On close examination of the existing Elm wood used in the barn, it's noticeable that the saw marks are not parallel and are unevenly spaced, indicating that it wasn't sawn on a machine but was done using a two-man pit saw. This clip shows how it might have been done:
One of the amazing features of such an old roof is that the principal rafters were all sawn tapered. From a modern approach this would be very complicated. But, looking at it through the eyes of the 17th Century woodworker, since the tree itself is tapered, treating it this way would be the obvious solution, requiring less work and making less waste.
It always strikes me how the barn's builders accepted the natural 'waney edge' features of the timber, meaning that in some places the beams were far from square. There really wasn't the modern demand for uniformity.
I would have loved to use like-for-like everywhere, but since the disaster of Dutch Elm disease decimated the species in the UK this wasn't possible. In the end I had to compromise and use what was locally available, which was English Oak. It is durable, readily available and a recognised suitable timber for old roofs. So, I selected an oak tree from a site about 10 miles away; a friend and I transported it to a sawmill where it was sawn to my specifications. Now the roof was stripped, the wood was on its way, and I was ready to start construction!