Formwork, bamboo tricks and mudding the corners

Plain ply formwork on the outside

Just when you thought it would never happen, I present for you a blog post that actually explains how to do things.  A bit.

I've been meaning to do a post on formwork.  Formwork was a feature of this weekend, so now would appear to be the time.

Winter solstice brought us one of those classic Central Victorian sunny weekends, so we made the most of it going for bush-walks and building.  This weekend we tackled some difficult bits we'd been procrastinating over.  

We also built a bit some more east wall.  Since this is one of the last simple wall sections we're going to build (yess!), it's the last chance to show the simplest formwork in action.

We start by screwing plain ply to the outside of the wall, covering as much as we think we'll build that day - that's the photo at the top of the blog post.  

Laminated chip-board door with pine plank at top

All the wall-building action goes on from inside of the building.  It's a bit of extra walking this way, but it protects the wildflowers that grow close to the walls from too much foot-traffic.

Ply on the outside, moveable formwork on the inside

The moveable formwork is made from someone's old laminated kitchen cupboard door, with a single bit of pine screwed to the top.  Laminate doesn't stick to mud.  But really you can use anything.

Once you've stuffed the bottom form with light earth mix and patted it down a bit, you're ready to slot the next panel in space.  

Isn't TJ filthy?!

Now you see why the pine board sticks up above the panel?  This prevents any gaps showing, and it's very strong.

If you want to build a lot of wall, you can pull out the bottom panel and leap-frog it up the wall for a third layer, and so on.

2nd layer slots into 1st layer

Light earth doesn't put very much pressure on the formwork (not like rammed earth or heavy poured mud) so you can get away with clamping or screwing the formwork in place.

You can remove the formwork straight away, or leave it for an hour or two to start to set.  Don't leave it in place for too long, because it needs to start drying out to prevent mould.  

When you remove the formwork, it's important to slide it off the wall, maintaining contact until you can feel the mud has let go.  If you pull the formwork straight out from the wall, some of the straw will stick and you'll lose the nice flat surface.

New mud section on East Wall

This weekend we spent most of our time fixing up some difficult sections of wall, some of which provide some interesting examples of formwork.

Before - slumping

This is the first wall we built, when we were still finding our way.

For some insane reason, I decided to put a heavy mud render cap on the top of the wall when it was still wet.

Actually I know why I did it.  At the time the verandah hadn't been built, there was no guttering and very heavy rain was forecast.   

The mud render capping helped the wall survive an extraordinary storm, but honestly, I have no idea why I didn't just cover it with a tarpaulin.

Anyhow, predictably the heavy capping caused a not very attractive slump in the wet wall.

Before - Flood damage

During the storm, a lot of mud washed out of the  wall, particularly at the top, exposing the straw and leaving it vulnerable to rot.

The capping itself started looking a bit mouldy - it was just too heavy to dry properly, and I was worried the wall below the capping would also have rotted.

Now we have a protective verandah on the west side, it was time to repair the damage and take away the stupid capping and any mouldy bits so I could pretend I got it right in the first place.

Specialised light earth cutting tools ... not.

I cut the top section of wall off using some gardening tools.  It really is amazing how Light Earth can withstand very harsh conditions, including driving rain, but be shaped with hand tools.  Of course it will be a lot more resilient to sharp objects once the render is applied.

Mouldy slumpy top removed, bamboo in place

And here we have the wall with the top taken off it.  If you squint, you can see two bamboo poles in place ready to help tie the new wall sections together and make them strong.  More on that later.

The extent of the mould damage.  Not a lot.

I was pretty amazed to find there was no rot in the wall sections that I removed.  Just a few patches of white mould that didn't seem to have damaged the straw at all.  There was no moisture for it to feed on.

Considering the length of time this wall was exposed to the weather, and the ferocious storm during that time (I think there was over 40mm of rain, which fell in sheets from the roof onto the wall), I was expecting to find at least some rot.  But none.  Mud really is extraordinary stuff. 

Once all the preparation work was done, it only took a few mixer-loads of mud and straw to top the wall up.

The wooden beam at the top doesn't go the full depth of the wall, so I was able to clamp formwork to both sides and shove the mud mix in from the top.

New light earth meets old wall

Shrinkage gap, showing bamboo pins at top

The next section of the West Wall presented some technical challenges.  

This was the second full-height wall we built, so we'd learned from our mistakes and were better prepared to fill the gap that forms at the top of the wall when it dries and shrinks.

When I built the wall, I drilled some holes into a wooden plate at the top and snipped up some bamboo garden stakes into pegs to hammer into the holes.

The pegs became partially exposed when the wall shrank, and gave me something to wrap handfuls of mud mix around.  It also connects the wall to the ceiling nicely, so it's much stronger.

This is a big improvement on the south wall, which was first full length wall, and the only one I've tried to render.  When I was trying to fill the shrinkage gap, I could feel the top of the wall move a bit, because it was only connected to the frame at the sides.  I still haven't got the gap filled properly.

When you get to the top of the wall, you have to leave a gap between the top formwork panel and the ceiling, so you can get your arms and mud mix into the cavity.

So the top section above the formwork is just a messy free-hand mud sculpture on one side.  It helps if you place the straw perpendicular to the wall, leaving a lot overhanging, then smooth it a bit flatter later with a plaster float.  But whatever you do, it will look a bit crap.

The west wall also had some tall skinny odd-shaped sections that we'd been procrastinating over, because the standard formwork doesn't fit them.  One next to the doorway, and one where the west and south walls meet. 

West wall section with gaps each side

The corner section presented a range of challenges.

Corner section. 

It's wide on the outside, but there's no room to put your hand in from the inside. So it's not like we could attach formwork to the outside and stuff light earth mix into the gap.

To add to the challenge, one of the posts is made of metal, which we didn't want to damage by screwing formwork to it.

But on the up-side, if you look closely you can see how we prepared to attach a new section of wall by leaving bits of bamboo sticking out of the adjoining wall sections.

These are held solidly in the dry wall.  

Door frame section

The spot next to the door was difficult, because the wall narrows from a double stud to a single door frame.  The bend in the wall meant I couldn't include it in the main run of wall - formwork doesn't bend.

Bamboo pegs

I repeated the trick with the holes drilled in the wood frame and hammering in some bamboo pegs, shown here.

There were already some pegs and bits of straw sticking through from the main section of wall, to help tie them together.

Dodgy formwork

We had some leftover off-cuts of laminated chip-board.

We screwed a tall skinny bit to the outside of the building to make one clean flat surface, and on the inside we screwed one small piece to the bottom of the frame, stuffed it with mud mix, then screwed or clamped another piece, and so on to the top, always leaving me enough room to shove my arm into the wall cavity and manoeuvre handfuls of mud around the bamboo pegs.

New mud meets old (drying grassy) wall

We re-used the same off-cuts on the corner section, with even more dodgy clamp work.  And le voila.

Outside showing corner section and more grass

West wall - check out the CURVES!

It was good to get those odd sections completed.  I've been pondering some of those West Wall problems for months - in fact I did a post of "west wall thought bubbles" a while back.  The solution was "bamboo!" and "don't worry if the formwork is crappy".  It just took an extra couple of months of building experience to give me the confidence to have a crack at it.