Trever Miller from Evolve Builders came out and helped us with some plastering. I asked him to bring his darby (a three foot long leveling trowel) to help us level the kitchen wall behind the cabinets. Trever has years of experience leveling walls with a darby, I filmed this video of him at work.
Archive for Natural Building Techniques
Window curves are one of the most distinctive features in straw bale homes, and are often a big consideration in the choice to go with straw bale over other forms of construction. But information on how to shape curves is sparse, so I thought I’d share some of what we’ve learned over years of doing bale work.
The first thing you really need to think about is radius of curve. To visualize this take a string nine inches in length, pin it at one end and attach a pencil to the far end. Now draw a quarter of a circle – this is roughly what a nine inch window curve looks like. Try the same thing with a 16″ length of string, and you’ll see how dramatically different this window curve would look. Generally we find large radius curves that open out quickly from the window are more popular because they let in more light and make the window seem larger. However many people really like windows that come out straight and then have a smaller curve at the end. The sides of windows usually have curves anywhere from 9″ to 16″ radius.
Window tops and ledges can be straight or curved. Flat tops generally need some extra wood framing.
There are three steps to shaping a window – carving, stuffing, and attaching mesh.
First you need to carve the bales to match the shape of the curve. We try to do the bale work fairly tight to the window, but typically the bales are set back 1.5″ because we push them up against the framing without notching. In preparation for shaping we cut one string, and carve the bale to the shape of the curve using a grinder with a lancelot blade. Cut a plywood template that you can hold up against all your curves as you grind them, to help keep them uniform over the curve and consistent from one to the next. Stand back and look at it from a few different angles. Try to get it perfect, but when in doubt err to the side of too low, you can always fill with plaster.
Next stuff loose straw in the voids. If the carving was done well, you hopefully won’t need too much stuffing. To finish the stuffing, you’ll need to attach the mesh to hold the loose straw in place.
We do a lot of our window shaping with tenax mesh instead of metal lath. If a curve is done well it is easily shaped with plastic mesh and a little stuffing, and is actually easier to plaster than curves shaped with metal. Curves that need a lot of stuffing are formed using metal lath, also called diamond lath (and sometimes ‘blood lath’ because it tends to draw blood from those working with it). Keep a consistent staple line with your mesh or lath, make sure it is on straight and even, and you should end up with an good curve.
People who are new to natural plasters sometimes think they are non-toxic: earth plasters are made out of materials dug from the ground, right, how could they be dangerous? In fact, while the end result is non-toxic, these products can still be hazardous to work with. Take clay for instance, which often contains large amounts of crystalline, or ‘free’, silica (fine quartz), which when inhaled causes silicosis (a debilitating lung disease) and lung cancer. Silica is also found in cement and fine sand, but not in pure lime (which nevertheless isn’t great to breathe in). The long and the short of it is that plasterers work with materials in fine powder form and need to be very careful about what they breathe in.
- Always wear a respirator when mixing, or anytime there is dust – including cleanup!
- Use a mop, or a vacuum with HEPA filter, instead of sweeping when fine plaster dust is present.
- Read the Material Safety Data Sheet (MSDS) for materials you are working with, including bagged clays and pigments
MSDS sheets can be found for most materials using an internet search. For example an internet search for “msds epk” finds that EPK bagged clay contains 0.1 – 4% crystalline silica, whereas the MSDS for Bell Dark Ball Clay shows it contains 10 – 30% silica. Kaolin clays often contain less than 1% silica, and are good for earth plaster finish coats. Ball clays and fire clays are more common in earth plaster base coats, and typically have large amounts of free silica. Site clays are typically processed wet, so they are hazardous only during cleanup.
Pigments vary greatly in their toxicity. Here’s a website that gives a summary of composition and toxicity of a few common pigments. The colour index can be helpful in trying to figure out what pigments are made up of.
