Sunday, 21 July 2013

Straw Bale Workshop 2: Groundwork

This will become obvious quickly enough, so I shall tell you now: I am not a builder. I've made a few structures out of wood before, re-tiled a roof very badly, done some wiring, plumbing and small patches of plastering, but I'm about to embark on the first serious bricklaying I've ever attempted. 

First, I had to get the hole straight.  I had given my digger-driving neighbour some poor instructions, and needed several days with a spade and a wheelbarrow to get the hole to the right dimensions and depth. Here it is:

I hope that's deep enough. It's definitely subsoil, and straw bale buildings are relative light-weights.

Next job (according to the book, which is: Building with Straw Bales; A practical Guide for the UK and Ireland by Barbara Jones) is to lay a strip of limecrete around the edge 50mm thick and 150mm wide (2 x 6 inch). I did as much research as I could on materials and method, and this is what I came up with:

Buy 4 bags of NHL 5 from a conservation builder's merchant. Walk past the ballast at the regular builder's merchant, because I wasn't sure it was well-graded. (It looked like 20mm gravel mixed with sharp sand.) Luckily we have sand and gravel pits just down the road, so I bought half a ton direct from there. Hire a regular concrete mixer. Batches start with 1.5 buckets ballast, then 1 bucket lime, mix for a while, add another 1.5 buckets ballast, mix for 5 minutes, add enough water to make a dry mix (4 - 5 litres), mix for 10 minutes, then add another litre or 2 of water to make a sticky mix in a further 5 - 10 minutes. Wear dust mask and eye protection when there is dust around. Once the mix turned sticky, it needed a certain amount of poking with a piece of 2x2 to stop it from sticking to the mixer drum.

It's not a great photo-point of the project:

According to the photo data, this was on the 20th June. Limecrete takes a while longer than concrete to harden, so I started bricklaying in the second week of July.

The advice I was given was to use engineering bricks below ground, so I bought 600 very cheap, new ones. I have a pile of recycled bricks and local flint that I am planning to use above ground.

Mortar mix for this level was 3 buckets sharp sand to 1 bucket NHL3.5 with around 8 litres of water. The method was very similar to that for limecrete, i.e holding back the last couple of litres of water (and the stickiness) until the last few minutes of mixing. It's beautiful mortar to work with: no plasticizer needed, and nice and sticky for dressing the ends.

This section ends with a rather impressive-looking photo of my foundation with 5 courses of below-ground brickwork. Before we get there, I will just confess to some problems encountered along the way. The limecrete levels went wrong on the foundation strip, and I had to gradually get back to level by varying the mortar thickness. More seriously, I managed to get the brick bonds wrong, with 37 whole bricks along one side and 36.5 bricks opposite. Luckily, it's all below ground and no-one will ever see it. Unless you look closely at the photo below:

There are plenty of books and internet resources on how to do this right. This is more of guide to muddling through.

Here's the finished foundation:

It's not perfect, but I'm reasonable happy with it. More or less level, about the right size, reasonably square. It's July 12th 2013. My wall needs 4 weeks to harden, and I then have a contractor coming for the next stage: 250,mm recycled foamglass insulation and 150mm limecrete base.