Dropped ceiling
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A dropped ceiling is a secondary ceiling, hung below the main (structural) ceiling. They may also be referred to as a drop ceiling, false ceiling, or suspended ceiling, and are a staple of modern construction and architecture. The area above the dropped ceiling is called the plenum space, as it is sometimes used for HVAC air return. The plenum space is also very commonly used to conceal piping, wiring, and/or ductwork.
A typical dropped ceiling consists of a grid-work of metal channels in the shape of an upside-down "T", suspended on wires from the overhead structure. These channels snap together in a regularly spaced pattern – typically a 2x2 or 2x4 foot grid in the US, or 600 x 600 mm grid in Europe. Each cell is filled with lightweight "tiles" or "panels" which simply drop into the grid. Tiles can be selected with a variety of materials, including wood, metal, plastic, or mineral fibres, and can come in almost any color. Light fixtures, HVAC air grilles, and other fixtures are available which can fit the same space as a tile for easy installation. Most tile material is easily cut to allow fixtures in other shapes, such as incandescent lights, speakers, and fire sprinkler heads.
The suspended ceiling was originally developed to conceal the underside of the floor above and to offer acoustic balance and control in a room. The acoustic performance of suspended ceilings has improved dramatically over the years, with enhanced sound absorption and attenuation. This is sometimes achieved by adding insulation known as Sound Attenuation Batts (SABs), more commonly referred to as "sound batts", above the panels to help deaden sounds and keep adjacent rooms quieter.
Concealed grid
An older, less used type of dropped ceiling is the concealed grid system. This type of dropped ceiling employs a method of interlocking panels into each other and the grid, thus making it difficult to remove panels to gain access above the ceiling without damaging the installation or the panels. Normally, these type of ceilings will have a "key panel" (usually in the corner) which can be removed, allowing for the other panels to be slid out of the grid one by one, until eventually removing the desired panel. This type of ceiling is more commonly found in older installations or installations where access to above the ceiling is generally considered unnecessary.
This system has some major disadvantages over the more common "drop panel" system, most notably the difficulty in removing and reattaching panels from the grid, which in some cases can cause irreparable damage to the panels removed. Finding replacement panels for this type of dropped ceiling is becoming increasingly more difficult as demand for them is slowing, as is production of the parts.
Safety Issues
The space above the dropped ceiling is often used as a plenum air return for ventilation systems, requiring only enclosed ducts that deliver fresh air into the room below. Return air enters the ceiling space through open grilles across the ceiling.
In the event that the dropped ceiling is used as a plenum, low-voltage cables and wiring not installed inside conduit need to use a special low-smoke and low-toxicity wire insulation which will tend to char and stop burning on its own. This helps to protect building occupants so that they are not poisoned with toxic chemicals sucked through the ventilation system in the event of a fire, and helps to prevent fires from spreading inside the hidden plenum space. This special low-smoke cable is typically referred to as plenum cable or Low Smoke Zero Halogen (LSZH or LS0H) cable. While networking cable is most the common form of plenum cable, coaxial cable and telephone cable also needs to be plenum-rated for safety.
In earthquake prone areas (e.g. California) diagonal wire stays are often required by building codes in order to ensure the ceiling grid won't sway laterally during an earthquake, which can lead to partial or total collapse of the ceiling grid on the occupants below during a severe tremor. Compression posts may also added to keep the ceiling from bouncing vertically during an earthquake.
Lighting fixtures and other devices installed in a dropped ceiling are required to be firmly secured to the dropped ceiling framework. In the event of a fire above a dropped ceiling it is often necessary for firemen to have to pull down the ceiling in a hurry to quickly gain access to the conflagration. Loose fixtures merely resting in the framework by force of gravity can become unseated and swing down on their armorflex power cables to hit the firemen below. Binding the fixtures to the framework assures that if the framework must be pulled down the fixture will come down with it and not become a pendulous swinging hazard to the firemen.
Desirability
In commercial and industrial settings, this allows the obscuring of fittings above the ceiling and access at any time. Drop ceilings may also be used in settings where obscuring fittings or the actual ceiling itself is desirable when wanting to hide problems, such as structural damage. Older, white panels combined with fluorescent lighting are common in many offices, and considered utilitarian, if not unpleasant by some people. More expensive, modern dropped ceiling styles are gaining popularity.
Advantages
Fire Safety
If fire safety is a factor, ceiling tiles made from mineral fibres, or fire-rated wood panels can be used within the construction to meet acceptable standards/ratings. These same tiles can provide the needed additional resistance to meet the "time rating" required for various fire code, city ordinance, commercial, or other similar building construction regulations. Fire ratings for ceiling panels vary based on the materials used and preparation of each panel.
Ease of modification
Another advantage of a dropped ceiling is that the easily-removed ceiling panels offer instant access to the plenum, greatly simplifying repairs or alterations.
Wiring and piping installed behind traditional plaster or wallboard ceilings is extremely difficult to modify once the finished ceiling is in place. Wires must either be fished through hollow spaces in the walls behind the finished ceiling, or the ceiling must be demolished in order for wiring or piping changes to be made.
In contrast, the tiles and other parts of a drop ceiling are easily removed to allow access to the area above the grid to do any necessary wiring or plumbing modifications. In the event of remodeling, nearly all components the grid can be dismantled and reassembled somewhere else.
In business, the drop ceiling is often used in conjunction with hollow steel studs to construct small office spaces out of a much larger cavernous space. Wiring and other services are run through the open ceiling, down through the hollow stud walls, and to outlets in the work areas. If business needs change, the office spaces are easily dismantled and the overall cavernous space reconfigured with a different floorplan.
Disadvantages
One disadvantage with this ceiling system is reduced headroom. Clearance is required between the grid and any pipes or ductwork above to install the ceiling tiles and light fixtures. In general, a minimum clearance of four to eight inches is often needed between the lowest obstruction and the level of the ceiling grid.
Dropped ceilings are frequently used by slumlords to hide structural and cosmetic damage, loose wiring, insect/rodent infestation, avoiding the need to carry out any repairs or maintenance. Additionally, sometimes walls do not extend past the grid to the actual ceiling. This can present a security risk when used in offices or areas where unauthorized entry may be an issue.
As a renovation tool, dropped ceilings are a quick and inexpensive way to repair a ceiling or reduce HVAC costs, however they tend to show their age quickly (and are sometimes discolored by excessive smoking), are damaged easily and are somewhat lacking aesthetically (which is to say, they have a cold and industrial feel – not something engendering a "comforting", "home-y", or "bucolic" feel). This can sometimes be avoided by using panels made of alternative materials, such as plastic or metal. In older buildings that have seen multiple renovations over time, it is not uncommon for a dropped ceiling to have been installed in one renovation and then subsequently removed in another, its installation having been an inexpensive fix to prolong the time between major renovations.