! Help! Applying your own clay plaster - How realistic is my endeavour?

Hi everyone,

I have been spamming the forum with questions lately, but have been doing so with good reason. We are currently in the process of deciding on the contractor to finish the construction of our house, and have been pouring over the prices they have offered for the work. The listed prices are unfortunately a tad higher than what we can currently afford, so we are trying to decide where we can cut the bill by postponing work, or by attempting to do the work ourselves instead.
One of those things we hope to do ourselves is putting in the flooring, and  I had posted about it already here. It turned into a really interesting thread, but also one that made me realise I need to lower my expectations of what level of fancyness I can achieve by myself with my limited skills. Which is why I once again want to get your insights on another possible project to see if it is doable doin git ourselves, or if we are underestimating the difficulty of the task at hand.

Is it realistic to apply our own clay plaster for the internal walls? we are not aiming for absolute perfection here, but we also don’t want the plaster to crack/ crumble/ make us feel like we’re living in a hut, rather then a modern home. We hope to be able to achieve something like the picture attached below, but have no idea how realistic that aspiration is.



We are starting from bare brick walls. You can see some pictures of the internal walls attached below.
I’m guessing we would need to apply a base clay layer first then more rough one, with silt, sand and fibers (probably flax or hemp). Could this be attached to the wetted brick walls directly, or would I need something to toughen/ strengthen the attachment. I’ve seen application of mats of reeds to the walls onto which the first clay layer is attached. I’ve also seen it attached straight to the brick, but with a reinforced mesh inside the clay plaster to prevent too much cracking. Maybe two or three applications of the base layer would be necessary? Then on top of that we would want a finishing layer of clay (without the fibrous material visible). This would just be a thin layer for the final appearance. We have no experience with plastering. I have experience with pottery but that is probably not very relevant. I am fairly handy and precise though, and generally a fast learner.

I’d love to hear anyone who has experience or insights with the matter! Please let me know if it is a good idea to do the plastering ourselves to save money on the contractor bill. Or would we end up more expensively if we do it ourselves (because we might need more material, would have to buy lots of tools we don’t have yet/ might fail to do so entirely)? Any help would be welcome. We have to decide soon on the contractor, so speedy advice would be greatly appreciated.

Thanks!

No experience with using clay as a plaster. I’ve seen a commercial version used in wall treatments that includes fibrous material incorporated and was considering it in a century house kitchen.  The technical docs list 4 sub layers in addition to the finish coat and that comports generally with my experience with frescos that were applied in four successive layers over some sort of support. Historically horse hair was mixed in and took the place of plastic fibers used today. I’ve used metal lath to hold the first scratch coat and I suspect to get it to hold some sort of support would need to added so the first coat can be thrown into it to secure it to the wall once it dries. Then successive coats, the brown coat and sand coat and finally a finish coat that would consist of like lime putty and marble dust in a fresco.

Thanks for your reply, James.
It’s hard to find conclusive information, but from what I understand, as long as your surface is rough enough for the clay plaster to attach to it, you could get away with 1 or 2 layers of the base clay plaster (brown layer), of about 3 to 4 cm, which would be a mix of sand, clay, water and fibers In would go for linen or hemp fibers), then on top of that would come the finish coat which would be a pigmented clay layer, without the fibers ( a few mm)
Maybe a base layer of lime, or mixing some lime trough the clay plaster could be good to prevent mould. So maybe clay plaster can be a little less complex then what you are describing?

I forgot to mention that we have a lot of heavy clay at our property, so obviously we would love it if we could use this clay for the base layers (which don’t have to be pretty, just structurally sound).
Fortunately I was smart enough to dig up a bit of clay and take it home with me the day before the quarantine started, so I have been trying to do a soil test to see what the soil is made of.
I had read to put the soil in a cilindrical pot, add water and shake it untill all of the soil is solved into the water. Then let it rest for a bit on a flat surface. The sand should settle first because it doesn’t solve into the water, then on top of that the clay, and the organic material would float on top. Then you can measure the ratio between the amount of sand vs the amount of clay.
Now I’ve done this with my soil sample, but I’m having a hard time seeing the difference between the sand and clay layer. Or maybe my soil just contains very little sand, so there is simply none to be seen?

What do you guys think? Would this clay work well to mix the plaster with?
Are there differences between the different types of clay that are relevant when it comes to making clay plaster?

