slip straw construction

I posted not long ago about the timber-frame bedroom Paul has been working on for us, over on the south side of the yurt. Now that the roof and the timbers are up, he’s starting to fill in the walls. And in his usual fashion (see his tool shed for an early example), he is doing it in crazy quilt style, patches of different natural building methods depending on moisture, aesthetics, and material availability. The south wall is mostly glass, the east wall will be masonry and cordwood, the west wall I think he said something about some kind of tiles?

And for the north wall, its slip straw.

Slip straw, also known as light straw or clay straw, is mixture of clay and straw, compacted into forms until it is tight and strong and stable—remove the forms and, poof! you have a wall. Slip straw comes in somewhere between cob, which is mostly clay with chopped straw as a stabilizer, and strawbale, which is mostly straw with clay (or other stucco) as a covering. The finished wall of compacted straw, glued together with clay, before it is plastered over, looks like this:

When my mother first saw this wall, she got this look of profound distaste on her face and said, “Why would you do this?” Cracked me up. But it’s a fair question. Number one reason for building with slip straw? $$$$$. The whole north wall, not including the timbers or the plaster (don’t know the cost on the plaster until he buys it) is about $25. Yeah, I’m not missing any zeros there. Twenty-five bucks for some bales of straw. The clay is free from a hole in the ground, because North Carolina soil around here is primarily red clay.

Another reason to choose slip straw is on account of how low-tech it is. Sophie, eight years old, can, and has, helped put up these walls. It doesn’t take fancy tools or a lot of know-how. Puts building into the hands of the people, yo!

Another reason is that the walls have a really nice feel to them—I don’t mean to the touch, although they probably will once they are plastered. I mean that they are made of natural, breathable materials. You don’t feel like you are in a plastic room, breathing formaldehyde fumes from the off-gassing building supplies. Because you aren’t.

Okay, let me show you the process. It starts with clay. And water.

You dig up some clay and you soak it to let the clay particles absorb as much water as they can hold. Some people do a bunch of filtering and shaking it through screens and such, but we (and by “we” I mean “Paul”) didn’t mess with all that. We don’t need a refined wall here because 1) its exterior and 2) the other side will be behind a wall of closets.

Okay, so you soak and stir the clay until you’ve got a nice slurry, about the consistency of heavy cream.

Then, while that’s soaking, you install “keys” onto the timberframe slots. These are bits of wood running along the inner edges of the timbers that the straw will be pounded around to kind of lock the straw into place. Here’s a picture of the keys in one spot of the wall:

This shot was taking after he had done one tier of straw packing, but you can see the keys sticking out from the sides of the timbers. As the straw dries over time, it shrinks, so you don’t want to be able to just push the whole straw wall through! The keys will help with that. Actually, Paul forgot the keys in one section and you can already see a bit of light around the edges as the straw has shrunken in. Hmmm. Might have to do something about that.

Anyway, next, you mix the straw and the clay slurry together. Paul used a wheelbarrow full at a time.

You want the straw evenly coated with the clay. Paul experimented with various ratios in different parts of the wall. We’ll see over the long term, whether one recipe or another works better, but in the short term, they all seem about the same. As long as you have enough clay to make it all stick together, you’re good.

Next you screw plywood sheets up over the space to create a form in which you are going to pound the straw.

You dump, then stuff, the clay covered straw into the wall-shaped space made between the plywood sheets…

…and then the primary work of slipstraw begins. The pounding.

Basically, you use a piece of 2×4 and you bang it into the straw, adding more straw by the handful as you go, pounding and pounding and pounding to compress the straw as much as possible.

Slave labor helps.

Seriously, little kids can do this! Easy peasy.

After you’ve hammered the heck out of a section, packing it with straw, you get to the Fun Part: taking off the forms and seeing the wall you made.

You then take those same piece of plywood and screw them in one tier higher up and repeat the filling/pounding procedure.

As you go up the wall, you need scaffolding or ladders in order to get to the higher sections of wall. And when you get to the very top, you have to figure out tricks to pound UP into the remaining hole, or sideways, until there is just a small hold left.

The last hole in that triangular section! This is the same wall from the inside. Paul filled that hole, by the way, with a glob that was higher in clay content, more on the cob side of things, and smoothed it into place.

Here’s how the wall looks at present….

Paul tells me he has a single rock that fits into one of those lower rectangles, and he plans to brick in the other. Leaving one triangular section and the top bits to go in slip straw.

When it’s dry, it will get the plaster layer and hopefully look all pretty and cool like this:

Straw, clay, plaster and timberframe can last for centuries!

All right, thats all the pluses, how about the down side? Paul says: it’s boring. Boring boring boring. Actually his exact words were, “it’s fucking boring and takes for-fucking-ever.” he clarified this with, “if you are unskilled and have a lot of time and not much money, it’s great. But if you are skilled and don’t have much time, its a pain in the ass.”

There you have it! He’s been surviving the boring with “This American Life” on the radio (i love that show) and mixing it up with other building tasks. But this is why the bedroom will have only one wall of slip straw. The price is right but Paul needs power tools to keep his interest. So be it!

Here are some other folks doing slip-straw construction and blogging about it, here, here, here, and here. Here is an interesting thread on the permies (as in permiculture, I think) forum. And here is a cute video of a guy and his kids doing the walls in their house. Very informative! It’s the same guys as in the second link in that list of blogs.

More posts on this when he finishes the wall and starts plastering. It’s exciting to have some walls going in, makes it feel like an actual building, like maybe we’ll have a bedroom one day. Cross my fingers!

3 thoughts on “slip straw construction

  1. Pingback: stuccoing over slip straw | mayaland

  2. Pingback: west wall of the bedroom is up! | mayaland

  3. Pingback: the bedroom is done! Or, what it feels like to finish building your own house, finally, after a decade of work – mayaland

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