I'm posting this here and at the Forestry Forum, I see that there's some overlap in members - my apologies to those who see it twice.
I'm building a timberframe house in a little off-the-road fishing village in Alaska. Our timber framer recently passed away, so I've lost a source of advice.
We're trying to figure out the best way to do the floor of the loft, we would rather not go with the "car decking" approach. We'd like to have drywall in the ceiling of the level below, it is the kitchen area and we would like to have some task lighting mounted in the drywalled ceiling in between the joists. The joists are 6x8 doug fir, and my partner does not want to bore through the timbers to run wire and mount fixtures directly to them.
I've been searching all over for another way to do the floor, but haven't found much, I have several timber framing books that aren't helpful there either, so I've come up with my own idea. The joists are 40" on center, so the solution I came up with is to run some 2x4 at 24" OC across the joists with the long side down, and then lay 1 1/8" plywood over that for the loft floor. That gives me a 1 1/2" gap in which to run wire and mount low profile electrical boxes (plus a space for underfloor heat in the upstairs bath). The wire can be protected by running it through routed channels with metal protection strips in the timbers the standard way where possible, and with a little bit of conduit elsewhere.
For the drywall ceiling I figure I can do a horizontal analog of the spacers for drywall between the SIP walls and posts- lay 1/2" plywood spacers on the tops of the joists to lay the 2x4 on to. Then I can screw the drywall to the 2x4s to prevent sag. Doing it that way we can also pre-paint the drywall which will probably be less painful and fidgity than painting the ceiling once installed.
I'm also looking into acoustic matting to lay on top to help deaden sound transmission between floors. There is another timber frame in town with 2x4 car decking floors and it's, ahem, a bit acoustically "bright"- it seems like the decking works basically like a drum head and really transmits sound. I don't think we'll ever be able to eliminate that, but might be able to diminish it a bit.
My reading on the specs for 1 1/8" sturd-i-floor plywood is that it will be just fine with the 24" OC 2x4 and larger 40" OC spans below them on the joists so long as everything is well-fastened. I.e. the 1 1/8 ply could span the 40" OC joists no problem, so using 2x4 "spacers" shouldn't compromise that. But it is an idea I haven't seen elsewhere so I'm wary of unintended consequences that I may have missed. I'd appreciate some opinions on the idea and if there's anything I might have missed.
I'm terrible at drawing so made a little mock up diagram to illustrate what I'm thinking, and am also attaching a picture of the frame showing the loft arrangement.
Answering my own question in case someone has the same problem some day.
I've had a lot of suggestions from people I took this to, online and off, and everyone had a different answer!
I took it to a structural engineer who thought my design would be fine structurally, and pointed out that since it's only a 40psf floor I can get away with 3/4" T&G.
Turns out I have a retired acoustic engineer among my in-laws, he suggests that I should hang the drywall between the joists on resilient channel for maximum noise reduction. That's a lot more work and would require sealing the drywall with acoustic sealant along the edges. That does have the advantage of being resilient to the frame shrinking a bit over time, and gives a bit more room for wiring.