26 July 2011

Clay Plastering Strawbales - FIRST COAT

I would like to share a few tips I have learned through trial & error over many years for plastering with clay on strawbale walls. In this post, I address issues and tips for the 1st coat of clay plaster applied directly to strawbale walls. I address 2nd coats of clay plaster in my post "Clay Plastering Strawbales - SECOND COAT". For information on finish plastering, see "Finish Clay Plaster & Wall Preparation".

Goal for 1st coat of plaster
The primary goal for the 1st coat of plaster is to create your bond between clay and straw.  The goal for the second coat is to shape & sculpt the wall the way you would like it to look.  And the finish coat is where you achieve your textures and sometimes color.  The strawbales provide ample surface area for the clay to bond to, so I do not use any lathing or metal mesh or anything else that may actually get in the way of working the clay directly into the straw.  Instead, I work a sticky clay plaster directly into the straw surface.

1st coat of plaster is often lumpy & bumpy

Because the goal is coating the bales with a well-bonded layer of clay plaster, your first coat of plaster often follows the contours of the bales.  This means the first coat is usually not that pretty...but rather comes out rather lumpy & bumpy.  This is OK!  You will shape the wall with the next coat of plaster.  It is more important that you set up great contact with the strawbales so that the weight of all the plaster to come adheres well to the wall surface.  You can shape the wall a little bit with this coat of plaster, especially in wedged areas, like the cracks between bales.  But remember that the clay is a liquid until it dries...and its stickiness can only support a limited weight of plaster before is flows off the wall.  If you find the plaster is slumping, this is usually a sign that your plaster is too liquid or that you have put on too thick a layer and the weight is pulling it off the wall.  Once the plaster dries, it is bonded to whatever it is touching, so best to be patient, use a thinner layer, work it into the straw really well, and wait until it dries and is strong before adding more weight with subsequent plaster.

WALL PREPARATION for 1st coat of plaster
1. Shape the straw - this means trimming the walls as closely as you can to the shape you want.  Spending more time shaping the straw pays back in magnitudes of time saved mixing and applying plaster.  I also cut niches and identify any truth windows prior to plastering the first coat.

2. Install electrical - I find that it is much more challenging to install electrical boxes after you have plastered the strawbales, so I get this out of the way before plastering.  This also makes an electrical inspection more straightforward, if you need one, since you have not covered up any of the wiring with plaster.

3. Make sure your walls are tight - this means achieving a pre-compression of the strawbales before plastering.  If you plaster before the bale wall is compressed downward, then the weight of the plaster can cause the strawbale wall to sag, leaving an open gap at the top of the wall.  A gap can be filled in with insulation, but it's usually easier, less stressful, and cheaper to compress the bales first.  There are many ways to achieve a tight wall, starting with purchasing tight strawbales to construct with.  (The looser your bales are to start with, the more they will compress in place.)  I make sure the top bale in any wall is installed very tightly up against some kind of framing above (floor, trusses, etc.).  You can also use temporary straps to compress your walls.  I'm sure there are many other solutions, but the general goal here is that the more compressed your straw is, the less your wall can settle from the weight of the plaster.

4. Trim the straw - the trimmed side of a bale, with all of the straw sticking out perpendicular to your plane of plaster, is an easier surface to plaster than the folded "fuzzy" size of a strawbale.  So trimming your bales with a weed whacker, chain saw, hand saw, or whatever works for you, saves you time applying the plaster.  Trimming also gives you the opportunity to sculpt the straw surface to the desired shape of the completed wall, which saves you time mixing and applying plaster or cob-like plaster to fill in voids and shape your walls with clay later.

5. Fill in voids - this also saves you time later!  Unless you want your wall to remain lumpy & bumpy, I find it is the least amount of work to make sure that voids between the strawbales (and at seams with other surfaces) are stuffed tightly with straw or light clay straw before I begin plastering.  Otherwise you are making additional plaster or cob-plaster to fill those voids later.  I use handfuls of long pieces of straw, align them all like a rope, twist the rope, and then fold it in half.  This lets you make relatively compact bunches of straw that you can then push between strawbales.

6. Wet the walls down - I think the most common error I see is applying plasters over dry substrates.  I hose down the surface of the straw until the water drips off, and then allow the water to absorb for 20 to 30 minutes.  The straw should feel pliable (not prickly), but all the water you spray on should be absorbed (not creating a water slick).  If you have a water slick, the clay plaster will slide right off the straw and drive you crazy.  If the straw is dry, it has a tendency to quickly suck water out of your plaster, which can cause the bonded clay to dry too quickly, which can subsequently reduce the adherence of the clay to the straw.


