This is a standard thread here on GJ.
Can you hang stuff off your trusses?
Sure!
How much?
Well now, THAT is the crux of the matter.
Structures are engineered to make them safe within an envelope of conditions.
Structural damage and even failures can occur when the envelope is exceeded.
Even though wood is graded, it has variability. So does construction methods.
And other forces like wind, snow and earthquakes also demand that we add factors of safety into the calculations.
Plus, for finished spaces, we make structure stiffer, so drywall ceilings don't deflect too much, causing cracking at joints, and nail pops.
So while many trusses and roof systems aren't specifically designed to handle additional loads, there are safety factors build into our structures, that can be used.
2 things concern me when you do this.
One is that you are using up some of the safety factor, and it is there for a reason. Damage and failures usually occur because of a combination of factors, an accident chain. Using up some of the safety margin just makes that combination a little more possible when combined with other causal ingredients.
The second is that you just don't know what capacity you have.
That is the whole point of engineering in the first place, to KNOW what the capability is, and where you will exceed it.
So you trial and error guys, go ahead. You may or may not have a problem. And the problem may just be some drywall ceiling cracks developing. Or if you put a point load in the wrong place, and it is sufficient, you could have a failure.
The point is, you don't know until you do the calculations.
There is one other method you might try.
Do your modification. Then load it to failure. Now you know where the failure point is, and you can avoid it in the future.
BTW, the example of the hanging racks sold for overhead storage.....The fasteners would pull out or the racks fail before the structure would.
Bill
