Yep, "bottom chord of an attic truss" is what I am referring to. Your
link is precisely what the contractor is using. After reading these
posts, I obtained the diagrams from the truss company, and it appears
they can bear 60 psf (40 live load, the rest I'm assuming dead load).
That's for the bottom chord as a whole. For the room area (10-12) live
load is 40 psf. Wouldn't meet code in most places if it wasn't.
Again, attic trusses have been in use for many years. I gotta wonder
how much experience you have if you've never seen them.
If the 2x8 is the bottom chord of an engineered "attic truss", it's
probably sized for the intended loads. The other chords of the truss help
carry the load, the same way a 2x4 is typically used to build a standard
truss. The 2x8 would essentially be sized to carry the load between the
truss chords (maybe 10 feet for a typical attic truss?). Assuming the truss
is being built by a truss company, they should be able to supply you (or
your contractor) with a copy of the engineering diagrams and the loads it
is designed to carry. In fact, that's one of the required items for
building permits around here, so it may already be part of the plan
On the other hand, if you're talking individual 2x8's for spanning 24'
you're going to have big problems. Even when sized for a ceiling joist, a
2x8 at 16" OC is only good for about 17 feet, and that's for a very light
load. You'll never get by spanning 24 feet with a 2x8.
Just for reference, I spanned 24' in my garage using 2x12's at 16" OC. Even
that is way undersized for a floor joist. It works OK for my light attic
storage (empty boxes, Christmas decorations, suitcases, etc.), but it is
far too bouncy to be considered a usable floor joist.
14" I-joists would probably work well if you have the vertical space to
"IF" you have a beam running down the middle of the garage, your span would
only be 12', in which case 2x8 joists would probably be perfect for floor
joists. You could even splice shorter joists over the beam if needed.
However, if you DO NOT have a beam, any splice in the joist is going to be
an immediate failure point (unless it's part of an engineered truss).
Yep - bottom chord of an engineered attic truss from a truss company.
After reading your post, got the truss company to send over the
engineering diagrams - turns out it can support 60 psf (40 ll +
remaining dead load I'm assuming). Wasn't required for our permit,
but I'm sure the building inspector will be happy to see it in
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