TIPS for 1st coat of plaster
In order to ensure a fabulous bond of the first coat of clay to the strawbale wall surface, I do the following:
  • high clay content - I used to try to control cracking in the first coat of clay by adding sand & sometimes straw, but this mixture is so difficult to apply because it is not as sticky.  So I have shifted gears toward a very high clay content, shooting for between 1/3 and up to 1/2 total clay content (the remainder is sand from the soil or added concrete sand).  I do not add straw to this layer, instead working the clay into the surface of the strawbale walls.  The high clay content means you will have shrinkage, and therefore probably lots of cracking, in that base coat of plaster.  But I find I am able to get a much stronger bond with the base coat if the mixture is nice and sticky, and you can easily address the cracking in the next layer of plaster.
  • sift soils through 1/2" screening - if you are using site soils, I recommend to sift the clayey-soil through a 1/2" screen to break up the soil into granola-sized chunks.  This makes the plaster easier to mix, and you don't end up with large chunks in your plaster that are difficult to impossible to work into the wall.  You don't need finer screening unless you are troweling onto a hard substrate in thin layers.
  • apply 1st coat with your hands - when I teach plaster workshops, I used to invite everyone use their hands or a trowel to apply 1st coats of clay plaster on strawbale, depending on their own comfort level with tools.  And invariably, someone with troweling expertise, would apply a vast amount of plaster on a wall, and it would look great, and then...it would fall off.  What was happening is that by using the trowel instead of their hands, they couldn't feel whether the clay was worked into the straw.  So now I always apply the 1st coat of clay plaster directly onto the strawbales exclusively by hand (or with gloves), and not at all with a trowel.  This way you can feel...with your hands...the tactile bond between the clay and the straw.  You can feel if the straw is loose, like at seams between bales, and you work additional clay into those spots.  You can feel if the clay is tight on the straw or if it shimmies like jello and is not bonded.
  • really work the plaster into the straw - this is probably the toughest to explain in words, but basically, you want to work the sticky plaster onto the surface of all the strawbales, but also into all of the nooks & crannies between strawbales as well as the joints where straw meets other materials.  I use the palm of my hand as a trowel to apply plaster to the surface of the strawbales, and I use my fingers to kindof massage the clay into any loose spots between bales.  Once you have some sticky plaster bonded to the straw, you can add lots of straw to your mix, and use that heavy-fiber plaster to fill in any big voids.  The straw-plaster will stick nicely to the clayey-plaster that you already worked onto the bales.
  • don't worry how it looks - the most important quality for the 1st coat of clay plaster is that the clay bonds well to the straw...it is not what the shape of the wall looks like.  That is not to say you cannot shape the wall with this coat of plaster, but the primary goal is the bond to the straw, not the look of the wall.
I'd love to hear other people's experiences and tips!  So please feel free to add comments below.

Resources

If you are plastering over strawbales, read our other clay plaster posts:
"Clay Plastering Strawbales - SECOND COAT"
"Finish Clay Plaster Recipe & Wall Preparation"

These are the most useful books I've found for information on making & applying clay plasters.

            

click the book covers above for more info or to purchase

      
Using Natural Finishes: Lime- & Earth-Based Plasters, Renders & Paints, A Step-by-Step Guide by Adam Weismann and Katy Bryce, Green Books Ltd, 2008

Clay Culture: Plasters, Paints and Preservation by Carole Crews, Gourmet Adobe Press, 2010

The Natural Plaster Book: Earth, Lime and Gypsum Plasters for Natural Homes by Cedar Rose Guelberth and Dan Chiras, New Society Publishers, 2003. 

27 comments:

  1. Thank you very much for your comprehensive description of the plastering techniques to apply over the straw bales.

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  2. Thanks for the good info! We are about to start with some clay plastering of internal Straw bale walls. Any tips for a)finding the right kind of clay and b) plastering in the cold??
    Thanks again,
    Bongo.

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  3. Hi Bongo, and thanks for your question.
    For finding the right kind of clay, I'm not sure where you live, so not sure how to advise there. If you want to purchase dry bagged clay, I recommend trying to find a local ceramic supply retailer.

    As for mixing in the cold...I would mix inside. If you are using site soil and your ground freezes, then I would sift your clay before it freezes, and then either bring it inside on a tarp or cover it to prevent rain from re-clumping the sifted soil. For mixing itself, I would just do that task completely inside. You can add warm water if you are using clay-soil and it is cold. Or you can bring several days' worth of sifted soil inside to warm up.

