Hi Ivanson...
Welcome to Permies!!
Let see if I can help some...
I was originally planning on using slabs for the roof also, but then I was thinking about it and slabs are all sap wood, which rots faster.What do you guys think?
Pine tar oil could address this as well as a good treatment of a product like exterior grade "land ark," though for roofs...pine tar oil is the best traditional treatment.
If not I'll use catalpa shakes for the roof.
Even without pto on a 16/12 pitch (53.13010235 degrees) you would get 50 years (maybe even 100 plus depending on other variables.)
In Ben Laws book Roundwood Timber Framing, he says oak is unsuitable for round wood timber frame, because there isn't enough heartwood.
Creator love him Ben is a great fellow...but reinvents way too many wheels...
I must say I read him as a very experienced novice and thinking of deep thoughts...not a traditional Timberwright or woodsman in many ways. Take him in small doses and often with a grain of salt as well.
From what I understand oak sapwood is almost as strong as oak heartwood, although less rot resistant.
And you would be correct in most cases as sap would in trad frames have little to do with the durability of the frame.
The sap wood in the ground will rot in a couple years most likely but there will still be 6-8 inches of heartwood left.
That is simple...don't stick it in the ground. Ben's "round wood" is romantic at best but not always the fastest modality. Try just a simple "Cruck" timber frame of the like and you would be better off (as well as have something that you could sell if needed and our move should you choose.)
Since the oak is only for the posts, with the exception of one wall plate, I don't know if it is an issue because the posts have the tenon which would be in the the heart wood.
Nope...not an issue.
My next question is the oak posts are still green and the cherry has been down for 3 years and probably drier then the oak. The oak will dry and shrink in the mortice but how much and is it an issue I should worry about?
Nope...not an issue.
For pegs I intend to use catalpa which is a fairly soft hardwood.
Not recommended even a little bit for your plan...stick with oak, cherry, or hard wood.
I think a timber frame could be built without pegs and unless a giant or dragon came along you'd probably be fine. I still think they should be used but is oak or other hard woods really needed?I am going to burn the ends of the posts and soak them in used motor oil or possibly linseed oil.
Trunnels (pegs) or wedges are a "must have" in almost all designs, unless you get into very advanced Asian timber framing systems that us "tessel tenons" and other "thread" tenon designs. Not worth the effort. I design/build many in Asian modality, as that is my specialty. I prefer no oblique braces (only horizontal methods of bracing) with either "trapping" gravity joints and/or drawing wedge systems.
That should get you thinking some more thoughts...good luck.
Regards,
j