Separating Floor and Ceiling System

A separating floor is everything between the walking surface of the upper room and the visible ceiling of the room below, taken together: finish, resilient layer, deck, structure, void and lining. It is treated as a single element because a change anywhere in that stack alters what arrives underneath, and because no layer in it can be judged on its own.

Dissimilar problems occupy the same construction. Airborne sound - speech, music, television - drives the whole build-up and is worked on by mass, by the void and by the way the ceiling hangs. Impact energy from footsteps, dropped objects and moved furniture is struck directly into the structure, bypasses the air on the way in, and is worked on by resilience and separation near the top of the stack.

The nearest sibling is the separating wall system, and the boundary is settled by asking whether anybody stands on the element. Nobody walks on a wall, so a wall has no equivalent of the impact route and no equivalent of the resilient layer that answers it. This is also why a floor can be altered for airborne sound and left untouched for footfall, and why occupants sometimes report no change at all after work that plainly altered one route.

The structural deck itself - joists, planks, composite decking - belongs with the floor structure entries. What this entry owns is everything settled by the transmission path: whether the walking layer is separated from the structure, whether the ceiling hangs free of it, whether the perimeter is bridged, and where services cross the assembly.

A resilient layer only does anything while it remains the sole route downward. If the floating screed touches the wall at the perimeter, if bedding runs down the edge, or if a skirting is fixed through the floating layer into the structure, the deck is no longer floating: a stiff connection now sits alongside the soft one, and the stiff route wins.

Because the airborne and impact routes enter the assembly differently, the parts that address them are not interchangeable. A soft walking layer works where the foot lands, so it can change what a neighbour hears from footsteps without altering what arrives from a television through the stack, whereas the resilient layer and the way the ceiling is hung both sit in the two routes at once. Identifying what is actually being heard, and from where, is what tells a designer which part of the stack is in question.

The ceiling lining and the way it is hung are a single decision rather than a pair. Board screwed tight to the underside of the joists is coupled to the structure, so its mass is driven directly. The same board carried on resilient bars, isolation hangers or an independent frame is driven mainly through the void. One screw that misses the bar and catches the joist reinstates the connection the bar exists to prevent.

The void, whatever is placed in it and the services routed through it also act on one another. Absorption damps air between masses that are already separated; packed tight against both, it stops being absorption and starts being a connection. Recessed fittings, ducts and pipework that pierce the lining plane remove mass locally, and where they are fixed back to the structure above they hand the ceiling a direct link to it.

Finally, the floor depends on the walls it lands on. Where a screed continues unbroken into an adjoining room, or where a wall lining runs past the floor line without interruption, sound arrives having travelled around the assembly rather than through it, and the stack drawn on the section is not the assembly the occupant experiences.