    Hope this helps. Let me know if I have spawned additional questions!
    Sigi

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  4. I am preparing to put the first coat of lime plaster on my exterior strawbale walls. Do you have any specific advice for lime, that would be different than the method for clay?

    Am I correct in my thinking that I don't need to worry about applying the burlap to window/wood edges until the second coat?

    I understand the lime needs 6 wks to cure before the first hard freeze. How crucial is it to get all 3 coats on the exterior the first year? I do have a wrap around porch, so rain/snow will rarely if ever blow onto the walls.

    In regards to the interior walls. I want to be able to have color (besides brown) on my walls. So, would you suggest using native clay for the first 2 coats and a bagged kaolin clay for the 3rd? Or would it be just as effective to use native clay for all 3 coats and then use a lime paint over top? I'm curious about the pros and cons. I may consider using bagged clay for all 3 coats if it isn't too pricey. Could you tell me how many sq ft a 50 lb bag of clay would cover?

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    1. For lime on the exterior, the preparation is actually pretty specific. You need to lath any joints between dissimilar materials, for example. Otherwise you will have cracks just from normal movement and expansion/contraction. If you have never done lathing before, I would get a stucco expert in your area to help you understand proper installation. Then you will need to wet your straw with a hose before you apply the lime. Be sure your lime is protected from wind and sun that will dry it out. And be sure not to exceed 1/2" thickness for your lime. It needs access to air in order to cure.

      For interior walls, I actually like to use site clay for all 3 coats. That is always my preference. The only reason to do kaolin is if you want a different color. Hope this helps!

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  5. Thanks so much. Do you consider clay plaster on the exterior to be acceptable if I have a wrap around porch that will prevent nearly all rain splash back? It's been raining nonstop for 3 wks and my unplastered bales are still dry. Only a wild and windy storm or maybe blowing snow can reach the walls. I'm thinking now that clay plaster would be way simpler to apply. I have ordered a roll of burlap already. Does burlap not work well with lime?

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    Replies
    1. yes, yes, for sure, you can do clay on the outside. The only downside is erosion over time if the clay gets rained on. If you have good overhangs to protect your plaster or if you don't mind the occasional replastering (or repair plastering), then I would definitely do clay on the outside. Just don't do clay base plaster with lime over it. Commit to one or the other.

      For the burlap, I have done burlap to limited extent with lime, but really for significant joints like wood-to-straw, you want well done metal lathing.

      Good luck! Let me know how it goes!

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  6. Thanks so much for the advice.

    I know using site clay is ideal, but since we have harvested our own wood to build the post and beam structure, we are a bit tired, so we would rather take the easy route and use bagged clay. I've not been able to find any info on specific varieties that are good for plastering. I have seen gray ball clay recommended for the base coat, but that's about it... Perhaps you could point me in the right direction.

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    1. I have located a ceramic supply store, but they have so many varieties that it's a bit overwhelming trying to choose!

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    2. For the first coats that you will not see, I would just get the cheapest clay they sell. Probably Red Art Clay or Gray Ball Clay. For the finish coat, you can select a clay based on color, or select white kaolin and pigment it any color you like. Does that help? (in other words, all pottery clay is sticky enough to make plaster with, so pick based on cost & color...and then you have to figure out sand proportions depending on the exact clay)

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    3. That helps greatly! Thanks so much.

      One more question... I have read of people adding small amounts of lime or borax to their clay mix in order to prevent mold growth. Do you recommend either of these, and if so, how much?

      Because of the humidity these past few wks, we have some small spots of green mold on the inside walls. I'm confident its only a surface thing, because our bales have never been rained on, and I expect it to be fine when we trim the bales... What do you think? This is why I want to add something to the clay mix to prevent mold growth.

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    4. you have to actually experiment with adding lime. There is some secondary reaction that I don't fully understand but that involves the lime and magnesium in the clay that can change the properties of the plaster. Also, lime is caustic, so if you add it, you need to wear gloves to work with the plaster. For borax, I usually add it to my wheat paste, which I do add to finish plaster, but I don't worry about it for any other coat. If you just let the walls dry completely, the mold will die, and you can just brush it off before you begin your next coat. If you really want to kill the spores, you can wash the walls with a mixture of baking soda and hydrogen peroxide, but I usually don't go to the trouble...

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    5. How much clay is needed for a 10x10 room. Approximately. ..."

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    6. For finish plaster: calculate the total wall area (width x height), divide by 200, and that's how many 50-lb bags of dry kaolin clay you would need. If you are using other clay or doing coats besides the finish plaster, then you'd have to experiment with your particular materials to calculate the recipe and the quantities.

