Winter Protection: How to Add Stucco to Compressed Earth Bricks [video]

Country stucco - Winter protection

Graham Robertson (Open Source Ecology volunteer):

My name’s Graham Robertson. I’ve been here for about five months, working on construction and getting the infrastructure set up.

So what I’ve been doing, as opposed to trying to fill the wall, like all the cracks and holes in the wall with stucco, which is a cement… it’s like a 10 percent cement and lime mixture, I’m going in and filling it with sand clay first, because you can build a lot thicker layers with sand clay.

And then, if you… to get the strongest finish with a cement lime mix, which is sand clay with a little bit of cement and lime mixed in, you want it to be the thinnest coat possible, because lime… hydrated lime, in order for it to cure, it has to be exposed to air and water, moisture in the air… and if it’s too thick of a layer, it’s gonna crack. It’s not gonna cure properly, and it’s probably not gonna be very weather-resistant, which is the objective of why we’re plastering these bricks in the first place.

So another reason that I’m filling these cracks is because it’s about to get really cold here. These cracks will let cold air…or, they’ll let the hot air escape… and will kind of ruin any attempt that you have at heating a building.

A wet mixture of sand and clay

Little cracks, anything from a quarter inch to, I’d say, an inch wide, you can just use a pretty wet mixture of sand and clay. So just go in… it’s best to wet your wall first, but for the sake of demonstration, I’m just gonna go in and show… just start from the bottom of the crack and smooth upwards, and just kinda go all different ways. Just kinda cup your hand and push it into the crack.

The stucco’s a lot stronger the thinner the layer is, so think of the objective like you’re trying to get like an eggshell coating on the wall, because that way, the lime and the stucco carbonizes more thoroughly. The more it’s exposed to air and moisture in the air, the stronger the chemical reaction will be.

But stuff like this, it’s just too, a little bit too big for filling with sand clay. Just push it in, get some like small gravel, like this… mix it up with some sand clay, and then just push it in. Sometimes it’s, the rocks are too big to fit in there but… basic point of this is the rocks add a lot of strength, they prevent cracking.

Mixing up stucco

OK, so this is instructions on how to mix up cement lime, earth and plaster, or the other known as stucco [it’s also known as stucco].

So, start out with just enough water to coat the bottom. The next thing that’s makes [sic] things a lot easier is having slip already mixed up, which is one part clay and one part water. What we do, our process for that is using a screen and taking dry clay, just any kind of clay-ish soil, and pushing it through a screen into a barrel of water or wheelbarrow, or bucket of water.

So it’s good to mix up your lime with your slip, lime and cement and slip together, mix it up homogenously and then add the sand. So this is like half a coffee cup of lime, or whatever that kind of can is. Just five percent, just keep that in mind, you don’t really have to be so precise with it, but… and the ratios will vary depending on your soil, and it takes some experimenting. So what I would recommend is mixing up a few different types and noting what your ratios were, and then doing a test portion on the wall, so… and then writing down what each ratio is and figuring out what’s the best for your soils.

So then we’re gonna mix this up, and then what I usually start off with is about three shovels of sand, and just get it to where it’s like watery ketchup, is what I think of.

So that’s a good consistency for my preference… you can kinda tell because the… the consistency of it, you can tell like how it stacks up on the surface, kind of piles, but it’s not completely thick like that stuff. So I might need to add a little water to this stuff. See that, how that sits up on your hand without flowing off, like this. That’s more what you want for a plaster sprayer.

This could, you could get by with that for trowelling… if it’s too thick with the plaster sprayer, for some reason, like this consistency cracks more than if it’s thinner, like that.

The plaster sprayer

It’s a [sic] air tool… so you can see there’s four holes here, now, those holes are not connected to anything, but there’s four jets behind these holes that are pressed… that are blowing compressed air. But you can get these at mortarsprayer.com, ideally you need a compressor that can throw air at… I think it’s at 90 PSI [620.53 kPa] at 12 cubic feet [about 0.34 cubic metres] per minute.

So the jets will basically just blow the plaster through these holes. It’s a pretty simple tool. Now the problem is, that’s another thing I forgot to mention in my stucco instructions, is if you’re using a plaster sprayer, it’s really important to screen all of your sand and clay through a quarter inch [6.35 millimetres] mesh, otherwise you get these rocks in there. So I’m dealing with rocks from clay that I mixed haphazardly like, a few weeks ago, en masse. So it’s very… it makes it a lot… a lot more efficient if you screen all of your materials before mixing it.

That’s what it should do. So I’ll usually do like two-gallon [about 7.6 litres], or, 10-gallons [about 38 litres] and then I’ll trowel, so I’ll just demonstrate. So you can kind of pick up the mud like that and put it wherever’s there’s not enough.

To read more about building with compressed earth blocks (bricks), check out our interview with Dan Powell of EarthTek»


video: Marcin Jakubowski; image: decar66

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