Trust Your Maths – green oak barn

Sunday – 31/1/21

I only did a couple of hours today adjusting the part of the truss I thought I’d finished yesterday. I wasn’t quite happy with the fact that the bottom of the principal rafter came in 198mm in from the cog joint and not at 182mm as I had calculated it should. Here’s the picture from yesterday.

New principal rafter position - 2

I did some further investigation and found that the diagonal measurements I’d taken on Thursday were out and that they would be corrected by lengthening the mortice in the tie beam to the 182mm mark (as per my calculations). So I disassembled the truss and made this change. I then put things back together and applied one of Ben’s lorry straps…

Lorry strap to pull in the principal rafter - 2

… to pull the lower principal rafter tenon across to its new shoulder at the 182mm mark.

Back to 182mm

This corrected the diagonal measurements but changed the angle of the king post remarkably little. Yesterday it was 90.5 degrees, now it’s 89.75. I have a lot to learn about angles.

Angle check 90 degrees

Anyway, I think this half of the truss is now truly complete.

I really like the fact that the principal rafter is narrower than the king post and tie beam. This makes for very pleasing inset joints at the top and bottom.

Principal rafter shoulder with king post Principal rafter shoulder with tie beam

I finished my short day by putting the second principal rafter in place and levelling it up ready for marking the joints tomorrow.

End of day 31/1/21




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