A suspended ceiling is one which is not directly fixed to the floor or roof structure above it but as its name implies is suspended by the means of metal rods and channels. This type of system is most commonly used in commercial/industrial buildings and is seldomly found in residential construction. The 3 main suspended ceilings that will be discussed will be the Jointless or flush finish type, the modular panel type, and finally the strip panel type.
The first system being the Jointless system is simply the suspended ceiling frame (usually manufactured by RONDO) with Plasterboard or Gyprock screwed into the furring channels which are connected and supported by top-cross rails. These systems can be altered depending on how many layers of sheeting is provided. The more layers that are placed, the more fire resistant the system gets.
The second system of modular panel has the suspension clips supporting main runners that are spaced at 1200mm centres, with cross runners at the same level at 600mm or 1200mm centres. This then achieves a grid that the modular tiles can be layed into. The most common materials for this are gypsum board lay in panels (1200mm x 600mm), mineral fibre boards (vary in size) and plaster acoustic tiles which are usually 600mm x 600mm.
The final type, the strip panel system consists of narrow strips of preformed metal fixed to horizontal carrier rails. These panels are then clipped onto the carrier rails and finished with insulation over the top. An example of this system is the drawing rooms on the 4th floor at Leederville TAFE.
Properties of Material
Suspended ceiling systems weigh less than conventionally residential ceilings as the ceiling joists would be much heavier than extruded sections of steel and aluminium. A normal Rondo suspension system without ceiling covering would weigh between 0.56kg/m2 and 1.50kg/m2. This all depended on whether main and secondary runners are used or if only a basic furring channel is hung off concrete slabs or other structural members.
The screw up or flush system has variable weights depending on how many sheets of Plasterboard/Gyprock material is used. The total mass for 10mm thick Plasterboard including the suspension system is 9.5kg/m2 while if the sheeting is 13mm thick the mass increases to 11.5kg/m2. Boral which has designed a 2 hour rated system (as well as 2 layers of 16mm plus metal lathing sprayed with vermiculite) for flush finish system will have a mass of 34.2kg/m2. This system is only to be used where the ceiling is suspended by furring channels which are hung off roofing purlins to provide greater fire resistance where it is needed.
Modular panel systems have roughly the same weight as the flush system for the suspension frame, but the panels have slightly less weight. Boral Articote, Paintcote and Vinylcote infill panels in the 13mm thickness have a mass of 9.5kg/m2 while the 10mm variety has a mass of 7.5kg/m2.
The strip panel system is the lightest of all with the facing material being made from lightweight aluminium and powdercoated. Most of these types of systems average 2-3kg/m2 with the difference coming from the suspension frame as well as minor variations in the strip panel design.
Although the weights of these
ceilings might look lopsided the general rule is the greater the mass the
better the fire and sound ratings.
The properties that the ceilings can transmit sound is measured and recorded as an STC (Sound transmission rating). This rating varies from 25 through to 50 with :-
25 - Normal speech can
30 - Loud speech can be heard easily
35 - Loud speech can be heard, but not understood
42 - Loud speech can only be heard as a murmur
45 - Must strain to hear loud speech
48 - Only some loud speech can be barely heard
50 - Loud speech cannot be heard
The flush mounted system has achieved STC’s of 38 without insulation and 45 for ceilings that are insulated. Both these readings are for 13mm plasterboard supported by a Rondo suspended ceiling system.
The modular panel style of suspended ceiling generally has STC’s of around from 35 to 37 for most panels depending on their final finish. The panels have slightly less sound insulation ratings as the gaps in the panels as well as at the light fittings and ducts will cause the sound to transmit easier.
Finally no figures in the form
of STC’s were available for the strip panel system but it must be noted
that this system of finishing to the suspended ceiling would have severely
lower figures than the other two systems as the slots in between the strips
are much bigger and therefore will transmit more sound up into the ceiling
Another factor to look at in the properties of the three suspension systems is the sag of the actual ceiling system. The strip panel system will not be likely to sag as much as the other two systems as it has a very light weight and they are supported at regular intervals. The flush finish system if screwed at enough support points, but some consideration would have to be taken when the weight of the ceiling becomes too great for the suspension frame to support and therefore sags. The modular panel system is the system that is most likely to sag depending on the infill panel material. The best material to use for infill panel would be one that has some metal in it to give it strength. 16mm mineral fibre board sags the most for the panel system while the 10mm and 13mm plasterboard infill panels tend to sag slightly less sagging about 2-3mm four days after installation while the heavier mineral fibre board achieves a reading of 6-7mm after the same period.
All in all each of the 3 systems have different properties depending on factors such as weight which would therefore increase the STC rating but would consequently increase the sag at the same time.
Performance Standards Required
When dealing with suspended ceiling systems their are certain standards that must apply governing the manufacture and installation of the suspended ceiling frame or grid as well as the actual ceiling facing material.
The main Australian Standard that must be dealt with whilst dealing with suspended ceilings is AS 2785 - 1985 : Suspended ceiling design and installation. This Australian Standard deals with things such as the strength of the whole system, fire resistance, the Acoustic properties of the system, as well as other factors like ventilation, the quality of workmanship and many other things.
ASTM C636-91 standard specification for metal suspension systems for acoustical tile and lay in panels deals specifically with the design and manufacture of tiles and lay in panels for the modular systems.
Other Standards that relate to suspended ceiling systems are AS 2588 - 1983 relating to plasterboard products and AS 1538 - 1985 cold formed steel structures code which deals with how the suspended ceiling frame is manufactured.
Compatibility with other Materials
A suspended ceiling system is really compatible with any sort of other material provided that the other material is structurally supported to withhold the weight of the suspended ceiling. The suspension clip can be directly nailed to timber that will hold the main runners or another clip can be nailed when using furring channels.
