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.
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.
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 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.
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.
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.
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.
We have just fitted the door. This was actually a very simple task (using the same company as supplied the windows), however it represented an important psychological milestone. Whilst your door is a sheet of OSB you don’t really have a house, you have a building site. It also means that we have a proper air-seal so it makes sense to light-up the pellet boiler – an important step forward given the sudden plunge into autumn.
I am now putting in the finished flooring on top of the chipboard sheets that sit on top of the joists. I have decided to go for scaffold planks. The problem with most conventional flooring products is that they all look too perfect (as well as being pretty expensive if you want solid wood as distinct from a laminate). For a project such as ours you need to have a floor that looks suitably rustic – and scaffold boards do just that.
Scaffolding Direct sells new, unbanded boards in various lengths, which work out at a cost at around £14 per square metre. This will give you a chunky 38mm, 23cm wide, board. The boards will obviously need to be sanded and finished, but the look is so much more solid and attractive. Interestingly, the price of new boards is about the same as recycled ones and since both would need to be sanded and the used ones are more likely to be damaged, as well as having bands on the end that would need to be removed, buying new seemed a better option.
I am nailing them down, rather than screwing them, because I think boards like this need to have visible nails to look suitably authentic. I would like to have used cut nails, but this won’t grip into the chipboard, so I am using good old ringshank nails. I got boards in lengths of 2.4m so as to get a good number of joins, since I think this also is a better look. I expect to get a little shrinkage over time, but this shouldn’t be a problem because big boards like this look better (more authentic if I may say that again) with slight gaps between them.
I have abandoned putting the first coat on with a trowel, relying instead on putting it on by hand and then really grinding it into the straw with the heel of the palm. This way you can put some real force behind it and actually feel when you have a really firm contact. I then trowel on a 10mm or so layer to build the coat up a bit – as well as mashing in some long-straw to dub-out the uneven places. We will now wait until this layer dries before making a decision on whether to use a mesh in the final coat.
I have also worked out a way to get good chopped straw for fibre. This involves chain-sawing the face of a bale and then putting the resultant chopped straw into a dustbin and then using a stimmer to shred it further. I got this idea from this video (https://www.youtube.com/watch?v=lEfmPKnzvZ0). I am also using a slip coat mix about the same consistency as the one in this video – which is more liquid than the type Barbara Jones uses in her plastering course video.
I have also slightly adjusted the mix – moving from a 1:3 clay to sand mix to one which has a little more clay in it. It came down to feeling the final mix. There is a fine line between a mix which is grainier than it is sticky, to one which is stickier than it is grainy. The sticky mix is what I am after – and there is not much in it in terms of how much clay to add. The technique is to start with about 3/4 bucket of water plus one bag of clay plus a handful of chopped straw, mix – then add a bucket of sand and mix again, then add two more buckets with a handful of straw – mixing between each one.
I have found an online supplier (TA Windows) who can provide windows to almost any customised specification. With a straw build you are likely to need customised windows because the basic measurement unit is the width and depth of a straw bale and this tends to rule out designing to use standard window sizes.
Rather than take the risk of ordering all the windows at once – and then finding there was some problem with measurement or fitting, I got one as a trial – which I have now fitted. The key lesson was the importance of undersizing rather than oversizing. Despite knocking-off the recommended 10mm from what I thought was my gap in the sub frame, the fact that the frame was not absolutely square meant that the window didn’t quite fit. This wasn’t a problem, it just meant that I had to fiddle around shaving some wood off the subframe. However, I wouldn’t want to have to do this with every window.
Having to fill a gap between the window and the sub-frame is not a problem thanks to the wonders of expanding foam, in fact having a bigger gap is slightly better than having a very small gap in terms of sealing it with foam. I know expanding foam is not a natural product and I am sure Barbara Jones would not approve, but I figured that it doesn’t intefere with the key natural characteristics of the building – which is breathability and flexibility. When it comes to getting a decent airtight seal in tricky places where straw doesn’t really work, either because you can’t wedge it in or you don’t have sufficient depth, you can’t really beat expanding foam.
The only drawback is that the cans sold in most builders merchants are effectively single use, because the nozzle pipe can’t be cleaned. However, it is possible to get a gun which has detachable / re-sealable cannisters which also comes with a solvent cannister that can be attached and used to flush through the gun nozzle. This makes it possible to do small jobs.
Following this trial window, I have gone ahead and ordered all of the windows and while waiting for delivery have got on with some small, fiddly but necessary jobs such as fitting guttering and putting a galvanised wire mesh around the bottom of the foundation beam to stop rodents getting under the suspended floor. This is a condition of building regs, but I don’t know if this will make a difference, since my experience shows that a determined rat can get around almost anything, including building regs.
This past week or so I have been getting on with the second coat, which I also intend to be the finish coat. The mix I have been using is firmer than that which went through the gun for the first coat, but still relatively sloppy – given the that it is serving also as a ‘float coat’ albeit I am not looking to get an especially smooth finish. The depth is probably a minimum of 10mm, although it gets much thicker in places given the uneveness of the straw and holes or depressions in the first coat.
Most important learning is the degree to which the first coat will rapidly draw the moisture out of the second coat, making it quite difficult to work – hence the need to ensure that you give the first coat a good drink before putting the second on. It is amazing how much moisture the first coat will suck-up.
I am not using the gun, just loading the hawk up with a good load of render and then pushing this on and up using a trowel. I then quickly work this to get a reasonably smooth and even coating and finish off by misting it and going over the surface with a sponge trowel. This helps cover the trowel marks and produces a relatively even but rough textured finish – which has about the right amount of ‘hand-madeness’ I am looking for.
This doesn’t require a huge amount of skill and can be done quite quickly – not quite as fast as I could go with the gun, but not far off. I reckon I can get through about 7 square metres per day. This is a relief, since I was worried that my attempt at a finish coat would either require a huge amount of time and/or end up looking like a bodge-job. This isn’t to say that plastering could be considered easy, but it is not too hard if you are happy with a what might be called a rustic finish.
I know that time is ticking on and I am now well out of what traditional lime people would consider to be the rendering season. This might be the case if I was using sand, but with the chalk and fibre mix I am confident that I have something that will be resistant to all but the hardest of frosts and that if it looks like a cold snap is on the way, putting hessian up should provide enough protection. There has certainly been no evidence of any sort of cracking developing on either the first or second coats thus far.
The only other job this week has been to order the window cills, since as you can see from the pictures, I haven’t yet finished-off the area around the windows and am waiting to get the cills in before doing this. I have set the windows quite deeply, which means the cills need to be 220mm deep – i.e. much wider than any standard widths, so I have had to order the wood (oak) from a local sawmill. I am going to use just a flat plank and set it into the frame with a downward angle, resting on top of the straw/render rather than going to the expense of having a custom-made cill, with flat base and angled top since I figured this is only necessary if you are fitting your cill onto a flat surface such a a course of bricks.
A few days ago, Martin Browne from Warmcote, came round to try out his Limecote mix and also see how this worked using a render gun.
All-in-all it was pretty successful. My main concern was to see how easy it was to get an initial coat on using a spray and also how much a coat of render would expose the imperfections in the wall, in terms of gaps between bales and inaccurate positioning (and thus how much dubbing-out work might be required).
Getting a coat on was straightforward. The beneft of Limecote (which uses lime hydrate, chalk and a heavy dose of artificial fibres) is that it mixes with a conventional hand-held mixer and also can go through a pretty lightweight (thus cheap) gun, powered by a small (thus cheap) compressor. We were actually using a spray paint gun, not even a proper render gun, with a 2hp compressor. As the video shows, it went on pretty easily and penetrated well into the straw.
We then made up a mix with quite a lot of straw in it, as a dubbing mix. Again, we were able to do this using the hand-held mixer. This didn’t fix well to ‘naked’straw, but was fine when applied on top of a sprayed layer. By pressing this mix onto the walls it was relatively easy to flatten-out imperfections and get to a pretty good finish.
One thing that was very noticable is that the cut and folded ends of the straw take the render in different ways. The cut ends present a very firm surface, albeit with lots of vertical ridges as a result of the layering / flaking of the straw as it was packed into the baler. The folded ends didn’t have this ridging (which needs to be filled-out with render) but were much softer and thus had a tendency to form a hollow. Since we had alternate layers of cut and folded ends this had the potential to create a slightly wavy surface, which required the long-straw mix to flatten-out the hollows on the bales with folded sides. Perhaps we would have been better having all our bales presenting the same way (say cut ends on the outside and folded on the inside) – but we varied the layers following advice from someone who had found that the folded sides were slightly thicker and thus made the wall start to bow inwards or outwards after a few layers.
After Martin had gone, I prepared a mix that was even more straw-heavy – i.e. pretty much just coated straw, rather than render with straw within it. This seemed to work well for dubbing out big holes and depressions.
Martin reckoned that the best technique would be to spray an area, gently even this out with a flexible edged float and then press-on the long-straw mix to get to a desired straightness of surface (accepting that I am not looking for a perfect, flat finish). This layer would probably vary between 10 – 30mm. This should then be ready for a finish coat of about 10mm.
The test layers have now been up a couple of days and seem to have got a good fix and be setting nicely. The one slight drawback was that the straw started to leach its colour into the render, especially with the staw mixes. I guess this is less likely to happen when using a sand-based render which is already more straw-like in colour and also where the sand is much less moisture permeable than chalk. If this comes through the final coat I will have to put a straw-coloured limewash, or sufficiently pigmented to drown-out the straw leaching.