davethompson413 t1_jeclleo wrote on March 31, 2023 at 12:56 AM

Reply to comment by me-chewbacca in Need to hang something from ceiling and unsure what lumber to use by me-chewbacca

Then don't hang anything more that the drywall that's already there. Engineered trusses are not designed to carry any floor load. They are designed only to carry the roof, and any expected snow load.

If you're considering modifying your trusses, you'll need to have a licensed engineer design the changes.

And they aren't joists. They're the bottom chord of the engineered trusses.

Seriously-- 2x4s aren't even included on span charts. Without a stamped engineer's drawing being followed, they don't carry weight when they're horizontal.

me-chewbacca OP t1_jecn8yk wrote on March 31, 2023 at 1:08 AM

Got it. So let's say you are right and it's not joists it's the bottom chord of the engineered roof trusses. Would that mean you can't put any load on it? I am genuinely asking because I know people walk on top of it in attics. If someone were to hang from the bottom of it (across more than one), wouldn't it be the same?

davethompson413 t1_jedq492 wrote on March 31, 2023 at 7:33 AM

It would be the same, in that a 2x4 bottom chord of an engineered truss is not designed to carry any live load other than the roof load. A person hanging from a 2x4 risks breaking the 2x4, because by themselves, 2x4s are not structural.

Doctor_Frasier_Crane t1_jee6t7c wrote on March 31, 2023 at 11:21 AM

I think he’s safe if he spreads the load across several joist. Plenty of people (me included) have many hundreds of pounds hanging from the garage trusses.

The risk is likely not breaking, but rather slight movement that can cause cracks in the drywall.

davethompson413 t1_jef6uof wrote on March 31, 2023 at 3:55 PM

They're not joists. Joists are structural, 2x4s are not. It really is that simple. When running horizontally, there is no span chart for a 2x4.

Doctor_Frasier_Crane t1_jefd3ox wrote on March 31, 2023 at 4:35 PM

Technically, that’s correct, they’re part of the truss assembly. The bottom chord.

A quick Google search does find plenty of results for span tables that includes 2x4’s though.

davethompson413 t1_jefj786 wrote on March 31, 2023 at 5:15 PM

Span charts for 10 pounds per square foot. Not really walkable. And at 30 psf, the 2x4s disappear from the chart.

Doctor_Frasier_Crane t1_jee6m3t wrote on March 31, 2023 at 11:19 AM

You certainly can apply load, it’s just not intended. The trusses form triangles so the snow load and wind uplift are actually causing tension and compression along the length of that bottom chord of the truss. Either pushing or pulling along the length based on the downward or upward pressure. That’s how they can get away with using a “little” 2x4. It’s not direct downward pressure like you get on a 2x8 or 2x10 floor joist (that is just a straight across board and no diagonals) from a load standing on top of it.

me-chewbacca OP t1_jefncps wrote on March 31, 2023 at 5:42 PM

Got it. Thanks for your thoughts on this.

Based on the comments in this thread and after confirming the type of roof I have, I gave up going ahead with this idea. It might be able to support it ok but I do not want to find out.

By the way, the plan was to hang a Hammock Chair like this which would not be bearing the load 24x7.