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.