Clay, Sand, Straw and Water...

These are the basic ingredients to the base coat of render that we are using to protect the straw bales.

All the straw bale walls are all up (with only a few minor parts to fix) and we are rendering. We have started with the outside first to help protect the bales from the elements. The roof overhang does do a pretty good job of keeping the water off the walls but no amount of overhang will stop the almost horizontal rain we sometimes get throughout the year.

First coat of render on the bathroom wall, the top part is cob which fills the gap between the top of the wall and ceiling

The amount of work to render the house:

• we have about 50m of straw wall that is on average 2.4m high
• which is 120m² (subtract 20m² for windows and doors, leaves us with 100m²)
• 100m² x 2 sides (inside and out) is 200m² of 50mm render
• or 10m³ of mud to mix and apply

This is not an insignificant amount. We are mixing and applying the render manually.

We have decided to go with a natural mud render rather than a cement render for a variety of reasons. The recipe we have chosen at least for the base coat is as above. So far we have been mixing up the render in a wheel barrow using a shovel, the wheel barrow makes it easy to get the render to the part of the wall we are working on. It is a lot of work mixing the render in the barrow though. The steps to mixing are:

1. Put four shovels of sand in the barrow
2. Add four shovels of clay (generally we get more clay in each shovel over the sand, I would estimate the ratio to be 3 part clay to 2 parts sand)
3. Mix the dry clay and sand so they are basically combined, chopping up the clumps of clay
4. Add a few hand fulls of straw (so far we have been using up the stuff that has fluffed out of the bails in the wall whilst putting them up)
5. Mix straw in
6. Add water and mix in
7. Keep mixing until the render changes colour slightly and is very sticky.

Preparing a load of mud render

The recipe does not have to be overly precise and during the render process we have tweaked it slightly, for instance we have lowered the sand content and added the straw. We did get hail on one wall which has made us realise we will need to add lime or some other weather proofing agent to the final coat. I have been using a short spade to "chop" the straw into the mud mix. The water content also plays a part in the ease of which the mixing is, a wetter mix for instance is much easier to mix and apply but it may also make it more susceptible to cracking during drying.

Wetting down the bales about to be rendered

Applying the render is pretty simple. First the bales that are going to be rendered are wet down slightly, this stops the straw from drying out the render too quickly and is supposed to help with adhesion. We used a garden spray pack and also wet the surrounding timer that the render was going to attach to. We then make balls about 15-20cm in diameter and squish them onto the straw. The mud is then worked in so that the mud penetrates the bail. We have noticed that the fluffy side of the bail it the hardest to work in but we get a much better mud penetration. We are leaving the first coat rough to hopefully help the second coat stick better

Working in the render

We have also been using the mud render mix to make cobs to fill in the gap between the top of the wall and the roof. We have been really happy with the result. The mud mix does have to be on the dryer side otherwise we have found that we get some slump which leaves a gap at the top (it is pretty easy to fix with a little bit of extra mud however). We also added a strip of wood (1/3 of a bit of 4x2) to the ceiling to key the mud to. A key was also added to above the doors and windows.

It is really exciting now seeing how the walls are going to look. We have been really happy with the colour and how it is all coming out. The rendered curved walls look fantastic. Before the final render coat is applied we need to finish off around the window and doors jams (we are thinking of using several planks of cypress glued and biscuit joined to fill the 550mm width).

Left side un-rendered straw wall, the right wall has the first coat applied

The rendering process is pretty slow and involves a lot of manual labour (mixing is a complete upper body workout) and there is soooo much to do, so a word of warning to any friends and family coming to visit over the next few months, wear old clothes ;)

And they huffed and they puffed...

We had the strawbales for our house delivered in mid December last year and there was a bit of huffing and puffing getting them unloaded from the truck and stored under cover. Would you believe that we had rain while we were unloading the bales from the truck, thankfully it was only light. On the morning there were 5 of us to unload the ~250 bales so it didn't take too long.

Now we have the bales it was time to start putting them up. There are a few ways to make a strawbale house each with advantages and disadvantages. We chose the framed house with infill walls type of building. One reason was because our outer straw walls are reasonably complex and we didn't want the stress of having to get all the walls up and roof on before rain (this usually involves getting help in to get it done quickly). The downside with infill walls is that you end up with a gap at the top of the wall and roof that needs to be filled (but i'll talk more about this once we work out what we are going to do with our gap).

The strawbale wall is basically made up of a bottom plate, strawbales and then top plate with straps from the top to bottom plate to compress the walls. All our outer straw walls are curved, which adds some extra complexity.

The bottom plate does two things, it raises the bales off the floor to give you a moisture gap and it gives you a solid base to compress the bales against. For the bottom plate we ended up using 90mm x 45mm pine framing timber made up into frames that are 450mm width (the width of the strawbale) and trapizoidal in shape with the long edge 1000mm long (just over the length of  bale). These were dynabolted to the concrete with 4 bolts each so that the frame followed just inside the edge of the concrete slab. Short pieces of strapping were then slid under the frame through notches made before bolting the frame down and the space in the frame filled with 25mm blue metal (bluestone gravel) so that the bales don't sink into the frame.

