Clay plastering a straw bale house

This is a post I have been meaning to write for a while: a review of the experience of clay plastering. The first point to make is that I don’t think I arrived at the absolute definitive method, finding myself constantly experimenting with different mixes and application techniques in order to find the optimum balance of the various considerations involved: these being adhesion, cracking, gap filling and time.

The considerations

Adhesion

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Adhesion coat

It was much harder to get clay to stick to straw than the Limecote I used on the outside which could be sprayed-on and built-out to possibly even 40mm in a single application. Using clay, it is necessary to put-on an adhesion coat. This should be mostly pure clay, to make it sticky possibly with a little sand added to give it a bit more strength. This layer can’t be too thick, because a clay rich mix, while sticky, will crack a lot and also will not be strong enough to allow it to be built-out. It s also worth noting that clay is more expensive than sand, so you don’t want to use more than is necessary. The issues with this adhesion coat are how sloppy to make it, how thick to apply it and how to apply it. My solution was to make a pretty sloppy mix (consistency of double cream) and apply it with a gun to a thickness where the straw was coated and stray sticky-out bits could be encouraged to lie flat but not to the point where the coat starts to build-out.

I did find that I got some areas where, after applying subsequent build coats, the clay was not strongly bonded to the straw and therefore was inclined to crack, but this mostly occurred where there was weakness with the underlying straw surface, either because the straw was rather soft and wispy or because there was a gap which wasn’t adequately packed-out. Where these were an issue I hacked off the clay and started again, making sure the clay was very well worked into the surface.

Gap filling

I probably wasn’t as diligent as I should have been on gap filling. This was partly because the Limecote was so good at bridging across gaps or soft areas and also because of the time required to do a thorough job packing out gaps with a straw mix. I therefore tended to rely on my first build coat to fill-in any gaps but this could lead to cracking and weakness if I didn’t sufficiently force this coat into the gaps. For this reason it would probably be best to spend some time really packing-out any gaps with a long straw dubbing mix.

Cracking

Cracking can happen for two reasons: either your mix doesn’t have enough sand and/or fibre added to it, or if the clay is insufficiently bonded to the straw underneath – because the straw surface is wispy and unstable or there is a gap beneath the clay that isn’t well packed-out.

I think I got the fibre issue cracked (as it were) by using concrete screeding fibre. This is much easier than using chopped straw because you don’t have to chop it. I did use chopped straw initially and found the best way of prepping this was to put a load into a dustbin and then plunge the strimmer into it, preferably with someone holding the dustbin lid over the top to prevent too much flying out. The only issue with fibre, especially screeding fibre, is that if you use a sponge to finish-off your top coat, this will drag bits of the fibre out, resulting in a hairy finish – but this can be dealt with by using a blow torch to burn these off.

As mentioned earlier, I got some minimal cracking of my first build coat, but this was mostly over gaps. I dealt with this by tapping the cracks with a hammer, and if the area around the gap sounded hollow, I would knock the gap out and ram clay (or sometimes dubbing straw) into it. If the gap seemed ‘natural’ and reasonably firm either side I just left it and filled it out with the second (and final) coat.

I didn’t get any cracking of my final coat – which I figured must have meant that the mix was right and the fibre was doing its job.

I did initially experiment using a fibreglass mesh which I stuck on top of my first build coat. This did stop any cracking but I was deterred from using it extensively because the supplier of it said it should be sandwiched into the final coat which effectively meant making the final coat two coats (as in two bits of bread if the mesh is the butter in the middle) and this was too time consuming. That said using it as a layer between the first and second coat didn’t seem to create any problems, such as creating a weakness or potential for ‘shearing’ between the two coats – although I guess only time will tell if this might develop as a problem.

