Dropped ceiling
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In construction and architecture, a dropped ceiling, also referred to as a drop or suspended ceiling, is used as a secondary ceiling formed to conceal piping, wiring, or ductwork, into an area called the plenum. It 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 - and each cell is filled with lightweight "acoustic ceiling tiles" or "panels" which simply drop into the grid. Tiles can be selected with a variety of materials, including wood, metal, or mineral fibres, and can come in almost any color. Fluorescent light fixtures, air supply diffusers, and return air grilles of the same dimension, and incandescent lights, various electrical devices, and sprinkler heads are then installed into the grid as desired.
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 sound attenuation. This is 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.
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[edit] 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.
[edit] Safety Issues
The panels are usually made of mineral fibers, and the dust is toxic to breathe.
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. While networking cable is most the common form of plenum cable, coaxial cable and telephone cable also needs to be plenum-rated for safety.
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.
[edit] Desirability
Modern dropped ceilings are gaining increased popularity, mainly in commercial and industrial settings where obscuring fittings above the ceiling is deemed necessary, with ease of use for access at any time. Of course, these 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.
[edit] Advantages
[edit] 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. Another advantage of a dropped ceiling is that the easily-removed ceiling panels offer instant access to the plenum, greatly simplifying repairs or alterations.
[edit] Ease of modification
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.
[edit] 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 to the original ceiling, avoiding the need to carry out any repairs. When evaluating a home for rent or purchase, it is recommended to inspect the space above for loose wiring, insect/rodent infestation and structural damage. 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. 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.
