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.

Preparing the posts

Since we got the timber a few weeks ago I have been trying to work out a) how to handle the timber and b) how to cut it up. The posts are unseasoned 200mm x 200mm (8" x 8") x 2.7m dense hardwood. As mentioned in an earlier post they weight around 150kg. The main beams are 300mm x 70mm x 4.8m.

The first challenge was to move them around and handle them. The came in packs, there were 2 packs of the posts and then the several packs of mixed beams, ring beam, purlin and fascia. We have started to sort the mixed packs out into their own stacks and in the process putting spacers between the timber so that air can get around the timber and help minimise uneven drying and warping. To move the bigger timber members we purchased a furniture trolley with big heavy duty wheels. Myself and Sharon are able to load the timber onto the trolley and between the two of us are able to move the timber to where we want.

The next step has been cutting the timber up to use. We are going to put up the center ring first and then the outer. So we started on cutting up the center posts. This requires cutting the bottom flat and the top on a ~7° angle and then a slot in the top and bottom that is 200mm deep and ~10mm wide. We tried a few different methods for cutting up the big timber

1) Chainsaw - this worked ok, it was the quickest cutting but the draw backs are you have to be careful that you are straight as it does cut quick. The chain tends to chip the timber at the cut and the last thing is that it covers the timber in chain oil.  It also requires maintenance, checking fuel/oil, sharpening and starting.  It uses fuel, is noisy and creates fumes.

2) Handsaw - this method is quite slow, but it is quite accurate as you can monitor the cutting as you go along (usually while giving your arm a rest). the cut is nice an clean with minimal chipping at the edges. I use a Spear and Jackson crosscut saw with 10ppi.

3) Circular saw - The biggest circular saw that you can get from a hardware store appear to have a cutting depth of 85mm, so drawback of this method is that you end up with a bit in the middle of the 200mm post that the saw can't reach. Also the post faces are not exactly perpendicular with each other and vary in size from 190mm to 205mm (so far these are the 2 extremes I have encountered).

The method I have ended up using is a mix of 2 and 3. so I circular saw as much as I can and then finish off with the handsaw.

Marking up the posts has been the most important thing, and as mentioned the posts are not square. The most important part of cutting up the posts is the height and that the slots at each end line up with each other, if they are at an angle to each other then the top bracket will not point to the center of the house which is a big problem. So to line up the slots properly I ended up choosing the face of the post which will face in toward the center of the house and then measure a constant 100mm (to the center of the slot) for all post no matter what the actual diameter was for each post. This will give me a constant distance from the center to the face.

To cut the slots we very carefully marked them up making sure that the top and bottom slots lined up. We did this by marking up the top and bottom together, the distance from the face was the same for the top and bottom and we used a tape measure from top to bottom when marking them. When both sides of the slot were marked we then joined up the side markings along the ends. To cut we used a circular saw. A cut was made on each edge of the ~10mm slot on both sides of the post, the big thing here was to make sure that the saw depth cut was inline with the end marking since each face of the post is not guaranteed to be 90° to the others. I did this by lining up the saw blade angle by eye with the slot end markings and then watching as the saw cut that it was staying true, if not I would account for the difference by slightly tilting the saw while cutting. The top of the slot was pre-drilled using a 10mm auger bit and the material left in the slot was hand sawn out using the crosscut saw (I will have to look around for a rip saw I think, since the crosscut while it worked took a while to get through).

The other issue we had was to drill a 'straight' hole through a 200mm chunk of wood. My drilling is not that accurate that I was confident of doing this for every hole (each post has 4 holes). The stirrups and the top brackets are all pre-drilled with 18mm holes. The bolts are M16 so there is a 2mm allowance for error. Since the timber is green this will allow for some shrinkage too. But because the steel is drilled we have to get the holes in the timber pretty accurate, a crooked hole would look bad but more importantly it will probably mean that the bolt won't fit through. To solve this problem we ended up marking the hole on both sides of the timber off the front face, this involved using a square on the face and measuring the opposite hole from the square, which should align the hole exactly perpendicular to the slot. We then drilled the hole about 3/4 from both sides. any slight variation on each of the holes was then 'adjusted' by re-drilling so that the two hole match up. We bought several sized augers and have found that they are brilliant for drilling nice straight holes through the timber, they literally pull the drill through the material.

We have now cutup the first 10 center posts and they are ready to put up, but before they can go up we need to cut up the ring beam timbers (240mm x 70mm) and get the top bracket made up. The top bracket joins the top of the center posts to the inner and outer main beams (these beams radiate out from the center) and the ring beams (they join all the center posts together).