maximum ceiling joist to beam span without joist hangers

Question

I'm building a tray ceiling and the assembly is easy to visualize.

In a 14' x 14' dining room, I'm coming in 18.5" to the inside of the beam on 2 sides with a 2x10, and then similarly cross-wise between them the other way, also 18.5" in. Those in-between beams are hung with a 2x10 joist hangar.

So there is a 18.5" "band" all around and then it goes up with new joists 12" above the base level.

My question is regarding the band area. The finger joists are only going to be 17" of span, with the other end sitting on the outer wall plate. Are 2x4 joist hangars necessary for such a short span? And bonus, do they need to be 16" O.C. or will 24" O.C. suffice? Thanks

Answer 1

There are two considerations, 1) vertical loads (including uplift), and 2) lateral loads.

1) 2x4’s at 24” o.c. are plenty strong enough to support a roof load of say 30 lbs. per square foot LIVE LOAD (snow load) plus a DEAD LOAD (framing, insulation, etc.) of 10-12 lbs psf for an 18.5” span. However, the connections at each end will be in the 140lbs. Range. So, 2-16d are required (each 16d will support about 80 lbs. each for toe-nailing,) but make sure you don’t split the joists.

If the 2x4’s extend over an exterior top plate, you’ll need to fasten them down by toe-nailing them to be he top plate, (if you don’t want to use clips).

2) Lateral loading (earthquake and high wind resistance) is different. Here you want adequate transfer from roof sheathing to supports. Unless you live on an extreme earthquake zone or high wind area, 2x4 roof supports at 24” o.c. Will be fine.

Answer 2

Joist hangers do more than take vertical up and down loads. Installed correctly and provided with sufficient twist nails, they reduce what is known as the effective length of the joist you’re hanging off, stopping lateral torsional buckling

Answer 3

I think I am reading this differently than the other answers. Contrary to the question, I don't find it all that easy to visualize based on the description.

The tray ceiling is basically just an 18" soffit

I believe you're running two 14' joists 18" from the walls, then running two 11' joists between those, 18" in from the other walls. These are the ones carrying all the load of the soffits / tray ceiling - the drywall and the some of the finger joists, probably no insulation, possibly lights / electrical - and the ones that need to be sized and installed carefully.

The finger joists are the shorties from the wall to these 2x10's. They are not structural, they aren't carrying any load, they aren't supporting a floor or a roof load. They stiffen the box and they provide an attachment for the drywall but that's about all they are doing.

If that's all correct, there's no need for joist hangers, I'd face nail them to studs, then end nail them to the 2x10 band. Assuming the wall's studs are spaced 16" OC, I'd either put one every other stud or even every third stud.

(It is possible I am misreading the question, in which case I'll delete this answer.)