Hard to tell for sure by the pictures but I know I've had samples like yours with really no distinction either. The best thing I would suggest is to do some patches on the wall with different proportions of the various things you would like to try. Be sure to etch a number or letter and record the ratios. If indeed you have mostly clay,  a ratio of 3 sand : 1 clay soil and 4 sand : 1 clay soil will most likely work out best without too much (if any) cracking.
A few days ago I just finished the final plaster layer on my rocket mass heater and used a 3:1 ratio with cattail fluff and a little wheat paste. Not a single crack so far.

EDIT:  Your wall bricks look like they have plenty of texture to them without the need for any further structural support. To be sure though, once your test patches are fully dry, try ripping them off the wall and see how good the bond is. Also, hit them with the claw side of a hammer and see how they hold up. Do they crumble or resist falling apart? When you slide your hand across them do they feel chalky or not leave any residue on your hands etc.

I think applying your own earthen plaster is definitely realistic as long as you have the time and you're okay with highly repetitive tasks. We're in the process of plastering our 400 square foot cabin. It's timber frame with straw light clay infilled walls. For our base coat we're getting fill dirt($20/yard) from our local landscape supply yard. Loads vary some but the mix is roughly 60% sand, 15% silt, 25% clay. We use a 1/4" screen and add chopped straw(~7.50/bale). For our final coat we're using ~ 2 parts screened sharp sand(~$45/yard), 1 part bagged C & C ball clay(it's around $12 a bag here but the final coat doesn't use much) and iron oxide pigment for coloring.

As for applying our bricks, it looks like there's enough of a mechanical key for the base coat to hold on. We plastered over some rough finished framing elements and across studs and the base coat held. The best thing would be to test a small section and see how it holds up

Responding to your other points, tool buy in is fairly low:
- Heavy duty drill and mixing paddle (~$100)
- Trowels (~$60)
- Buckets
- Concrete/Mortar Mixer (Optional)

Here are some video links:

This is an amazing 7 minute video that I've easily watched a dozen times


This is video from applying our base coat. We're currently working on the finish coat. I'll have something up soon.

I did some repair on house I owned about 15 years ago, it had lathe and clay on the walls, built up about 5/8" to 3/4" thick. I used what was on the lot, simple red clay. I added sand, but I forget the ratio. I do remember that it took quite a lot of the clay, more than you'd think anyway. I mixed it with hands and a shovel and toted it inside in buckets, I applied each coat with a tile layers float. It went well and when it cured I painted it to match the rest of the wall. Looked good, and lasted as long as I was there.

I say go for it, life is an adventure.

I have some experience, with rendering with natural products. My friend who's done a lot if it quit giving advice, mainly because the earth people use is different everywhere, the clay content, silt content,sand etc. So you've got to do your own testing, that takes time. To properly dry and so on.You can go and rush it on. No problem, but later you'll have to do it again or add another finishcoat.

If you're new to rendering, start in places where you are not going to be constantly, start in a hall way, not the living. The walls, not the ceiling.. Put time into making a perfect mix, the amount of water being used is of major importance. The difference between you really having to push the stuff, it dripping off your trowel onto the floor and having something that really sticks to the trowel and you can smear on fluidly all day long to never look back. You can always add more water in the mixer, taking it out is impossible, you can add more ingredients, but when you have a complicated mix, that is very time consuming. Get good measuring equipment for the finish. Even near perfect measuring results in big differences.

Some people take a liking to the continuity of the work after a while. Me and my girlfriend at the time, she did beautiful restive strokes, repetitive, beautiful work, with a lot of love and be here now-ness, singing softly to herself , while i just impatiently slapped it on in anger, thinking about the next job, bills and evil things people had done to me. It's turned out ok-ish, but that's only because i did the big surfaces most people's eyes don't register and she did the details, around roof windows, connections with wood.
Over the years i got better at it and have done the front of the house, with local natural materials, even getting compliments of the locals.
If you are in a place with many romantic foreigners who are doing local houses up, there must be someone who is doing this job for a living, they might give a tip or two, where to get the best clay for cheap, coloring etc. Don't rush it. Do it when you can't work outside, in the heat of summer. If your house doesn't get scorching that is... Cause the hotter, the faster the drying, the more it will crack.
Any way, here some pics.