The door frames were dynabolted to the floor, nailed to the bottom plate and attached to the ceiling with a block that was glued and screwed. The door frames were made up of 2 bits of 90mm x 45mm with noggings (like a ladder) and then a piece of 12mm structural ply glued and nailed to brace it and to give the strawbales a solid end to press against.

The window frames are made like the bottom plates and the door frames, they are designed to sit inside the bales. We will probably attach the windows to the ceiling to add stability to the walls. The windows are curved with the walls. We also designed to windows to fit within a bale width. Later on the door and window frames will be lined (most likely with floor boards).

We have half a wall to finish as of writing this post then we start on filling the gap between the top of the straw wall and the ceiling.

Lining the ceiling

Its a few weeks now (time really does get away from you sometimes) since we finished lining the ceiling.  We decided to use ordinary pine lining boards (V-groove), as a reasonably eco-friendly material (Radiata Pine is plantation grown - and although there is some potential that the plantation was planted on what was previously old growth forest, most of the pine plantations I know are on what was previously degraded farmland, and it is renewable).

nailing the first board!

The lining boards were hand nailed to the bottom of the purlins, which allowed us enough space to put in some serious insulation (R5 batts), which  added to the wool blanket on the sisalation that was put in when the roof went on gives us (we think) a total of R7 insulation in the ceiling.  Not too bad, and will complement the high insulation qualities of the strawbale walls.

putting in the insulation batts as the boards go up.

It took us about one day to line each 'wedge' of the ceiling, working between our main beams that form the post and beam frame.  So sadly now much of that heavy timber is not visible, but a good insulated roof is more important that showing off big timber.  We got quicker in our methods, pre-cutting the angles on the boards (since each wedge is the exactly the same there were only two angles to cut, the centre angle and the outer facia angle).  The full ceiling is lined completely, including what will be eaves.

So about ten days in weekends, and its another big job done.  Its nice to start seeing 'finished' parts come together instead of digging away at trenches that will be filled in with concrete!

All done!

Now onto the most exciting part of building a strawbale house, the walls!

The great concrete pour

It was a very foggy and gloomy day (one of the few we have had all winter) that we finally poured the concrete slab for the house.  That was about a month ago now, time flies when you are having fun.

We had a crew of about five volunteers, and a paid concreter who was happy to work with us (harder to find than you would think!), and two guys who operated the concrete pump.  We had to hire a concrete pump (a line pump) as the slab site was just too far from the driveway to be able to shoot the concrete straight out of the truck, and while we contemplated it for a (very short) moment, we decided against wheelbarrowing 40 cubic meters of concrete!

the pump truck with concrete truck

pumping the concrete, first into the trenches, and then over the whole slab

it was a fairly relaxed affair!!

It ended up being a fairly relaxed day, nothing really went wrong, and the pace was calm.  The man who held the hose for the concrete pump certainly earnt his wages, he worked like a machine!  So now months of work is covered up, all those trenches and curved reinforcement....  at least we have photos to show for it all!

the boys 'hard' at work

screeding the concrete

our curvy slab, just finished.

its dry!

So now we have a lovely flat area, that will be our thermal mass in our house.  We designed the house to recieve winter sun (but not the summer sun) and its lovely to see how far into the house the sun reaches at its lowest point (to the halfway mark in the house).  This will be integral in our passive solar climate control, gently warming the house with winter sunlight. 

Now to save up for the next stage which is most likely going to be lining and insulating the ceiling. 

And your not going to see any of it!!!

Was the mantra that was repeated over and over by my labouring wife over several months of preparing the house site for the pouring of the slab. This is the last expensive thing we need to do before getting on with the relatively cheap mudbrick making. I really can't blame her it has been a bit of a slog but we can now see the end of this stage of the house build.

To ready the site for pouring the concrete slab it is simply a matter of digging the trenches, adding a layer of sand (road base?), boxing up, covering it in plastic and setting out the steel reinforcing. We have done these steps but of course we added an extra level of difficulty.

Firstly we put up the frame and roof before the slab, which is not normally the way it is done. The main reason we did it this way was to get the house build started quickly so that we would be able to extend our planning permit (ie. frame stage finished). It has had an advantage in that we don't have to worry so much about the weather when preparing for and when we eventually pour the concrete. But we have had to dig the trenches by hand (or pick, shovel and wheelbarrow), which added up to moving about 16m³ of dirt. This was done over a period of a month on the weekends. Because the outer edge is curved half the trenches ended up being curved. The total length of trench was about 160lm. The dirt we moved was dumped next to the house and will be used for the mudbricks and render later on.

