole day to do the hallway, and two shorter days for each of the bedrooms. It is important to remember that the amount of open, large wall space is not so much a factor of how long it will take to complete, but rather the amount of windows, edges, and sculpture. Those are the features that slow down the process and thus require time management considerations of each person’s role, as the working areas of plaster need to be monitored for dryness so the completed area will have a uniform, continuous surface. It is also good to keep in mind the changes in colour as the plaster dries, so attempting to fix up areas that have already set will result in colour patchiness. We are very happy with these walls, the hallway is indeed nice and bright and the warm orange tones in the bedrooms are cozy.

We are looking forwards to completing the last section of the addition this year!

Sculptural Walls

The wonderful thing about natural plaster, whether it be on a cob wall or on straw bale walls, is the potential for creative sculpture. Cob is even more so, as 3-D shaping can be accentuated into the depth of the wall, while with straw bale, the sculpture tends to be more on the surface (unless one wants to cut into the bales). In the bedrooms and the hallway, we added some sculptural elements before applying the final plaster.

In the hallway, we had rather lumpy bales, which I managed to make less lumpy with the rough coats of plaster, but there still were definite hills. Either side of the opening that leads to the upper level of the house were really rather bulgy because of the small cavity within the framing that we rather awkwardly stuffed mini-bales into. We decided to build up a kind of crossing of flowing ribbonish lines that would frame the sides of the opening and go up and over the arch we had built at the top. These lines also widened out around the staircase landing, and down the length of the hallway. We drew lines on the wall with chalk and then used water and some clay slip to moisten the wall before beginning to add the cob.

In the bedrooms, each of the kids wanted to design and create their own sculptures, so we have a bird above one window, and a tree in the corner of the other room. We built a small half moon shaped shelf in our room that is wide enough to hold a tea light candle.

The sculptures look so much more effective when the final plaster is done, but that will have to be another post! The additions of the sculptures definitely makes the work of the final plaster a lot slower and more tedious, but in the end, having these personal touches of creativity enhances the intimacy of the house. Too much sculpture, I think, can be visually overwhelming, so finding a balance between a nice smoothly finished wall and a specific placement of sculptural detail is good to consider. However, considering how much work it is to build a house in the first place, I think it is equally important to take the creative opportunities when they arise- and cob/natural plaster is a wonderful way to incorporate such creative moments.

Second Coat Plaster- Filling and Detailing

Second coat of natural clay plaster

The second layer of plaster for this strawbale house consisted of layers of fills and details to level out the lumpiness of the bales, which was a result of not doing any trimming to the bales once they were installed. This is because of the orientation of the bales as they were stacked into the framing of the walls, which meant they were string side out and unable to be cut. (See this post for details!)

Generally the same process for second coat plastering applies that I described during the building of our first straw bale house (see this post…). The main difference here is that I did a round of filling in the sections that were significantly dippy, (which was generally around the perimeter of the bales), letting that dry, and then covering the walls fully with the attempt to keep the plaster thinner in the lumpy spots and thicker in the indented areas. If the plaster is too thick in the dips, it will crack too much as it dries. The window edges also had burlap which needed to be embedded into the second coat to keep the plaster from pulling away from the wood, and there was some extra shaping needed in the curves of the window openings as we couldn’t trim the bales ahead of time into the curves we wanted.

We created wire forms above the hallway openings to create a curved archway, which then needed two coats of plaster. We also plastered over the interior bedroom walls that are constructed with 2×4 framing and rock wool insulation covered in metal lath. The bottom of the upper story exterior walls needed flashing installed, which we plastered into the wall also using metal lath.

The final purpose of the second coat of plaster other than protection of the bales, is to create a surface that supports the final clay plaster, especially at all the various edges. Once the windows and doors were installed, more plastering details were attended to- wherever the clay plaster meets window sills, frames, counters, kickboards, ceiling boards, etc., there needs to be a tight edge to within 1/4 inch for the last layer to fill. This can be finicky and involve a lot of unique areas that need extra attention and thought, and will be different in each build.

First Plaster Layers

With the baling done by September, our goal was to get the first layer of plaster on the bales before the winter rains began.  Plastering has to be done while the weather is still suitable for quick drying, since it is not advisable to have wet clay against the straw for long enough to develop mould.  The faster it dries, the better.  Because it was September when we started, we decided to make this first layer no thicker than 1/2 inch.  If it had been June, we would have taken the seasonal opportunity to fill the dips, which would mean much thicker plaster in some areas.  However, we will probably go around in the spring and add plaster just to the places that are dippy, so the next layer will begin to smooth out the surface of the walls.

Once the baling was done, there commenced the job of stuffing.  This entails going over each wall and looking for spaces that push through the bales, generally where one bale meets up against another bale.  All along the tops of the bales needed stuffing, and sometimes where the framing separates the bales.  Tucking in tight little twists of straw to fill these places becomes methodical but doesn’t take too much time.

After that, comes the messy job of applying clay slip to all exposed straw.  We collected waste clay trimmings from a pottery studio, and soaked it in buckets so it becomes soft and scoopable.  Then we mixed it with more water to dilute it to a thick chocolate milk consistency, using a paddle mixer.  It was thick enough to not drip very much off a dip stick.  I created a little wearable square bucket to hold the slip while I slapped it on the bales with a big brush and a gloved hand.  The bucket, equipped with old belts that went over my shoulders, could catch any excessive dripping and free up both my hands to massage the slip onto the straw.  It dries a much lighter colour of pink.

