Suspended ceiling

Frequently asked questions

Why is the void above a ceiling a fire and air question, not just a gap?

Because it is continuous, and anything continuous through a building is a route. A ceiling void usually runs uninterrupted above the partitions below it, so a fire starting in one room passes over the partition heads into every other room the void reaches — which is why cavity barriers are required in the void on the line of any wall that is meant to be a fire or smoke barrier, and why a partition built only up to the ceiling grid provides neither fire separation nor the acoustic separation people assume it does. In North America there is a second layer: where the void is used to RETURN air to the plant, it is a plenum, and everything in it — cabling, insulation, pipe lagging — has to meet a smoke and flame-spread rating, because the air in that space goes back into the occupied building. That is why plenum-rated cable costs several times the ordinary kind and why substituting it is a code matter rather than a saving.

What actually decides whether a partition stops sound?

Whether it goes past the ceiling, in nearly every case where a room is disappointing. A partition built to the underside of a suspended ceiling leaves the void as an open path over the top, and sound takes it: the flanking route over the partition head bypasses the wall entirely, so a wall specified at a high sound reduction performs like the ceiling tiles and the void. The fixes, in order of effectiveness, are to take the partition up to the structural soffit and seal it, or to fit a barrier in the void on the line of the wall, or — least effective and most commonly attempted — to lay heavy pads over the tiles either side. This is the same flanking principle the sound-rating concept describes, and the ceiling void is where it bites most often, because the partition looks complete from the room.

Why are the hanger and grid spacings not just a matter of holding the weight?

Because the failures they prevent are stiffness and movement rather than collapse. The dead load of a tile grid is small; what the spacing controls is DEFLECTION — a main runner spanning too far between hangers sags visibly against a straight line of light from a window, and the eye is very good at seeing that. Spacing also governs what the ceiling can carry as point loads: a light fitting, a diffuser or a projector mount is not a tile's job, and each needs its own independent hangers rather than sitting in the grid. And in seismic regions the rules are different again, because a ceiling that is merely hung swings: bracing wires and a perimeter detail that allows movement without the grid walking off its wall angle are required, which is why the American installation standard has a separate seismic section rather than a note. A grid installed to a European layout in a seismic zone is a life-safety issue rather than a finishing one.