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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A (psychological) door

DSC_0008We 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.

Rain skirt

One of the many finishing-off jobs is putting on a rain skirt at the base of the building. This has two functions: to protect the base plate and throw any rain that runs down the building away from the foundations and also to protect the car tyres from direct sunlight, which can cause them to degrade.

Before rendering I had screwed a 2×2 timber through the woodfibre and into the timber of the base plate. This acted initially as a render stop, although the sticky nature of the Limecote mean this wasn’t really necessary. What I have now done is screw a piece of standard black stained weatherboad at an angle into this timber and nailed onto this a piece of 90 degree angled plastic beading. This beading can act as a new render stop for a a strip of render that overlaps the top of the weatherboard (see pictures).

I am pleased with the result. It finishes-off the base of the wall and gives the the whole house a slighty suspended feel.

Elsewhere, I have been finishing off the flooring and been back on the tyres and gravel again, but this time for the less critical job of providing a foundation for decking. I will use the scaffold boards so that the same material that is the interior flooring also then runs-out as ‘exterior flooring’ albeit with a small gap between the planks to allow rain to run-off as well as having set the decking on joists that are on a 1% angle to further encourage run-off.

 

 

 

More flooring

DSC_0696I 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.

 

Window detailing

It has been a long time since the last post – mostly because we have been doing ‘non-straw’ jobs, such as installing bathrooms and kitchens and painting which are not especially interesting or noteworthy tasks. I have been meaning to write a review post covering my conclusions from clay plastering, but haven’t got round to it yet. However, last week I got back onto a straw specific job, namely finishing off the exterior render around the windows.

I had left the window area without a finish coat because I needed to get the windows themselves in before doing this and I wasn’t able to do that job before the winter lime rendering window closed. As luck would have it I received a surprise visit from Arthur Phillip, the lime render man, who happened to be in the area. Arthur is probably the country’s foremost expert on rendering straw bale houses and he introduced me to PVC window frameseal beads. These are basically a strip of white PVC channel with a sticky back to it that you can put around the wooden frame edge of the windows allowing you to render up to and into the channel – thus giving you a stable and watertight edge to your render. They also have on the a tear-off sticky plastic strip which you can use to attach polythene sheeting to in order to protect the windows themselves from render spalshed – important if you would be using a gun to apply the render.

I couldn’t actually source these in sufficiently small quantities, so I made my own by buying standard PVC channeling and runing a bead of silicone up the back of them to stick these to the frame while also creating a weathertight seal. I then mixed up a batch of lime and straw to create the shaping into the window frame and finished-off with a coat of lime that then ran into / up to the window edge fo the PVC channel. This seems to work fine.

 

 

Second coat

I am now close to finishing the second coat – having been at it for nearly a month. Fortunately I have been able to borrow a cement mixer, which makes the mixing much easier and has probably come just in time to preserve the life of my hand-held electric mixer which has been showing the strain.

The basic procedure that seems to work is as follows.

First put a dry mix of one bag (15kg) of clay plus ten shovels of plastering sand into the mixer together with a couple of good handfuls of concrete screeding fibre. I have opted to use this instead of creating lots of chopped straw, simply because it is easier (and seems to work).

Then leave this to turn-over for a good 15 minutes to get it well mixed and also to tease-out the fibres (they come out of the bag as short strands, but are in fact pieces of mesh and by stretching the out you greatly increase their volume/area).

Then add about half a bucket of water to get a consistency of mix that slumps well in the mixer, but doesn’t stick to the sides. It is also important to ensure it is wet enough to avoid turning into small pellets because if this happens it can be hard to get these to blend-out. Leave this for a good 10 or 15 minutes to get smooth and well mixed. Then add the remaining water to produce a consistency of mix that is close to that of butter icing. This will tend to stick to the sides – so may need a little prompting with a spade to ensure the water works its way through. While the clay is mixing give the wall a good soaking. It is astonishing how much water a dry clay wall can suck-up and if you don’t take the edge of this it will pull all the moisture out of your final coat. I put the mister on the hose and wet the wall down just to the point where water is about to start running down. Then I leave it for a minute and do it again and perhaps even give it a final mist just before starting to plaster

I then apply the clay in quite a rough-and ready way using a plastic trowel and once I have done a decent-sized area I use a sponge trowel to smooth things out. The advantage of a sponge trowel is that it can move the top 2mm or so around without dragging the whole coat.

Once the coat has firmed-up a bit I then rub it down with a regular sponge. The use of the two types of sponges can pull some of the fibres out of the mix a little, leaving a slghtly ‘hairy’ finish, but these can either be brushed-off when the coat is nearly dry or I am hoping they can be encouraged to ‘lay down’ when the wall is painted. Even if some still poke-out I don’t mind the fact that the fibres are visible because, to come-over a little bit Kevin, it “tells the story of the material” (we can see the horse hairs in the 16th century plaster on the inside of our house for example). It gives a slightly rough finish, but I think this looks fine. It would be possible to give a smoother finish if you worked the surface with a plastic or wooden trowel, bit this would take ages and probably look a bit odd, given the natural irregularity of the walls. You could try and use a flexible rubber-edged trowel, but this would probably only work if you were able to get a much flatter finish, which would involve a great deal more clay.

Results from the walls which have dried out are encouraging – very little cracking, such that it will be easy to hack-out the one of two  that have developed and patch them over, possibly with a strip of hessian of fibre-glass mesh.

It is, however, very slow-going, especially doing the detailing around the windows. Renderng and plastering will be by far the two biggest jobs involved in the build.

The last straw

I am now getting round to the fiddly detail bits. One of these is fitting notched bales between the top the main interior wall and the roof  (the bales have to be notched to allow for a purlin) and also putting bales in place above the doorways. It was good to return, one last time to ‘straw work’ athough cutting the notch with a chainsaw meant becoming swamped in loose straw. It also reinforced that by far and away the best tool for cutting or shaping straw is an electric chainsaw (not a petrol one because the straw gets in the air intake and it overheats).

I have also been finishing off the clay around the windows and door frames – using a ‘cobby’ mixture of straw and clay. I want to plaster right up to the window frames, rather than put decorative wooden frames and window ledges in place. I think this sets-off the window frames to best effect, but it does mean plastering over the window sub-frames as well as sticking clay ‘upside down’ to the sub-frame above the windows. To do this I covered the wood in a flour, water and clay paste to give it maximum adhesion. This appears to have worked – although for the final coat I will also stretch hessian or fibre-glass mesh around curve, both to stop cracks developing over the join and also to hold the upside down plaster in place should it feel like falling-off. The job feels a little like being a housemartin, sticking clay and straw to the underside of eaves – and the resilience of their nests gives you some confidence that the clay, with added flour paste, will be sticky enough.