Recently my friends asked me for advice on building a small straw bale building (a garage with a living space above). They asked me how it will compare, timewise, to conventional framing. The answer: it will definitely take longer. They will save money on materials (some) but it will only be worth it if they really want the extra insulation value. They do, and they want to try building with bales as a test run for a future house. Also they want to use post and beam anyway, which means they need some kind of secondary wall system – for this reason, and aesthetic reasons, bales and timber frame go well together. I wanted to give them a good idea of what they were getting into, so I compiled a step-by-step checklist of exactly what you need to do to build an infill straw bale wall.
Here it is:
Bale building checklist
Before bales (after foundation and basic structure)
- Roof must be finished. Really finished!
- Bale curbs installed
- Top plate (never OSB – plywood OK)
- Framing for doors and windows
- Pre-hang mesh if / where needed
- plan air sealing details which may need to go in before bales
- Bracing for windows or guides for bales if needed
- Scaffold? Set up before baling if possible. Plan tarping accordingly
- Tarping (tarping that rolls up can be nice). Attach to soffit area, not facia
- Get bales
- Check bales for moisture as you go
- Pre-fill voids where corner of bales meet
- Notch / install bales. Check plumb as you go. Tie into posts as needed (frequent)
- Cut curves / angles for windows – can be done before or after bale placement
- Dipping bales? Need to pre-mix clay slip and mask finished wood
- Straighten walls / weed whack if not dipped
- Stuff all voids / check stuffing everywhere
- Cover all wood that will be plastered with typar / tyvek,
- Lath over wood & typar, hanging over bale transition by 1.5 to 2″
- Lath over air sealing materials
- Tenax mesh / stitching (recommended if not dipping bales)
- Flashing completed everywhere plaster will meet
- Plaster stops?
- Process site clay?
- All materials and mixer on hand
- chopped straw (save from windows)
- Make wheat paste where needed
- Apply scratch coat inside and out
- wait for drying / curing (1-3 weeks earth / 3 weeks lime / 1 day cement-lime)
- Finish coat in and out
- De-masking / clean up / detailing
- Caulk joints
- Crack filling (minimal with earth / more with other plasters)
Earth plasters are wonderful because you can use soil that was dug from your foundation, or a truck can deliver a load of clay soil that was dug from another construction site. But anyone who has a pile of clay soil sitting on their jobsite knows that it’s hard to process. It can be rock hard, or incredibly sticky globs, and often contains stones that need to be sieved out. Here’s how to process clay into slip for use in earth plasters.
You need a very good drill that can withstand running for hours, and a barrel with one end open. Fill the barrel about 1/3 full of water, measure exactly how much you are using because you need to keep the slip consistent if you’re going to use it in earth plaster recipes. make a setup to hold the drill in place (some drills or paddle mixers have attachment points for this, others you may need to use All Round steel strapping (found in plumbing section of stores) or some other ingenious method.
You’ll also need a very good paddle mixing attachment, I used the Kraft Tools DC303, a 24″ mixer with three propeller blades to keep the slip moving even as it gets thick. Since the blades are removable from the shaft, which is somewhat too short for barrel mixing, I bought a separate metal rod which I cut to length and used that as the shaft. Alternately you could cut the barrel down to match the shaft length. I know someone else who uses a boat propeller attached to a shaft on his drill to stir the slip – which was the best setup I’ve seen to date.
Start the mixer spinning in the water before you start adding clay to the barrel, once you have a vortex start adding the clay. Keep careful track of how much clay you are adding relative to the water. I find about 2 clay: 1 water is quite manageable, and that is about the minimum you want for your recipes. If you can get more (up to 3 clay : 1 water is possible) it will give you more flexibility in your recipes, since thin clay slip will make for wet plaster.
Once all the clay is in you’ll probably want to leave it spinning for at least an hour, and sometime in the middle you should stir up the bottom, either by moving the mixer around, or pull it out for a minute and stir it up with a shovel.
When the slip is well mixed, pour it through a screen (diamond lath works for rough plasters, you’ll want something finer for finish plasters) into a holding tub or trough. You can easily make one with a new tarp and some bales. You’ll want to process a couple of barrels of slip before you start mixing plaster, or you’ll get behind on slip. And that’s it – keep it well covered when you’re not using it to keep rain, frogs, leaves etc. out of the slip. Happy mudding!