I have finally finshed making the foundation box beam that will sit on top of the tyre pillars. This was made in 13 pieces to make it easier, with the joins designed to rest over tyre pillars (although moving each one out to the site could not be described as easy). The beams are now all sat on the pillars and, pleasingly, I had to make very little adjustments to get them all level – I was 20mm out in one corner and was tempted to just to wedge a cap of OSB board underneath but decided to make the extra effort and put in place a new slightly larger tyre. Next job is to bolt the beams together, put in insulation and glue/nail the top plate on.
It now finally looks like a proper building project.
I am now into making the individual elements of the foundation box beam. All-in-all I need to make 13 separate elements – varying in size from 2 metres to 4.2 metres. I am lucky enough to have a lean-to covered area next to the barn, where I have set up a box beam production line. Eight beams down, five to go.
The car tyre foundations are now complete and signed-off by Building Control. Having filled in the space either side, they look rather unimpressive – as though someone had just laid out some tyres – belying the amount of time an effort involved in getting to that point.
It is now time for some serious, abeit basic, carpentry to make the foundation box beam. There have been two main issues here: the precise design of the beam, taking into account the position of car tyre pillars and management of joins and then selection of wood. in selecting the wood the debate has been whether to use a durable timber such as larch, or the much cheaper option of usng standard treated softwood. In the end I have gone for treated softwood, partly because when speaking to the owner of the mill from which I was proposing to get some larch, he actually recommended it. My thinking is that none of the beams are going to be exosed to the weather or in contact with the ground. The outward face will be covered by a woodfibre board and render and the bottom element of the beam was always going to be OSB3 board, irrespective of whether the solid timber elements are larch or softwood. The chance of getting any deterioration is therefore pretty small and, in an case, if any problems emerge in future years it should be relatively easy to remove and replace a problem section. I am also going to give the beam a couple of extra coats of a natural preservative treatment.
The second issue is design of the beam. The longest length of beam required is 11 metres. Clearly it is not possible to get single timbers of this length and even if you could, working with something so massive would be pretty difficult. Some form of joining of timbers is therefore necessary and Barbera Jones’ book and online drawings gives guidance on how to do this, avoiding having joins that are within 600mm of each other for example. However, the foundation beam doesn’t have to act in quite the same way as the roof beam. The main structural issue, aside fom the dimensions of the timber, is bridging the span between the tyres which is best achieved by avoiding having any joins between the tyre pilars.
I have therefore divided the beam into smaller sections that correspond to where the car tyres are positioned and added in some extra pillars to provide additional, or wider, bearing points (as you can see in the picture) . That means the beam is divided into 13 sections, none of which is longer than 4.2 metres, and which I can make separately and then bolt together. As mentioned above, it should also make it more feasible to remove an element of the beam in future years if any problem with rot emerges.
So, I have now opened an account at Jewsons and got busy with a circular saw, hammer, nails and glue. I reckon there is about 40 hours of work ahead before then moving onto the base plate for the bales and then the slightly more complicated (but not quite as massive) roof ring beam. This is more complicated because it will have to take account of the positioning of the vertical window and door positioning posts and also because it will have to work as a single element from the perspective of spreading the roof load and distributing compression.
Once all that is complete, we face the tricky moment of devising a way to raise the roof beam onto scaffold supports, so we can then fix the roof timbers onto it and (probably) a temporary plastic or tarpaulin cover to provide weather protection which will allow us to do the straw work during the winter. The reason that we (probably) won’t be able to put the finished roof sheets on initially is that we need to have access to the top of the straw to drive hazel pins down into the wall.
Filling tyres with gravel is not as easy as simply pouring gravel into your nicely positioned tyres. Firstly, it is very important to ensure that the gravel is compacted throughout the whole tyre – especially under the rim. This means pulling up the inside of the rim and then stuffing the gravel under it – tough on fingers. Second, packing out the tyres is the equivalent of inflating them, thus they get fatter (higher). A tyre can easily ‘swell’ by 10 to 20mm when filled, which can then play havoc with your carefully stacked and positioned un-filled tyres especially because different tyres swell by different amounts. A tyre that has a square profile and flat outer walls won’t swell by much, whereas a tyre with a rounded profile and ‘flabby’ inner rim can easily put on 20mm or more.
