Loft floor detail

[RWC_CDV]

 Hi everyone:

I'm posting this here and at the Timber Framers Guild, 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 rafters.  The rafters 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 rafters are 40" on center, so the solution I came up with is to run some 2x4 at 24" OC across the rafters 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 timbers with metal protection strips 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 rafters 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 rafters so long as everything is well-fastened.  I.e. the 1 1/8 ply could span the 40" OC rafters 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've made a little diagram to illustrate what I'm thinking,  and included a shot of the rafters from above.

Thanks for your help,
Rob


 

 

[Don P]

Rafters hold up the roof, joists hold up the floor, semantics but you can lose a guy on the job, he'll be getting a tan on the roof while you're sweating down below.

I don't see a problem with that, turn the plywood 90 degrees to the way you have it drawn, sheet goods lay across their bearings rather than along them, you'll sag between sleepers as drawn.

Build stiffer, shorter if possible, then add mass. Mass damping, if you can support the mass  :D

[canopy]

That's a nice looking and solid frame. The floor joists will shrink and expand and thus the drywall between them will need control joints or trim boards to look good. Another option is rather than drywall is lay tongue and groove decking over the floor joists as the finished ceiling. You could apply acoustic panels to the bottom of the decking like you see in movie theatres if sound is a problem. Make the cavity in the floor thick enough to fit substantial sound dampening material inside. Also lay a sound dampening underlayment under the finished floor. Consulting an acoustical engineer would be ideal to assure goals are met.

[RWC_CDV]

Thanks Don and Canopy,
I noticed I'd used rafters throughout when I posted at the tfguild and changed it, of course I managed to post the older version here!

A good couple of points, I was focusing on the direction of the big beams, not the sleepers.  I tried it out with some 3/4" I had around, that was still pretty stiff, I'm sure with the 1 1/8 it'll be fine.

I hadn't thought about expansion and the drywall, will think about how best to deal with that.  A T&G ceiling would be great, but the cost of the drywall is hard to beat- turns out building during a pandemic-fueled supply shortage was a dumb idea and our budget is shot.  I am talking to some acoustic people about how best to deal with that - that's another cost/benefit optimization I'll have to do!

Cheers, Rob

[RWC_CDV]

Interesting thing about this question is that I've gotten a different answer from everyone I've asked!   Thought it was time to take it to a pro, so I asked a structural engineer.  He thought that laying the plywood along the sleepers like in my diagram is the way to go, so the non T&G butt ends can be supported by the joists, like is called out by the sturd-i-floor specs:



 

He also pointed out that since it's only a 40 psf floor I can get away with 3/4" plywood.

I'm also talking to a acoustic engineer, he suggested to hang the drywall ceiling inside the joists with resilient channel.  A lot more work but the best option for sound reduction.

[Don P]

And as I look at it again, I agree with him, that direction is across the real support. The non t&g ends need to land on 2x blocking over joists. At 40" joist spacing that means 80" long plywood, can you move the joist spacing to 32"?

We've been talking about the correct resource for plywood and engineered wood. At apawood.org, they have a pdf on span ratings and applications, E-30.