P&B cathdral roof design

Main frame is 40'x28' 8x8,posts and beams,with knee braces.2-8x8 rafter ties at the wall plates,spaced at 12' from each end of frame.Roof design thus far is a 12/12 common rafter gable,with no ridge board or beam.Rafters are 4x6- 20' in length and have 4x6 collar ties at the 10'mark.Build up will be:Rafters,1x6 t&g pine,vapour bar.,2x8 on end over rafters to box in 6 1/2" rigid ins.1x strapping,metal roof.My concerns are:with no beam or purlins are the ceiling boards enough to hold the roof laterally?(gable to gable)Perhaps 2x for the ceiling boards?We are using mostly plate joinery.Can any one suggest the best way to connect rafter to the wall plate?(rafter end stops at the plate,with an extension added later for overhang)
Any insight would be helpful Thanks

Hey Webster,

I think you would be well advised to consult with an engineer. Just a casual review raises flags. You are quite right that laterally there appears to be inadequate stiffness in the roof, although a plywood or osb membrane might be an adequate and simple solution. Your rafter sizes might be ok for Ottowa if they are no farther than 24" o.c. However I would be concerned about spreading especially at the 16' un-tied middle bay. But this depends significantly on the joint between the 8x8 and your plate as well as your plate sizes.

Your engineer will need to know your local ground snow load, wind exposure, the species and grade of timbers, and more detail about the frame and joinery. Especially plate sizes, post locations, bracing, rafter spacing and so on. Good luck and don't skimp on your design. An unbraced, 28 foot span without structural ridge or purlins is no trivial undertaking even in areas without snow.

Good luck!

Thanks Paul

I think I will eventually be seeing an engineer.The reason I have not yet is,this is a cottage in Quebec and no stamped drawings are required.Also taking red pine from the lot(no grading required)That said ,I want to do things correctly to avoid future problems.The 4x6 rafters spaced 4' is a concern.Your comment about spreading of the 16' middle bay.When you said plate sizes,did you mean-"joint connection plate"?(the wall plate is 8x8)With regards to lateral stiffness.Plywood or osb on top of the ceiling boards?would plywood or osb on top of the 2x8's(over the insulation)be as effecive?This is still in the planning stage so I am open to change to avoid headaches. Maybe a ridge beam is the better way to go?I belive I can eliminate the collar ties when using a ridge beam.

Regards