I now realise why the Strawworks’ plans for car tyre foundations show a 20mm extra OSB plate between the top of the tyre and the box beam which is there for levelling purposes. I had previousy thought this was a bit of a cop-out and that it should be possible to get all your tyres level to within a 20mm tolerance – which it is: before you fill them. Clearly setting the final level is something you do when you put the box beam down and will involve fiddling around with bits of wood here and there to get everything right. All this fiddling and stuffing means that it takes about 20-30 minutes per tyre which, if added to the time for digging and levellng, probably means a total time of about 1 hour per tyre pillar.
The other observation is that it is easy to underestimate the amount of gravel required. I would say, that with a two-tyre stack, one tonne of gravel will do between seven and 8 pillars, although you can save on this a little by putting stray bits of hardcore into the mix. Suffolk seems to have a shortage of 10mm gravel at the moment – most builders merchants are out of stock – which is why I had to go direct to a quarry to get mine (Betts Aggregates at the Shrubland Estate Quarry), which did at least mean that it was a little cheaper (£29 plus VAT per tonne).
Builder Tim did indeed have a solution, which was to lend us a magnificent device called a Cowley Level. You can actually pick these up on ebay for around £25 and I would recommend getting hold of one of these in preference to buying or renting a fancy laser device. We spent the weekend playing with this, scrapping bits out of the holes and messing about with tyres of varying widths. This taught us the value of having lots of tyres of width 165mm or less – which unfortunately are relatively rare – so my one piece of advice (other than getting a Cowley Level) is to start early in keeping a look-out for narrow width tyres. Anything over 175mm in width is easy to source. (Note: tyres have three measurements on them e.g. 205 55 R16. It is this
However, we now have all tyre pillars assembed and to a level correct within about 10mm, Building Control have come out and signed them off, so the next step is to use gravel in the bottom of the holes to get a fine level of accuracy, and then start the fun business of ramming gravel into the tyres.
We now have building regs approval. Mid Suffolk Building Control are happy for the tyre pillars to sit on top of the clay sub-soil, the only stipulation being that the ground level has to come a reasonable level up the final tyre in order to provide some stability. We are therefore entering the time of spades, sticks and string (and spirit levels): marking out the site, digging the holes and working out the levels.
I have actually completed digging the holes to an approximate depth and position, i.e. to within about a 20-30mm tolerance. This was basically a two-day job. Fortunately I didn’t encounter anything too horrible, just a small area where there was an old slab, about 8 inches below the surface. However, one of our neighbours has a heavy dutry masonary drill that made relatively short work of it.
The tricky job (for this weekend) is going to be getting an accurate position and level. Building Control will need to sign-off on this before we then move to fill the tyres with gravel. Tim Moss, who is the builder supervising what I am up to is going to come round and give some words of advice on this one.
It is amazing what one 7.5 tonne digger and someone who knows what to do with it can achieve in the course of a few hours. The cowshed, that I have been labouring for weeks to demolish by removing the roof and recycling all the roofing sheets and timbers, is now rubble: blockwork walls and concrete slab a neat and tidy pile, ready for crushing. I was hoping to do the crushing at the same time as demolition, which would allow us to create an area of hard standing (useful for access and mud limitation) and then get the excess off-site, but the machine we need is not available for the next couple of months so this will have to wait. Not a major problem.
The speed and tidiness of the job is largely due to Robert Garnham (01359 242302), a man who has been described to us by a local farmer as ‘an artist with a digger’. To see him at work is quite something.
We were also able to dig out the water pipe and take it back to where we will route a new trench to take it to the straw bale house. In doing so we were able to take a good look at the sub-soil – which, as suspected, is pretty much pure clay with a few chalk pebbles in it. This has kept alive my interest in trying to mine this clay for use as the internal render.
We are now just awaiting input from Building Control, because until we have their sign-off I can’t start on the car tyre foundations or start work on the base beam or wall plate ring beam since we need to agree on size of timbers, spacing of pillars and depth of pillars (or even if we can use car tyres on this sub-soil at all).