Craftsmanship in Wood

My project is an 18' x 24' movement studio. I've been pondering the placement of splice joints along the purlins and ridge beam and I could use some input. Rafter spacing is yet to be determined and king and queen posts are 8' apart. I was planning to use a version of the Kanawa Tsugi joint placed to one side of the king/queens.

Dimensions (width x depth):
Purlins and ridge: 8" x 9.5"
Rafters: 4" x 5"

I'm worried that putting a bevel on the purlins/ridge might compromise the splice joints, so I'm leaning toward notching in rafters on both the ridge and purlins and spacing them as to avoid the joints.

I have just one purlin on either side of the ridge, so I'm wondering if I should place the splice joints in the same locations on the two purlins but locate them on the opposite side of the king/queens along the length of the ridge so, in effect, they're staggered?

I would appreciate any feedback,

Thanks,
Lonnie Kaechele

Lonnie,

the ridge is not a heavily loaded component in virtually every roof frame system, so I wouldn't worry too much about loss of strength from notching. The rafters push up against the ridge and hold it in place, so it is in compression. A thinner ridge will shrink less across its width, and that is a good thing. You could put a secondary block atop the posts which support the ridge if you were worried about strength or wanted to diminish point loading.

Kanawa-tsugi is a fairly standard choice for splicing ridges and purlins. It's a good option and I wouldn't worry about the bevel compromising the joints. Simply cut the joints, assemble the pieces, and then do the backing cuts. When using that joints, it is generally advisable to design the framing sequence so that the ridge and purlins are assembled on the ground and lifted into place as a complete beam.

Staggering the splices between the ridge and purlins probably isn't going to make much of a difference, but it wouldn't hurt anything either.

Chris, when you say "I wouldn't worry about the bevel compromising the joints. Simply cut the joints, assemble the pieces, and then do the backing cuts" are you referring to a bevel along the entire length of the ridge/purlins, or the backing cuts of individual rafter notches? Assuming you're referring to a bevel along the entire length, I suppose how much wood was removed in cutting the bevel would affect how much the joint is compromised. For instance, if the entire width of the purlin were beveled and the roof pitch were steep, a lot of meat would be removed. On the other hand if only a third of the width of the purlin/ridge were beveled and the pitch were around 4 or 5 in 12, as I am considering, much of the keys of the kanawa-tsugi could be avoided.

Good point about assembling the joints on the ground and lifting the ridge/purlins as a complete beam, though it seems a bit tricky to lift a 29' beam with two splices.

Lonnie

It's a question of rigging;

I suppose how much wood was removed in cutting the bevel would affect how much the joint is compromised. For instance, if the entire width of the purlin were beveled and the roof pitch were steep, a lot of meat would be removed. On the other hand if only a third of the width of the purlin/ridge were beveled and the pitch were around 4 or 5 in 12, as I am considering, much of the keys of the kanawa-tsugi could be avoided.

You didn't specify the rafter pitch in your original post, so, given the use of Japanese joinery I wasn't envisioning a steep slope. Your assumption is correct: a steeply pitched rafter will remove a lot of meat from a supporting purlin. In such cases the normal solution is to raise the HAP (Height Above Plate), so that the rafter's lower surface does not meet the centerline of the purlin, but is raised some distance above.

That's a great photo. I was trying to imagine that so It's helpful to see it for real.
My purlin/ridge dimensions will actually be 7.5" x 9" rather than 8" x 9.5" (I had rough dimensions in mind). I think I'll build a model joint and try and find a good HAP and bevel angle that work well with the splice.
Thanks Chris