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  7. Thanks again! You set my mind at ease. We have finally had some sunny weather and everything is dried out and smelling fresh.

    Can you give me an idea of how much coverage I will get from a 50 lb bag of dry clay? I know it will vary depending on how much sand I add and how thick the coat is, etc..., but a ball park figure would be very helpful. Also, what thickness do you recommend for the different coats?

    Your finish coat recipe using kaolin clay appears to cover 100 sq ft with only one cup of clay... I'm having a hard time wrapping my mind around that... so maybe I'm reading it wrong?

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    1. Sorry for the recipe confusion. I just changed the recipe in the post with actual measurements, so hopefully it's clear. Basically, one 50-lb bag of dry clay makes just under two 5-gallon buckets of clay putty. And the recipe is for one 5-gallon bucket of clay putty. So, your 50-lb bag should make enough plaster to cover 200 SF of wall. Obviously, it depends on thickness too...
      Hope that helps.

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  8. A very good informative post....Sigi your knowledge of the strawbale wall plastering is marvellous i tell you. You have clearly explained each and every minute requirement of plastering a strawbale wall ..really nice.....i like it....

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    1. thanks! Glad it was helpful to you. :)

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  9. Thank you so much for sharing this beautiful, helpful & clear information.
    I'd mega-appreciate some of your insights/guidance with my situation...?

    We live in Eastern Canada (zone 4), and the timing for our bale installation will be Autumn- when the temperatures will change.
    We intend to clay plaster the exterior walls & install wood siding with a vented rainscreen. Our interior walls will also be clay plaster.

    I'm wondering:
    1) Should we do all 3 regular clay plaster layers for the exterior, (given it will also be sided), or is there a different technique for plastering in this case?

    2) Given the upcoming cold, are there risks/limitations in completing the exterior plaster before winter?

    3) If we can get the exterior done in autumn, would it be possible to then do the interior plaster as winter sets in- with a woodstove as our heatsource? (Or might wood heat be too drying and mess up the curing process)?

    Appreciate your time, if you're able.
    Cheers!
    Jenna

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    1. Hi Jenna,
      I'm so glad to hear that this info is useful to you! Sounds like you have a spectacular house underway. For your questions:
      1) If I use siding, I only coat the straw with clay...usually a single coat, worked into the straw pretty well. If you want it to look nice, you can go 2 coats...but that's bonus.
      2) You absolutely do not want any plaster to freeze when wet (or even damp). If it freezes, you will see crazing cracks all through the surface, and what's happened is that all the sticky clay bonds have been broken when the water expands from freezing. This makes your plaster weaker. So I try to apply when you know your plaster can dry completely without risk of freezing.
      3) For sure! I've had many many clients do this. A couple tricks to help your sanity & comfort: if you can, stockpile your sifted clay inside so it's not cold, and you can also use warm water to mix the plaster so it's not miserably cold.

      best of luck to you!

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    2. Thank you so much Sigi,
      I really appreciate this advice!
      :)

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    3. ... oh, one more thing, if I may.

      To ease my anxieties: any ballpark estimate to drying time, in the fall?
      I realize a million factors (temp, wind, sun) come into play, but I just don't have any idea what type of timeframe I could be looking at.

      Thanks so very much!

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    4. depends so much of the variables...thickness of the clay plaster, humidity, temperature, air flow, etc. That said, an averagely thick plaster applied in average temperatures should be dry enough for the next coat in about 1 to 3 weeks. The final coat takes less time...usually a few days. Hope that helps!

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  11. I’m very confused by all the contradicting information about plastering strawbales. Yours has been the most helpful and simple!! Last night I tested a spot with clay and it won’t dry. Too much water?? One person on permies said clay takes weeks to dry. Help?

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    1. clay plaster can take days or weeks to dry. drying time depends on the following variables:
      1. how warm it is (warmer = faster drying)
      2. how humid it is (less humid = faster drying)
      3. how much airflow there is (more airflow = faster drying)

      When you are plastering clay over clay, it also depends on what your substrate is, how damp it was when you plastered, and how absorbent it is.

      It doesn't really matter on how much water you've added to your plaster, since wetter plaster will also inherently be applied thinner (and so dries more quickly).

      Hope that helps.

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    2. Thank you. This is more complicated than I thought it would be so I appreciate any help I can get. I’m battling time now as well with winter coming. And for whatever reason I can’t get my clay to be clay anymore instead of watery clay. Very frustrating.

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