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.
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.
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.
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.
We 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.
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.
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).
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.