This short blog post summarises hours of anxiety, thinking and internet research and concludes with a simple and, in hindsight, a very obvious solution.
The problem is that when you place a straight edge across the two hip rafters in this shot, it rocks on the common rafter (king rafter) in the centre. So either the common rafter or the hips are not sitting on the end wall plate correctly.
The first few hours of thinking concluded that it must be the hip rafters that are out of place and this conclusion was based on the fact that there are common rafters elsewhere in the roof structure which are sitting fine and that the common rafters have much simpler geometry.
Then, I had a breakthrough when I came across the concept of HAP – Height Above Plate. This the vertical distance from the top of the wall plate to the top of the rafter as measured from the top of the heal of the birdsmouth.
According to the roof design, this should be 152mm. It turns out that this is correct for the common rafters but not for the hip rafters.
This means the hip rafters are sitting lower than they should be and it’s this that’s causing the rocking. Tomorrow, I will try using a spacer to lift the hip rafters so that their HAP matches that of the common rafters and hopefully this will solve the problem.