Saturday 30/1/21
Great day today buoyed by an advisory visit from Ben .
Firstly, I realised why I was having such problems with the top king post joint yesterday. What was happening was that the king post was not staying straight and level. It was moving every time I took the partially completed joint apart and put it back together again. Up until now the pieces of wood we’ve been working with have been so huge and heavy that once you level them up they don’t move. So this morning I levelled up the king post and clamped it to its trestles. Then I checked it regularly. After this realisation, it wasn’t long before I had a half decent king post/principal rafter joint. Note how the shoulder of this joint has a kink in it. This is because the shoulder on the principal rafter is longer than the angled surface at the top of the king post – this turned out to be significant – read on!
My attention then turned to the other end of the principal rafter. Here I found a surprise.
The nose of the principal rafter now came in short. Instead of being 182mm from the cog on the tie beam it was 198mm.
[Edit 31/1/21: The following explanation – now in red – is wrong. See Trust Your Maths for the true explanation and the correction made.]
I think this is due to a combination of two things:
- the faffing around trying to get the top joint to fit properly slightly shortened the principal rafter and …
- the 182mm was calculated (see Roof Geometry) and the calculation assumed that the underside of the principal rafter came in at the point where the king post widens to form the top king post/principal rafter joint. However, it comes in slightly lower than that (see above).
I’m not sure this is correct but it’s the only explanation I can think of.
So I proceeded with the principal rafter nose at 197.5mm and will reflect this on the other side of the truss.
I cut the tenon on the lower end of the principal rafter and flipped the tie beam through 90 degrees and cut the mortice.
Then came the moment of truth. With the principal rafter pinned to the king post, I moved them both towards their respective mortices in the tie beam. I pulled the king post down with linked sash clamps.
Et voila! It came together beautifully.
The angles were pretty close to what they should be: tie beam to king post – 90.5 degrees; tie beam to principal rafter – 39.6 degrees; king post to principal rafter – 49.1 degrees (total – 179.2).
Tomorrow I should get the other principal rafter and so finish my first king post truss!
Hi Simon. In the connection between principal and tie, how did you size your tenon? I’m presuming its somewhere around 38 to 50 mm thick. How long is the tenon from its shoulder? Did you choose this based on the joinery in the house or did anything else factor in the decision. I have read elsewhere of the heel of the principal joint being notched into the tie but I’ve seen plenty of examples just like yours too and it seems an age old accepted connection. Thank you for considering these questions!
I was using a 40mm chisel (my friend who I worked with most of the time used a 1 1/2 inch chisel) so this defined the size of the mortices we each worked on and this the thickness of the tenons. And yes, this was based on the dimensions used in the timber frame of our house extension.
Our tenon lengths were 70-75mm.
Note for all the braces (there are 22 of them) which are 75mm thick, we used ‘one-sided’ tenons. Even the trained eye would need to look hard to spot this. This saved a lot of time and the joints look better than the house extension frame where the braces have double-sided tenons.
In our house extension the heel of the principal was not notched so we didn’t notch ours either.
I hope this helps.
Thank you as always. It’s very difficult to quantify forces in these joints and my wishing to use clay tiles has caused me quite a bit of analysis paralysis. It’s great to have real world examples and it seems, I think as you allude to in an earlier post, that the members have to look big and strong enough when using time proven methods. Presumably in the house, it is also a trussed roof and they’re holding up slate or clay tiles? so unnotched connections are certainly up to the task. Feel I’m slowly getting there. My oak is arriving tomorrow which is exciting and terrifying! I plan to use bare faced tenons on quite a bit of my joinery including braces as the building will be sheathed externally, the oak only exposed internally.