Comprehensive New RoofDek Brochure Now Available

We have just launched a new brochure focusing on our RoofDek range of structural roof decking and trays.

The brochure is a component of “The Works”, our new design, specification and construction manual.

Well illustrated throughout, it highlights the UK’s most comprehensive selection of structural roof decking solutions.

There is a detailed overview, including diagrams and all key product data, of RoofDek shallow and deep deck profiles, liner trays and fully tested acoustic solutions.

Dedicated sections deal with sustainability issues, safety, technical information, structural, diaphragm and acoustic design considerations.

In-depth case studies show a diverse sample of major UK projects that have successfully deployed RoofDek systems. They include the UK’s largest curved green roof, two new university campuses and a combined cycle gas turbine power station.

RoofDek decks range from 32mm to 210mm, with a wide choice of structural trays. They support all types of insulated roof systems, including single ply membranes, standing seam systems, green roofs, slates and tiles, three ply felt and asphalt.

Our structural design service provides deck diaphragm design assistance and full support calculations for all profiles.

Get your copy of the RoofDek brochure now by clicking here.

A sound solution for acoustic absorption

Profiled decks present a hard surface that reflects sound back into the internal environment. This can lead to longer reverberation times. The longer the reverberation time, the more ‘echoey’ a room sounds. Reverberation time performance of a room can be calculated from the areas of walls, floors and ceiling, and their absorption coefficients. Typical requirements for schools, recommended by BB93, fall between 0.6 seconds and 1.5 seconds. 

Sound absorption is the reduction of sound energy. The sound absorption coefficient indicates the fraction of energy absorbed on striking any surface, stated as values between 0 and 1.0. If a surface absorbs no sound its coefficient of absorption is 0. If 100% of sound is absorbed the coefficient is 1.0. Absorption is frequency dependent, and is tested over a range from 125 to 5000 Hz.

Ratings for Sound Absorption classifies materials into bands, Class A to Class E. Insulation achieves Class A, the highest level of absorption, whereas plain steel, or aluminium, sheet reflects sound with no absorption and is unclassified.

To enhance acoustic absorption decks can be supplied perforated. Fully perforated this would reduce the structural strength of the deck; therefore perforation areas are limited to the side webs.

Perforating the liner sheet, allows sound to penetrate into the cavity, to be absorbed into the soft insulation, formed from web inserts, or rigid slab, at 45 Kg/M³ minimum density. Insulation to be tissue faced, to prevent fibres detaching and penetrating the internal environment. By allowing sound to escape into the cavity this does have the effect of worsening the over all reduction value. However this can be compensated for by the addition of dense acoustic membranes.

Structural Trays offer a wider flat soffit thus enabling a greater perforation area, which achieves the best acoustic absorption result.

Where perforated decks, or trays, are installed these cannot act as a vapour control check, therefore a separate vapour control membrane layer must be installed.

A range of acoustic absorption tests have been conducted on our perforated decks, and trays, at Salford University Acoustics Laboratory, with either open troughs or with the troughs in filled with preformed insulation. The test results are indicated below.

Profile	Description	aw	Class
D46 (10.7% Open Area)	With Trough Fillers	0.60	C
	Without Trough Fillers	0.55	D
D60 (13.9% Open Area)	With Trough Fillers	0.60	C
	Without Trough Fillers	0.50	D
D100 (13.3% Open Area)	With Trough Fillers	0.60	C
	Without Trough Fillers	0.55	D
D159 (7.6% Open Area)	With Trough Fillers	0.60	C
	Without Trough Fillers	0.40	D
HLT 130/600 (17.3% Open Area)		0.95	A

A Sound Approach to Achieving the Best of Both Worlds

In building envelope designs calling for high levels of acoustic performance, there is often a potential trade-off between the conflicting requirements for controlling sound reduction and sound absorption. Sound reduction refers to the control of either internal or external noise in a building.

Typical applications include concert venues, where one is trying to avoid excessive noise levels from breaking out and buildings in areas near noisy facilities, such as airports, where one strives to achieve precisely the opposite. Sound absorption is the more common acoustic feature required by designers.

It is the control of attenuation, or internal reverberation of sound against building surfaces. A school sports hall is a very typical, everyday application requiring attention. Although most projects usually require one method of acoustic control, some still need to address sound reduction and absorption in the same envelope construction.

A typical example would be a concert venue situated close to or underneath a flight path of a major airport and with nearby residential properties. Decks are typically perforated to break up sound waves and assist absorption. However the perforated open area also allows sound to escape into the roof cavity, thus lowering reduction values.

There may also be requirements of a project where longer spans are required and  perforated deck  is unsuitable.

How, then, can these conflicting requirements be achieved at a minimal cost?

The answer is to install a plain deck with a  perforated under lining sheet, as indicated:-

Click to enlarge

Installation of a D159 plain roof deck provides maximum spanning capability, whilst the addition of RL32/1000 perforated liner enhances absorption values with a 22% open area, and provides a more aesthetically pleasing finish. Sound is absorbed into the trough infill fillets, as for a perforated deck profile. This liner may be installed directly below the deck, or positioned under the beams to hide all support structure.

Comparison of ex works prices, based on 1000m2, indicate that there is no additional cost increase, for profiled elements, by offering an underlining system.