Time

I underestimated the length of time it took to do the clay plastering, possibly because the experience of the external rendering was easier than expected (Limecote being such a magical material to work with) or perhaps because the internal walls meant there was a considerably greater area to deal with. Using the gun didn’t seem to speed things up, other than when using it to spray the adhesion coat. When I used the gun for the first build coat, I found that I could go faster by hand, especially if I kept this layer relatively thin and when it came to the final layer pushing it on with a trowel was also faster than standing there with the gun waiting for it to build-up to a sufficiently thick layer. This was probably because I was using a basic gun and compressor. If I had more powerful kit I probably could have sprayed thick coats on quite quickly.

Because of the length of time things were taking I also tended to miss-out the gap filling stage, as mentioned above. This was probably not ideal, but I don’t think it has caused any long-term problems (i.e. clay dropping off at some point in the future), but I guess only time will tell on this one.

Recommended process

If time (or labour) wasn’t an issue the absolute best process would probably be the following.

1 Adhesion coat

Apply a slip coat of 3 to 1 clay to  (plastering) sand with a gun, mixed to a consistency of double cream. Apply this to a thickness such that the straw is well coated, but not starting to fill-out.

2 Gap filling

Make up a batch of long straw dubbing mix. This is straw with a clay / sand slurry poured into it. You could make this slurry by taking the slip coat and adding some sand to it to firm it up a little. This mix should have the characteristics of straw bound together with clay, rather than clay with straw added to it. Use this to pack out any gaps or soft areas, ramming it in as firmly as possible. Unlike the lime (a chalk fibre lime such as Limecote), clay is not very good at bridging across gaps and soft areas and even if it appears that you have covered a soft areas well, the likelihood is that it will be weak and crack easily as it dries. You could (probably should) do this while the slip coat is still wet. This should get you to the point where the surface is relatively flat and firm, but still basically strawy.

First build coat

Make up a batch of clay that is 3:1 sand to clay with fibre added. I used concrete screening fibre (a couple of handfuls per 60kg batch – ie one 15kg bag of clay powder plus 45kg of sand). I mixed this in a regular cement mixer, tumbling the dry mix first and then adding sufficient water to take it beyond the golf ball clumps stage, past the big lumps stage to the point where it starts to slump. Something like the consistency of cream cheese. Then apply this by hand, really ‘grinding’ the mix into the straw, using the heel of your palm, having first dampened down the straw with either a mister on a hose or by using a pump-action sprayer. This layer need not be that thick, but you should get to the point where the wall no longer looks strawy, but more clay like, with odd bits of straw poking out.

Let this coat dry out so that any cracks can fully develop. Then test the cracks by tapping them with a hammer to see if they sound hollow around then. Those that are hollow, knock-out and fill with a clay mix or possibly by ramming a long straw mix into the gap. Alternatively you could wait until you start to see cracks develop as the coat dries and then push the still damp clay into the straw along the crack line with your finger tips.

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First build coat on right, second/final coat on left

Second build / final coat

Get the wall good and wet with a mister. It is amazing how much water the dry clay wall will suck-up. I tended to give it a good soak, to the point where water is just about to run-off the surface, and then wait five minutes or so and give it another go. Apply a 3:1 sand to clay mix with fibre using a trowel to a sufficient thickness to give a decent covering to any straw poking out. I find descriptions of coats by thickness rather unhelpful with straw, since its irregularity will mean that a coat which in one place may just cover the straw, will in others be 30mm thick. I used the trowel to get the surface reasonably flat and smooth, but tried not to work it too much, and then used a wet sponge trowel to further smooth over the surface. This produced what I guess you would call a rustic finish, which I was happy with. If you wanted a better finish you would probably need to do a third skim coat which you could spend any length of time working-up to whatever finish you want, rubbing it with a wooden float or a stone or whatever. The great thing about clay is that it doesn’t set, it simply dries-out so you can rework or revisit the surface as much as you like.

Finally, once this coat is dry, get a blow torch and ‘wash’ the surface with flame to burn-off any bits of fibre or straw.

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