Thank you all for your wonderful advices. This is really good affirmation of my hope to be able to do this ourselves. Hopefully with the clay we have at the property.

Hugo Morvan wrote: Some people take a liking to the continuity of the work after a while. Me and my girlfriend at the time, she did beautiful restive strokes, repetitive, beautiful work, with a lot of love and be here now-ness, singing softly to herself , while i just impatiently slapped it on in anger, thinking about the next job, bills and evil things people had done to me. It's turned out ok-ish, but that's only because i did the big surfaces most people's eyes don't register and she did the details, around roof windows, connections with wood.

I can totally relate to this. I suspect the situation with us will be very similar. I’m the patient and precise one, while my husband enjoys the more hard physical labor, but gets frustrated easily whit things that have to be precise and tedious. So I can imagine he will be the one mixing the materials and hauling it up 3 flights of stairs, and I will be the one applying it. I actually look forward to doing the clay plaster, I can really enjoy doing repetitive tasks. Puts my mind at ease knowing I have to focus on nothing else but the task at hand.
By the way, Hugo, your house looks beautiful!!

@ Aaron, thanks so much for sharing the clip of how you are applying the cob at your own property. I’d love to see the results once you’ve applied the finish coat. What are you planning for the finish?

In any case we will definitely be doing a few tests to see if the material we have on our property holds up. We are thinking of using our own clay for the base coat, and then buy the material for the finish coat and spend a little more money on having a nicer quality finish to look at.

Could any of you help me calculate the amount of clay, sand and fibrous material I would need for the base coat? How much would you need per square meter?

Thank you for the compliment S.Bard! Great you already know the divide of labor roles. If i were you when you're doing the finish, which usually is a matter of a very thin layer, have him haul up all the materials to where you work, do your own mixes, in a bucket with a mixer on a battery drill will get you a long way, and send him off to do something else if he doesn't mind too much..
While the two of you're getting into the rendering habit,earlier on look and feel the material. Scrape crazy thing sticking out off, when it's dried enough to have sucked onto the underlying material, but not dried enough to be unworkable at the surface. Smooth it off, with a sponge if bits are too rough and sticking out, roughen the surface up with a scrapbit of wood or a trowel, add a tiny bit of the material if there is a crazy dent. Sometimes you should wait for it to dry completely, because you can't add layer on layer, only when it's properly dry.
Be careful, be thought full, but not afraid to mess with the stuff. Sometimes with materials, the perfect point to work the material is at ten o'clock in the night. I've worked at those times.with chalk based mixes. Get to know the material you're working with. Put a few small sticks in it on an undercoat, see and feel if you can still work it at different times and pull it out. Feel how it behaves during drying in the beginning.
Good luck and photo's please!

Thanks for the wonderful advice Hugo.
I will definitely share photos once we get started.

Any idea on how to calculate the amount of material necessary?

Thank you S. Bard.  Glad to help. Depends on how thick the layer is you put on it. If it's 150 square meter and you put 1 cm of the layer on. You will need 150 times 0,01 =1,5 m3(cubic meter). If it is going to be 5 cm, 5 times that. More specific i can't get, because i wouldn't know the formula you are going to use.
Depends as well if you have a lot of dents in the walls that are present now. You're probably going to use a very straight piece of wood, i usually use a aluminium ruler of 2 meters. You lay it flat on the wall and measure to the wall every ten cm amongst that ruler, then you take the average. If you do this at different places you get an idea of the amount of filling you need to do to get the wall filled flat
I usually put a lot of my material in a straight line on the wall from up to down, push the ruler into it and use the level to get it level. You kind of wriggle the ruler in. On both sides the material which sticks out you wipe it off, You do this every rulerwidth minus 10 cm along the wall. Then you've got some measurement lines to work with on the wall. Then you fill it in exaggerated in between those lines. By that time your vertical perfect lines will have dried a bit, then you use the ruler to level it out between the standing level lines. You easily see where some extra is needed.
I hope this makes sense to you! I totally understand if it doesn't though.

I will be eagerly following, as we are in the same boat!  I look forward to seeing your pictures.  We are just starting to test & play with our on-site materials, and will try some test patches.  It all feels like high-stakes playing.  Fun to explore, but with the pressure to get it right before committing.  All the best to you!