Here’s what I have. “Room in attic” trusses placed 24″ oc. with 3/4 t&g OSB as subfloor. Trusses span about 32 feet. Living area (room in attic) is 16′ in center of truss (8′ on either side of “room”) engineered as “non-living area/no load”. “Room” will essentially be like an efficiency apartment with small kitch, bath (shower stall, no tub), etc. Plans are to install a free standing fireplace weighing right around 300 lbs. Trusses engineered for 40 psf live load. Finished floor will be carpet with some light weight resilient flooring like “Allure” in kitchen and bath. What I’m wondering about is whether the 3/4 OSB will provide a sturdy floor or should I look to add say a layer of 1/2 OSB on top of the 3/4 OSB. Thanks for any feedback.
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This is a can of worms, but I'll throw out some thoughts and info for you to chew on
First, having it engineered as non-living space, tells me that the bottom chord is more likely intended to support a 15-20# live load is all. At the most.
The 40# is for live load on the total truss - in other words, the wind and snow load on top of the roof. That is different from the load that bottom chord can carry.
So to place an apartment in there without proper engineering is a scary proposition! Could be dangerous and/or illegal/unethical, IMO.
So the sufficiency of the subfloor is really a minor matter. It is at least as good as the rest of the design. OSB would add nothing in terms of strength, IMO. Plywood might- or Advantec, but they add more weight to an already cchallenged system
I took "room in an attic" to
I took "room in an attic" to mean that these were the type of trusses designed to have the center area occupied.
And I took
(8' on either side of "room") engineered as "non-living area/no load".
to be an unfortunate placement of the closing paren -- the 8' sections are the "no load" area.
My post probably wasn't too clear, obviously. The trusses were engineered as "room-in-attic" trusses. Center 16 feet of bottom chord supports live load of 40 psf and an additional dead load of 10 psf. Bottom chords are LVL's (11 7/8"). Dan H has it right . . . 8' on either side of the 16' "room" are designed to support only 20 psf. Roof live load is 45 psf. Anyway, I was just wondering if the 3/4" t&g osb supported 24" oc provides good support or would additional plywood or osb be a better way to go. Thanks
My thumb suck is that 3/4"
My thumb suck is that 3/4" OSB at 24" will be a little bouncy but would work.
In that case, i would add 1/2" ply glued and screwed, tho minimal codes might allow what you have now
A 300 lb fireplace on a 4 ft by 2ft footprint would pretty much speak for that 40 psf live load rating. I hope you're planning to situate it perpendicular to the trusses. I'd add 1/2" extra subfloor.
I've built a couple of buildings with attic trusses, and we used 3/4 T&G for the floor. I thought it worked fine. You had to work pretty hard to find the trusses by bouncing up and down on the subfloor. You couldn't tell just walking around.
The 300# fireplace bothers me. It depends on how it's oriented to the trusses, but it could cause problems if the load isn't spread over several of them.
You should really get away
from thinking OSB is the answer. It IS NOT structural. Use plywood if you use anything in the 1/2" dimension. Stagger the joints from the first layer, glue, screw, and know it's right as opposed to adding more dead weight which won't hold up to the test of time. Advantec is structural and has been engineered as a viable flooring material. Just because it looks like OSB doesn't mean it has the same characteristics.