In some of the build pictures I see a rafter doubled up as in 2 2x6's side by side making a 2x12. (I think those are the dimensions) I think this is only on one side of the cabin. What is the purpose of this? I hope I am explaining it well enough and hope it's not a dumb question.
David.DiPietro's pics show this detail. Not sure how to link to the pic.
Thanks
Strengthwise, stacking lumber one on top of another does not create the deeper dimension, it creates 2 of the shallower dimension, which is a fraction of the strength of the deeper dimension... 2- 2x6's do not make a 2x12. Sometimes you'll stack lumber to create insulation space after the needed strength dimension has been taken care of.
So it is just for additional insulation depth? Doesn't make sense to have on one slope of the roof and not the other if it were just for insulation. I understand 2 2x6's does not equal the strength of a 2x12 but it does equal the size (depth wise) of a 2x12...approximately.
Is this the link?... http://countryplans.com/smf/index.php?topic=6311.0
Only David can really answer the question of "why" .... but I would suspect it made a better, more sturdy, surface to land those siding board-ends on. A different siding plan might not benefit from it.
.... but you guys have me wondering: How much strength do you get from two similar boards, say 2x6's, placed edge-to-edge?
If we think about a simple beam of 2x12 dimensions, placed horizontally, ... uniformly loaded on top ... supported by two end supports: .. and we examine the cross section of that 2x12 somewhere the middle of its length, we will find top fibers in compression and the lower fibers in tension. And there somewhere, perhaps near the center top-to-bottom, we will find a "neutral axis" ... that is doing very little for the strength of the beam ... doing not much in compression or shear.
So it might take very little to glue, or pin, or whatever ... to attach two 2x6's together on edge, and get a fair substitute for a 2x12! (... or for a 1-1/2 x 10-1/2!)
That's a glulam, or in older work a keyed beam. You can use split rings between plies in timbers. Using mechanical means you can approach a solid beam's strength, using appropriate glues and controlled conditions in a plant they can exceed the strength of a solid sawn beam because they have distributed the defects around more randomly.
If you're familiar with the beam diagrams for a simple beam it'll show very clearly in the shear diagram where to do your best pinning of things together.
Thank JRR, that's the thread. A lot of good pictures. The pic with the rafters hanging down and ready to go up shows one side is doubled and the other is not. In some of the other pictures none are doubled but the plywood that holds them together look like is there and ready for the 2nd 2x.
I am wanting something similar to the 20 x 30, 1 and 1/2 story (just a loft really) but I would like a back entrance so it may have to be a little wider than 20'.