The clips can also be fixed to roofing purlins and concrete suspended slabs. The first of two ways to fix to purlins is to fold over a metal tab under the purlin to hold the clip in place that is clipped over the purlin.(See diagram below). If it cannot be fixed this way, a rivet gun must be used to rivet the clips into the desired position.
The system is compatible to be fixed to suspended slabs through the use of a bolt/nut that is drilled through the concrete. A suspension rod angle bracket is held in pace by the nut from which the suspension rod can hang.
The facing materials that are used for suspended ceilings are also compatible as the plaster, wood, metal and mineral fibre based ceiling sheets/panels do not have any negative reactions with the ceiling frame.
A suspended ceiling can have much better aesthetics than an ordinary ceiling that is finished off with Gyprock/plasterboard with cornice. The variety of different ceiling infill panels means that there is most probably one that will suit its purpose.
The ability to change the facing colour and style with suspended ceilings means that it is very popular with commercial properties as new shop/unit tennants can change the panels to suit their own particular interests. No cornice is needed with the suspended ceilings to cover up the joint. Instead a wall angle which either has a shadowline profile (see below), or just the normal `L’ angle.
The actual metal used in the suspended ceiling frame is either galvanized metal or extruded aluminium, while the sheets are either of a flush, flat surface or in the case of infill panels can be perforated or powdercoated metal or like the plasterboard sheets have a smooth finish.
The strip panel system is also quite good with the panels not allowing anything to be seen behind them as the insulation provided a good backing. This system much like the modular panel system can allow itself to be changed in a relatively short period of time.
A good suspended ceiling system should meet the purpose that it is built for, in the building. The main problem with the suspended ceiling is how accessible it is. This is of paramount importance to the tradespeople and specialised trades(eg. air-conditioner installations, Duct installers etc.)
Obviously the modular panel system of ceiling is the quickest and easiest to demount and access. If desired only some or all of the panels have to be removed to get to the ceiling space. This is good as the trades which have services in these ducts do not have to worry about future placing of manholes etc.
The flush system probably has the worst in terms of access for maintenance. When designing the system it must be worked out were the manhole(s) will be placed. This all depends on were ducts etc. will be positioned in the ceiling. If something is required to be done in other places than where these manholes are the actual ceiling would have to be removed to get to the problem which could be a costly process. This reason is why many people are turned off using this system.
Finally the strip panel system is like the modular panel system but when removing the strips, one spanning all of the room would have to be removed taking more time to remove it. For this reason ducts and other services should be laid parallel with the ceiling strips.
Another good thing about the suspended system is that the height of the ceiling can slightly be altered by using shorter suspension clips or loosening or tightening the nuts on the bolts to either lower or rise the ceiling to the desired level. The only re-fixing would be for the wall angles.
As stated earlier the fire resistance of a ceiling can be increased by adding more layers of ceiling material. By doing this the sound insulation properties are also increased but the weight factor then must be taken into consideration.
The flush finish system can be the most adaptable in getting a 2 hour (120/120/120) fire rating. To do this 2 layers of 16mm Firestop/Fyrchek plaster based product needs to be fixed to furring channels with galvanised mesh sprayed with vermiculite plaster in two coats reaching 19mm in depth. This system will provide 2 hours resistance to fire although if this is done the suspended ceiling rods and other fixings would have to be placed at more frequent centres so as to provide a greater weight distribution of the ceiling. Alternatively 2 layers on their own would achieve a 1 hour rating.
The modular system does not have as good FRL levels as the flush system due to the entire system being able to demount easily. Under early fire hazard tests carried out on the infill panels the vinylcote, articote and paintcote panels all achieved 14 (14/20) for ignitability, all achieved 0 (0/10) for spread of flame, 2 (2/10) for heat evolved and 4 (4/10) for smoke developed. These figures are quite good as most plaster based products are not combustible and therefore can be used and meet the fire resistance requirements. The best possible material for use in an infill panel would be a metal based product that was backed by some sort of fire resistant insulation.
Finally the strip panel system could achieve fairly good FRL’s if it had good insulation laid both in the strips and behind the facing material. Nevertheless the F.R.L. for the strip panel ceiling would be less than the other two as the slots in between strips would not resist the fire at all.
The cost between the three systems is fairly the same, but if the ceiling was designed in mind of fire protection and sound insulation then the costs would be far greater. Also the greater number of penetrations in the ceiling such as light fittings, (which need extra support) and other things like manholes will increase the final cost of the ceiling. As a general rule the plaster based products are cheaper than selecting woodtex panels or other modular panels such as metal based ones.
Another factor to be pointed out is the initial cost of the suspended ceiling, even if it is more than other types (eg, conventional ceiling) the suspended ceiling can be altered and changed for future needs (modular & Strip panel only).
The suspended ceiling is a quick, easy and relatively cheap way of installing a ceiling in commercial/ Industrial/High rise structures. Its ability to be demounted and changed in a matter of days would suit owners of buildings as new tenants could change the ceiling to their liking. In addition to this the fire and sound properties can be increased if the area of the building is to be in a high noise or high fire risk situation. The overall aesthetics of a suspended ceiling is a reason why they are so popular.
Þ Building Construction
- Vol 2: Chapter 22, R. Matthews & R.L. Ward : Trust
Þ Rondo Building Systems
File : Suspended Ceilings, Studs Partitions, Metal Framing
Þ Profil - Vertrieb :
Fixing systems for partitions and Suspended Ceilings
Þ Pioneer `Plastamasta’
Þ Boral `Plasterboard’
Þ CSR `Gyprock’ File
Þ In-class Notes