We've decided to go with a metal roof for these reasons - cheap, doesn't leak, and long-lasting [the roof comes with a 40 year warranty]. It's also the choice most compatible with rainwater harvesting and a cistern, which will be placed on the NW side of the cabin.
Everyone in Tennessee, from builders to the actual metal roof vendors themselves, had told me I could use purlins to attach the metal to. These are basically boards [often 1x] laid horizontally across the rafters every 2', and they're what the metal sheets are screwed to. It sounded easy, cheap, and practical - I could actually install the sheets from the comfort of the loft because I could stand and reach through the rafters between the purlins.
We'd actually ordered all the 1x material for the purlins and had it delivered. But literally the day before I put the purlins in, my wife did some research on how metal roofs should be installed, and told me to STOP! Every single authority on the internet, sites like This Old House, contractor sites, metal roof sites - all recommended going with a conventional roof approach - which means sheathing, tarpaper [30lb roofing felt], then metal. The reason? Sheathing makes the roof stable and strong, it won't dent if something falls on it or someone walks on it, it's a must if you plan on insulating the roof, and the tarpaper stops condensation. Apparently a metal roof drips underneath from condensation when only purlins are used. My next door neighbor has installed a metal roof over his barn, with only purlins, and he says it drips all the time from condensation.
I called back the metal roof people I'd ordered my sheets from, and asked if you could use sheathing and roofing felt. They said that you could, that that was the best way to do it . . . and I wondered, why didn't you tell me the best way in the first place? Apparently for the average owner-builder, what's cheapest and easiest is what they do, especially in a land of no codes. I'm happy there're no codes, but . . . things should be done the right way, especially by the 'experts'. They shouldn't be giving out half-baked advice.
So now that I know my roof is going to be much heavier with all that sheathing, I first beef up the frame. I do a little preliminary wall framing, essentially a picture frame with 2x6's to create a lower and upper story all around the building - I don't want to be putting in 13' studs.
So I put in 2x6's up against the posts on the lower story, to support a top plate [to be doubled later] of 2x6's at 7.5' in height - where the bottom of the loft joists is. And all the 1x4's I now have no use for, I attach to the building as a temporary diagonal bracing until the wall sheathing goes in. Here's a pic:
The first step in installing the 7/16ths sheathing is to straighten the rafters across the middle with furring strips. Once this is done I nail up bumpers to the tails of the rafters, so that when I set the sheathing up there, it won't slide off the roof. I put in sheathing at the outside first, then move towards the peak, staggering the joints on different rafters.
On the first row of sheathing, I measure from the peak down to get the exact location for the sheets. I clamp the sheet and bump it around with a mallet a little to get it right where it needs to be. I then tighten the clamps and hammer in ringshank nails, which have a great permanent hold, but can be somewhat of a pain to get in, as they want to bend.
Here's me working on the first sheets:Here's a shot of the barn (a.k.a. Tent City), where we're living as we build:
Here one side of the roof is done: