Yet more refinements

DSC_0178We have now had a chance to see what happens as the base coat dries-out. There are some cracks developing – but not many. I have probed any large crack to see if this indicates any fundamental weakness in the straw or adhesion beneath, and for those where there is significant softness I have pushed my fingers in more strongly and forced the clay back into the straw / gap – and then filled this out with more base coat.

I have also changed the approach to the first coat – moving away from a very liquid pure clay slip, to a mix that is 1:2 clay to plastering sand (no fibre). This gets over the probem of pebbles blocking up the gun because the sand is finer and better sieved (albeit more expensive). I get the mix to the point where it is a cross-over between liquid to solid but where if you dip a piece of straw into it you still get a good amount clinging to it. I have then sprayed this on just to the point where you stop just covering each bit of straw and start getting ‘fill’ between the bits. This we will then leave to dry-out a bit before putting the 1:3 base coat (plus fibre) on. DSC_0176

I have also noticed there is a marked difference between the bales whch present a cut face and than those with a folded straw face. The cut face is already pretty rigid and stable and therefore doesn’t require much to cover it, whereas the folded face is more wispy and requires more material to really get into the straw and stabilise it.


More refinements

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 ( 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.


Refining the process

We have now refined the process. First up, I have decided to abandon using the render gun to apply a 10mm coat of 1:3 clay to sand mix in the first pass. The reason for this is that, as good as the gun is, the sand I am using has too many rogue 10mm stones in it, which block up the gun and the time I was saving was used up by frequently having to unblock it. If you have a better-graded (but therefore more expensive) mix this would not be an issue. Instead I am using the gun to put a clay slip-coat on – reverting to the technique recommended in the holy book. Putting this coat on with a gun is very fast.

We have also experimented with the mesh, working this into the top of the first / main, 20mm coat. Once this is on it is possible to ‘move’ the render around underneath it and flatten things out because the mesh stablises the surface. However, having discussed this with Chris from Back to Earth he recommends not putting the mesh onto the first coat, but rather allowing this to dry somewhat and then put the mesh into a 10 – 15mm top coat. We will try this from now on. I have also decided to add straw into the first coat, despite not necessarily needing this with the mesh, on a belt-and-braces approach.

I have also worked out a good way to get hessian to stick onto the wooden window sub-frames – namely to make a glue from flour, clay and water.

Finally I have made up a batch of ‘sticky straw’ to build-out around the windows. Since this involves creating a layer at least 50mm thick, this is too much for a render layer – even with lots of straw in it. Instead this is more like a very straw-rich cob. I used a mix of around 1 part flour,  2 parts clay and 2 parts sand – plus quite a lot of water and sufficient straw to get a mix to a point where the straw readily sticks together and forms a sticky, maleable material.



Experimenting with clay

A couple of days ago we took delivery of ninety 15kg bags of powdered clay from Back to Earth. We spread the bags out in several piles within the building so as not to put a single 1.5 tonne load in one place, since I am not sure how happy the floor joists would have been with this.

I then started experimenting with mixes and methods which I will leave a few days to see how they dry out. The most important thing was to assess the suitability of using the render gun for getting the first coat on and also to determine whether to go for an initial pure clay slurry coat (as the holy book recommends), or whether using a gun would mean you could go straight to a thicker 3:1 sand to clay mix. Chris, from Back to Earth had reckoned that the sand mix should stick pretty well, if applied with a gun – and so it proved. I made quite a sloppy mix and it stuck very easily to the straw (see close-up picture). However, if you are using your hands, as the book suggests, I could see that it would be pretty difficult getting a sand mix on – once again emphasising that spray application, for both lime and clay, is definitely the way to go.

Having sprayed a thin coating onto a test area about 1m square, I then played around with building up different thicknesses – both to see how well this sticks and also to see how it performs as a base coat once dried-out. The answer was that it stuck pretty well – but not quite as well as the fibre lime I used for the outside, which was quite magical stuff. The main difference in terms of stickiness was that it was not possible to get the straw to ‘lie-down’  quite as well when smoothing the first layer off with a trowel.

I ended up with three types of thickness – a slurry-type coating, then a depth which was similar to my first lime coat where all the straw was covered but with plenty of gaps due to the unevenness of the surface (probably equivalent to a 10mm layer) and finally a small area (top left of big picture) where the straw was covered creating a reasonably flat surface with no gaps, such that only a few tufts of straw poked through (probably equivalent to a 20mm coat). I will now see how these develop as the dry out.