Then came plastering!  We used local clay from an excavation job on the island, pitt run sand, (also from the island) and all that straw we collected from the bale trimming.  We built a soaker pit from damaged straw bales, lining it with tarps and then filling it with clay and water to make it soft.  On tarps, we mixed 4 or 5 shovels of the soaked clay with 10 shovels of sand by rolling the tarps around and then foot mixing, adding water as we went to get a good consistency without allowing the mix to be sloppy.  Adding a few handfuls of straw (equivalent to 2 or 3 shovels)  soaked up a bit of water, so sometimes we would add a bit more if it became too stiff.  The test is to make a ball, and not have it ooze or slop, but also not have it be too stiff and crumbly.  The other test often comes from actually applying it, and becoming used to the ideal consistency while working with it.  The hands on experience of plastering generally becomes the best way to really learn!  We had to adjust things as we went- our ratios changed as we dug into our clay pile and found less high quality clay.  We noticed that our mixes were more silty, difficult to apply, and orange in hue instead of blue/grey.  It was a subtle change as we filled the soaker pit, but after a few mixes we realized we needed to amend our clay from a different pile we had saved.  (After so many years of natural building, we have all sorts of little and big piles of resources hanging around!)

The clay slip gets sprayed with water before applying the plaster, just lightly, not so that water runs down the wall but the slip turns darker in colour.  A handful of plaster is then smeared onto the wall with the heel of the hand, and massaged in so that it doesn’t peel back off.  Everyone finds their own method, but the important thing is that it isn’t too thick, or too thin, ideally the same thickness, and it doesn’t peel off! Our first group of plasterers were my son’s school class, a group of 24 kids ages 10-14.  They spent a whole day rotating in groups between mixing, plastering, and hiking around our part of the island while birding and geo-caching.  They got so much done!  We also had a visitor staying with us for two weeks, learning all the steps of plastering.  For the next month, we had a variety of friends and community people coming by for a few hours or a day to get their hands muddy on our walls.

Partially dried wall

We focused first on the north walls, and the places where there is less sunlight and wind movement.  Then we moved to the inside when the weather got rainier in October.  Luckily, the fall was generally sunny late into November, so when we finally got to the south walls, the warm sun was still shining most days.  We used a fan to help dry out some of the inside walls that weren’t getting much sun.

plaster over clay slip with insulation at the top covered with metal lath

Plastering the bales in this build was different than the last house we did because the bales were oriented differently- instead of placed like bricks and plastering the sides of the bales we have stood the bales up and plastering the faces of the bales, where the strings are.  In the last build, we trimmed the entire bale walls, shaping the curves of the windows and removing any shaggy straw, so the plaster went onto the evenly cut ends of the straw.  This time, we couldn’t do any trimming because of the baling twine, and the plaster went onto the length of the straw.  While we didn’t need to take the extra step of trimming, it was a little more challenging to apply the plaster.  The walls are more lumpy and will need some extra work in the plastering to get a nice smooth wall.  Smooth walls, of course, are a matter of aesthetic preference over function.  Maybe in the end we will incorporate more creative sculptural elements into our walls.

first coat plastered dried. Burlap will be plastered into the next layer of plaster.

This winter will be a time for these seed ideas to take root while the house building projects are on hold.  We can take some time to sit in the house and imagine the different possibilities for the next phases.  It can be frustrating at times to have to be patient, but often the results come with better decisions and a renewed sense of creativity following the flurry of building.

If you read all this and want to see more photos and more descriptions of plastering, please refer to my previous post on the last house we did~ Natural Plaster

Workshop Update

cob workshopThings have been progressing slowly but steadily in the workshop since we finally got the roof finished in the winter.

Colin focused on getting the doors in right away, refining two old doors that came from his Dad’s work of deconstructing old houses on the island, and refitting a pair of salvaged french doors.  He also built a raised stone hearth from the pile of stones we have on the property, put in the stove pipe, and wired in the rough electrical circuits.  We got the insulation in the ceiling and the plastic vapour barrier secured over that and ready for the finished ceiling layer, which we think will be some kind of bamboo or sea grass mat.

rough sculptureI spent a couple of weeks on and off getting the rough coat of plaster on the inside and outside of the building.  This layer of clay plaster is the only rough coat needed on a cob wall, and is the same layer as the third layer that we put on the strawbale house.  (See my previous post called Natural Plaster for full details).  A mix of 4 parts sand, 2 parts clay, and 1 part straw with water to mix it to a troweling consistency kept me busy in the warmer days of late winter. rough plastered windows I cleaned up and shaped the sculptural aspects of the walls so they were ready for the final layer of plaster, which Tracy and I completed earlier in May, while the temperatures were still cool enough to give the plaster time to dry.  The final clay plaster took us 2 days to put on, a seemingly record pace for any one job to be completed in.  We mixed up the kaolin clay, sand, straw, paper pulp and whiting in the buckets the day before, and then added the rice flour paste, and borax and mixed multiple buckets together the morning we started plastering.  (See my older post Final Plastering for more exact details).  We got much of the simple walls done on the first day, then completed the wall with the windows and arched shelf the next day, cleaning up our tools by the afternoon.  I am always so amazed at how quickly the finishing layer goes, compared to the time it takes to get all the other layers beneath it built.final plaster

Last weekend, Colin and I built the one interior wall that separates the workshop from a small storage room.  It will also be the wall against which will be built Colin’s work bench.  It holds the majority of the electrical outlets and is a standard 2×4 construction filled with rock wool (Roxul) insulation and covered in plywood.  final plastered windows

There is not too much left to do in this simple building for Colin to finally move in and use the space for his wood working.  Right now he is coming to the end of a giant stone facing job and is looking forwards to working with the more forgiving medium of cedar wood.  I can turn my attention to finishing the outside of the building later this summer, playing with some creative designs for sculpture as well as getting the last coat of lime plaster done when the weather begins to cool off once again.     interior wall

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