We have a existing suspended ceiling that we need to seal off from the breezy roof area above it. We thought we could attach 18mm plywood to this by screwing every 200mm into the metal suspension rails and cleaning the painted metal surface and applying construction adhesive for extra fixing.
I would not do that. Suspended ceiling is not designed to support that type of weight. Apart from being unsafe, it is most likely also illegal: check your local building code, or call the manufacturers of the ceiling, to find out.
If you need to seal off that area, then just take out the suspended ceiling, put in proper joists, and attach your plywood to that.
The rods are rated at 40kg load each so we put in about 30 which means it can take an extra 1200kg.
Live load, or dead load? What deflection?
The weight of the ply will be about 550kg
You are not calculating dead load correctly: The structure itself is part of the dead load; the rods, framing, screws, glue, paint, sealant, etc. Everything that goes to make up the structure is dead load. Also included is all the insulation that you need to put up there, plus anything else that rests on it: conduit, electrical, HVAC, lights, etc. All of that is dead load. To me it looks like you have about 18 sheets of plywood up there, meaning roughly 53 square meters. Assuming an even coverage of 20cm of mineral wool as the insulation, that's 514 kg of insulation up there (48 kg/m3).
I think your safety factor just flew out the window... (plus, I think you are underestimating the weight of the plywood itself by about 10%...)
and also considering that the outer perimeter that meets the wall will have a supporting baton attached to the wall for extra support, so the hangers wont be taking the full weight.
Actually, that would only be supporting a very small percentage of the entire weight. The center of the ceiling would be supported entirely by those "rods".
We have not been recommended to do this due to the ceiling not designed with that extra weight
I agree with the people who are "not recommending" this as a good approach!
:)
But what you need here is not my opinion: Hire a qualified structural engineer to come in, look at what you have, look at what you plan to do, and give you the answer in writing. If he says it is OK, then go for it. If not then absolutely do not even think about it any more.
You are talking about hanging several tons of mass directly above your head: Are you sure you want to do that based just on a guess, and against the advice of people telling you not to? If it were my head under that load, I would spend a few dollars and check with a qualified expert...
- Stuart -