There would be costs for supporting zed spacers, additional fasteners and installation time. However the combined benefits of enhanced acoustic absorption, reduction and aesthetics will more than justify the modest additional expense.

Diaphragm design and construction in Green Roofs

In our previous green roof focus, we noted that the growing popularity of a hitherto “alternative” solution has not been at the expense of the advantages of more established options.

Green roofs are heavy in comparison to almost every other roofing method. However, if they are used in conjunction with structural decks and trays, they can still provide engineers and building owners with the economies and aesthetic enhancements that can be gained from diaphragm, or stressed skin, construction.

Diaphragm action essentially concerns the judicious selection of a structural deck or tray and designing the roof layout such that horizontal wind load forces are transferred into the deck and the structure.

This complex technique, usually accomplished with the use of the Corus Roof Decking Software package, can replace cross bracing and reduce secondary steelwork.

Not only does this method save money, it also provides a cleaner internal building appearance that is commonly considered to be better looking than standard techniques.

As if all this were not enough justification, there are also no real additional costs associated with the diaphragm/stressed skin method, save for the use of relatively low-cost primary fasteners around the building perimeter.

So with all these very attractive benefits to be gained, why on earth doesn’t every project go down this route? Well, as always, the term “horses for courses” applies.

Straightforward, square buildings are the easiest to design in this way. It gets more problematic, however, when rectangular , long, thin structures are necessary, for reasons of location and/or function. It is also essential to have at least three braced walls.

Other essential roof requirements, such as rooflights and other roof penetrations, can also quickly count against the viability of the diaphragm option. Stressed skin construction only allows for 3% of the total roof area to be “open”. If we take the example of a building that requires considerable natural roof lighting, this factor alone is going to eliminate the option.

Armed with your drawings showing your proposed deck layout and braced walls and line loads at diaphragm edges, our technical support team is able to quickly advise you as to exactly what is possible on your own project, via email or by telephone on 0845 30 88 330

Green Roofs – Acoustic Considerations

As green roofs become an increasingly popular solution, their image as an “alternative” method of construction has not completely disappeared. One might therefore consider that caveats lie in wait and that they do not offer all the common benefits of more established options.

In practice, however, nothing could be further from the truth.

And nowhere is this more apparent than in the area of acoustics where green roofs, supported by Corus decks, offer a completely comprehensive solution, with additional advantages of their own thrown in for good measure.

Although acoustics has moved more into the mainstream of desirable building features in recent years, it is still something of a “grey art”. We often talk to designers who are confused between the sometimes conflicting requirements of sound reduction and sound absorption.

Sound reduction refers to the control of either internal or external noise in a building. Typical applications include concert venues, where one is trying to avoid excessive noise levels from breaking out and buildings in areas near noisy facilities, such as airports, where one strives to achieve precisely the opposite.

Sound absorption is the more common acoustic feature required by designers. It is the control of attenuation, or internal reverberation of sound against building surfaces. A school sports hall is a very typical, everyday application requiring attention.

Green Roofs installed over Corus decks and trays can be fine-tuned to achieve sound reduction and/or sound absorption. They actually offer two significant advantages over other types of roof construction: -

• Green roofs contain soil and sedum which gives them good mass. This is of critical importance in a situation where sound reduction is required, as they provide a natural sound barrier that can be further tuned by careful attention to detail with the underlying construction.
• Reduction of the drumming noise created by rainfall on the external roof surface is an important acoustic consideration in many buildings. Green roofs offer a naturally soft outer surface that provides a “built-in” solution to an age old problem.

Choice of deck or tray plays an instrumental role in the success of a properly executed green acoustic roof project.

We offer a full range of perforated decks and trays, for applications requiring sound absorption. The perforations help to break up sound as it hits the underside of the roof construction, thereby reducing reflectivity.

Dense mineral fibre is the general insulant of choice for acoustic applications. It is therefore common for a tissue or foil layer, to prevent fibres from falling through the perforations, to be inserted in the pans of decks or trays, or directly above the deck.

The “open area” – the degree of perforation of the deck surface – is critical to the success of sound absorption. We can provide decks and trays with open areas between 5% and 30%. In an ideal world, everyone would specify the 30% maximum. However, as the open area increases, so the structural performance of the deck or tray in question reduces. This has implications for the amount of bracing and secondary steel that will be required. Therefore compromises often have to be made, either in favour of better sound absorption levels and the consequent increase in steelwork, or vice versa. Cost is clearly a key driver here.

Moreover, if a project requires both sound absorption and reduction, the bigger the open area, the more sound breaks into the roof construction, increasing noise break in and/or break out levels.

As mentioned earlier, the superior mass of the green roof construction helps to reduce the deployment of other sound reduction “counter measures”.

Bespoke mineral fibre acoustic infills that fit flush into the deck/tray pan profile are the most beneficial option, because they act as a direct barrier to sound entering the construction.

Flexible, high density polymer mass layers with exceptional sound reduction properties are also commonly used in conjunction with insulation, particularly when very high levels of control are required, especially where very low frequencies need attenuation.

We hope that this blog entry has given you some general food for thought when considering acoustic performance in green roofs. Please contact our technical support team if you require any specific advice on 0845 30 88 330.