I will then experiment with a final finish coat on top of the thickest of the test patches once it is firm to see what sort of finish I can get. I have a suspicion that the finish I got for the outside using a sponge trowel may not work with clay because a sponge trowel produces a slighty gritty finish which is fine when you have a lime set which will hold the gritty pieces into the render, but with clay I suspect the set will not be firm enough to hold the sand grains and I will end up with a loose (rather than a firm), gritty surface – albeit then rubbing this down with a stiff brush might solve this problem.

I have also decided to go against the advice of the holy book and not put fibre (such as chopped straw) into the mix, relying instead on putting a fibreglass mesh into whatever turns out to be the main ‘building’ coat, such that the fibreglass sits within the last third of final depth. This is following the recommendation of Chris from Back to Earth who is a cob building specialist and said this was the best way to guarantee no cracking. However, I will use a straw mix to build out the plaster around the windows because I have decided that I want to plaster close-up to the frames, rather than have a boarded-out window recess. This will involve plastering over the existing window sub-frames so to this end I also experimented with sticking hessian soaked in plaster onto a piece of frame to see how well this works to provide a key for the clay over wood. I didn’t use an adhesive for the test piece, but if it looks like a good solution I will soak the hessian in either a solution of PVA or possibly flour and water paste to make sure it sticks well. I may also try this against the fireboards and for use on the studwork so that I can get a clay finish through-out the building.

One other thing that I might try, just to see what happens, is to get some artificial fibre and add this into the mix, to see if these helps both with the adhesion and stickiness as well as allowing for the application of thicker layers (and as a preventative to cracking).

Coming on in (drainage) leaps and (electrical) bounds

Last week my brother-in-law (who is a proper builder) and sister came across for a couple of days to tackle the drains and also the studwork for the bathrooms. Drains are relatively simple, but given it is so important to get them right it is very useful to have someone who knows what they are doing to set them out properly. The electrician also came across – so in the space of a day and-a-half the the drains were put in, the flooring was finished, the studwork for the bathrooms (the only internal walls other than straw) was put up and first-fix electrics was done.

It has just remained for me to chase-in the electrical conduits and secure the switch and socket boxes into the straw. The chasing was best done with a claw hammer and the boxes were secured using hazel pins about 30cm long as massive rawl plugs (see picture). These provided a reasonably firm fix – but I wouldn’t use them for fixing anything too large or heavy to the walls. For things like kitchen units I will probably revert to chasing 4×2 uprights into the straw and fixing these to the floor plates and then screwing horizontal batons into these. (Note the picture shows the boxes and conduits as the electrician left them – they are not secured into the straw).

For holding the conduits in place I used some smaller hazel pins and put a bent nail into the end and hammered these into the straw beside the conduit (see picture).

I have also had to prepare one corner for the installation of a pellet boiler. The view from HETAS is that you need a fireproof plasterboard and a 12mm air gap between this and the straw – so to be doubly sure I rendered the corner with one coat of lime, then put a 2×1 frame onto this, screwed top-and-bottom into the ring beams and also secured against the straw using hazel pin rawl plugs and then attached the fireboard. Final job to do here is make a hearth out of quarry tiles and we will then be ready to get the pellet stove in – which will provide heat and be very useful for helping dry out the clay plaster (next big job).

Boring flooring

I have spent the last couple of weeks putting in all the flooring and ceiling joists as well as the actual flooring in the living area. The flooring in the bedroom / bathroom area will have to await putting in the drains and be coordinated with creating the studwork for the bathrooms. This will happen next Thursday, under the direction and assistance of brother-in-law Nick – who is a builder and therefore knows about these things.

The one piece of advice worth passing on for the non-builder, concerns the slight absurdity of the fact that sheeting products such as floor sheets are generally available in imperial sizes (i.e. 4×8 foot whic is approx. 1220mm x 2440mm) whereas plans always specify metric gaps – such as 400mm or 600mm. However, this should not lead you to space your joists at imperial-friendly gaps (e.g. 610mm) because, if you are using tongue-and-groove flooring sheets these are 8ft long but this includes a 10mm tongue – so the actual length of the sheet is less, making it sit perfectly on joists at metric 600mm spacing.

I have also become an even greater fan of expanding foam, especially the type with the attachable gun, because this makes it possibe to do ‘precision welds’ along joins (see picture) rather than the rather messy results you get with the conventional single use cans. No exactly natural building, but you could say eco-friendly in terms of creating good airtight seals.

Creating a view

We spent the weekend attacking brambles and some small hedgerow trees (but mostly brambles) that run down the field boundary that is one edge of the area that will be the garden. The result is that we now have a view out over the fields from the house. It is a good view.