Today I set about fitting the righthand hip rafters. Because we decided to orientate the end wall plate differently to the side wall plates, the geometry of the corners are interesting. We knew this would happen and decided that this was a small price to pay for the benefits gained by reorienting the end wall plate. These corner ties will not be seen once the barn is complete as they will be hidden in the far corners of the mezzanine above the ceiling of the open, righthand bay.
I decided to take a different approach to the construction of the hip rafter birdsmouths on this side of the building. Instead of cutting a bevel on the corner of the wall plate and cutting the birdsmouth square, I left the wall plate corners square and cut into the heal of the birdsmouths. This was easier to do than I expected. I started the cutting of the heal with the circular saw set at 45 degrees and finished off with a chisel.
Unfortunately, I cut the hip rafters slightly short and had to extend them to get them to sit in line with common rafters on the sides of the roof and sit at the right angle (30.6 degrees). So far so good.
Then the real trouble started. When I put the central common rafter (the king rafter) in place I got the dreaded ‘rocking straight edge’ problem. When I put a straight edge across the roof it rocked on the king rafter. The king rafter was too high. To this moment I am not certain why this is.* I then set about trying to sort this out and it took the rest of the day! It would probably have taken a profession roofer about 30 minutes!
In the end I resorted to running a line between ‘height above plate‘ points on the hip rafters.
I then adjusted the birdsmouth on the king rafter to this line. this all sounds very simple when written like that but this took me a couple of hours. The heal and seat of the birdsmouth, the length of the rafter and height above plate are all inter-related in ways I don’t fully understand and changing one changes the others. Anyway by the end of the day I ended up with a set-up I can live with.
Tomorrow I will start work on the jack rafters (creepers).
* [Addendum 13/4/21. The mystery of the hight king rafter solved! When I took the softwood roof down, it became clear why the king rafter was high. It’s because the end wall plate is bowed outwards – see image below.]