The next step was covering the bits of land with about 8m³ of sand. This was done by wheelbarrow and shovel. We compressed the sand by hitting it with a bit of 4x2. The sand needs to be slightly wet to do this well, too dry and it won't hold it's shape, but too wet and it is too sticky.

The boxing up was also a bit different and I think a bit easier (and cheaper) than if we had a square slab. The entire outer edge of the slab is curved, so I decided to use 3ply because it is easy to bend instead of 6x2's that you would normally use. I used scraps of 4x2 and 3x2's upto about 500mm long to stake the 3ply. The bits of wood were staked to the ground with 250mm galvanized fencing nails (these will be re-used for fencing once the slab is done) and then the 3ply nailed to it. The slab is 150mm above the ground level so the 3ply was cut into 200mm strips. Because the 3ply is curved this adds strength to it and hopefully it will hold up on the day of the pour.

Laying the plastic we thought would be a bit of a nightmare seeing as how half the trenches are curved and at odd angles to each other and the added difficulty of working around posts. But it wasn't all that difficult and only took a day. We used about 400m² of plastic to cover an area of 200m², I guess it was in the up's and down's of the trench walls, the overlap of the plastic and a little extra in the trenches so when the concrete is poured it doesn't pull the edges in too much.

The last bit of the preparation really highlights one of the reasons most people don't make a round house. Trench mesh is straight and is really, really hard to curve. So we had about 80lm of straight trench which was easy to lay standard 3xL11TM in. The outer edge and inner trench which made up 80lm of curved trench needed to have 3xN12 bar, which is 12mm notched steel bar, which needed to be curved. This ended up being not too hard to do, but did add a lot of extra time over installing standard trench mesh. There also ended up being a lot of extra cutting of the slab mesh (SL82) because it has to follow the edge of the slab. We also had to navigate the 10 central posts and cut more than half the 18 6000mm x 2400mm sheets used to fit around the posts. But myself and the wife got the job done in about 4days with a bit of help from the kids.

So now we are ready bury the whole lot in about 40m³ of concrete, we just need to organise the concrete, concrete pump and helpers for the day.

Framed signed off (possibly), and a roof!

We did get the extra timber to finish off the frame and had it up in September. Some cross bracing was then added and all the bolts had to be tightened.

Once done we called the building inspector to come out and check the frame for sign-off. Because we used the coastal grey box which is rated pretty well instead of what the engineer specified in his documentation for the frame the inspector wanted the documentation changed to reflect what we used was in fact ok. For instance the engineers report specified for the main beams 2 290x35 nail laminated to make a 290x70 beam, We used a single 290x70 beam. The engineer also specified mostly F17 kiln dried hardwood, we used unseasoned coastal grey box which is rated to F22 unseasoned, but when it eventually dries it goes upto F34.

We have submitted the updated engineers report to the inspector but to date and after several attempts to check that the extra information was adequate we still don't know if we have the frame signed off, but taking a positive outlook on it and assuming that no news is good news we have moved onto the next step, the roof.

We got several quotes and finally settled on one from a local roofing company, a bonus is that the owner of the company lives just down the road and is a nice guy. One of the things that we did want is to either put the roof on our selves or at least be involved in putting the roof on. As owner builders we want to do as much as we can, the roof is one of those jobs that the authorities want a certificate for. The roofing company after some negotiation were happy to knock off some of the cost of the roof and let me take part as a general laborer.

In total it took 4 working days to complete the job, and there were three of us. Myself and two qualified roof plumbers. The guys did a great job of the roof even though they constantly made cracks about the wonky frame. We ended up putting on wire mesh, then foil with 50mm fiber insulation (fire proof, and an >R2 rating) and a "Surf mist" colourbond corrugated roof. We also got a round gutter (called smoothline) which looks great. The head roofer was in charge of cutting all the gutter joints and the roll top flashing. He did a fantastic job of lining up the flashing as you can see from the picture.

Now that the roof is on we have to save money up for the next big jobs, lining and insulating the underside of the roof and doing the slab (or maybe strip footings). While we save money we will start making mud bricks which will make up the inside walls, this is a relatively cheap job but pretty labour intensive.

The frame almost finished

The frame is almost finished now, and would have been finished if not for a change of plans. Originally when ordering the timber we were going to leave the two north facing outer segments without purlins since the roof was not going to extend to the fascia, but when we started putting the purlins up I made the mistake of putting them up in the two northern outer segments and upon reflection decided it was good and to leave them. This change though meant that we were 5 x 6m lengths of timber short.

I made the decision to leave the 3.6m and 2.4m lengths out of the last 5 segments (the ones either side of the ring beam). I did this because I should be able to fit those lengths onto the trusty old ute and pick them up myself, saving on delivery charges.

Due to the weather the timber mill has been unable to get extra bits of the Coastal Grey Box for the last few months, but today I got a call to say that it is in and ready to be milled (yippee!). Which means that we should have the timber within the next week.