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Consider a house with a gable roof that looks like this:

    /\
   /--\
  /    \
 /      \
/        \
====[]====
|        |
|        |
|        |
+--------+

(the [] is a load bearing beam coming out of your screen)

Roof rafters need ties at the bottom to form a truss. In many cases, ceiling joists act as the rafter tie. For long spans, the ceiling joists might overlap on a wall, and will be nailed together, still forming a truss.

But if the ceiling joists are hung from a beam using joist hangers, is that still considered a truss? Or do rafter ties needs to be nailed in above the ceiling?

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  • It probably depends on the joist hangers and how they are attached to the beam. Some of them hang from the top of the beam and have just a few nails down the side of it. A strong force pulling sideways away from the beam would easily pull those nails out. Even hangers with lots of nails into the side of the beam don't seem to me like they could resist strong forces in the "pull the nails out" direction.
    – jay613
    Aug 2, 2022 at 12:43
  • Perhaps a joist hanger could be designed that would fulfill this purpose. It would be a solid ring of metal that wraps all the way around the beam and does not rely on nails to resist the forces you are asking about.
    – jay613
    Aug 2, 2022 at 12:44

2 Answers 2

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What you want is something like a tension tie. They come in various flavors. Here's a picture of one being used from the Simpson catalogue.

enter image description here

Note that standard joist hangars are not rated for pull-out strength/resistance, along the axis of the joist, as this table from the Simpson catalogue shows:

enter image description here

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The question is: "Can the joists prevent the lower ends of the rafters from spreading?"

If they can then the rafters + joists form a truss, if not, then they don't.

Specifically, it would depend if the joist halves are fastened together (with nails, bolts, nail-plates, or some other means), and if the ends of the joists are fastened to the to rafters (as opposed to just sitting next to them).

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  • To add an even more general description: it's only a truss if all of the constituent components can transfer significant compressive and tensile loads, as well as torque, to each other. A couple of nails won't do, and 'hanging' implies no torque transfer (and likely also no compressive load transfer).
    – MiG
    Aug 2, 2022 at 12:59

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