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.

Fasteners are a vital component to ensure the safe installation of structural decks or trays. The correct fixing, to be specified by the fastener manufacturer, to suit the application of fixing to cold rolled thin gauge purlins, hot rolled steel sections or timber supports. Self drilling / self tapping screws (commonly known as drill screws), must be a minimum 5.5 mm diameter, available in carbon steel or austenitic stainless steel, and must resist all pullout, pullover and shear forces.

The load acting on a screw can be calculated thus:-

Screw Load (kN) = Wind Load (kN/m2) x Support Centers (m) x Screw Centers (m) x Factor of Safety

Example:- To find the fastener load for D60 deck spanning 3000 mm with an applied wind load of 1.5 kN/m2
Screw Load = 1.5 x 3 x 0.2 x 2 = 1.8 kN

Fastener manufacturers can confirm:-
Pullout, pullover and shear values, to suit gauge of deck / tray, type and thickness of support structure, and provide information on the correct torque setting for screw guns, to ensure that the screw is not overdriven or underdriven.

For further information please do not hesitate to get in touch with our Technical Department at Tata Steel, E mail: roofdek@tatasteel.com

The correct orientation for deck installation is for the narrow trough to be down onto the support structure. This ensures that the widest crown is uppermost, providing maximum support for insulation. Direction of lay can be left to right or right to left, ensuring that the correct engagement of side lap is maintained.

A running check should be taken during installation to ensure that the closing dimension ‘X’ is maintained. A tolerance build up due to discrepancies in steelwork and deck cover width manufacturing tolerance can occur.

To determine setting out dimension ‘X’ (Example based on D100 over 20000 mm roof width)

Where sheets are cut down to make up a finishing section, the unsupported edge requires to be supported by a zed spacer.

Alternatively the closing width can be adjusted by overlapping the deck by the profile pitch:-

Based on the previous example, if the number of sheets is increased to 29 with a single one pitch overlap at each end, the setting out dimension will be:-

To determine setting out dimension ‘X’ (Example based on 20000 mm roof width)

To produce a make up section, trays may be cut down their length, overlapped on the micro ribbed pan section, and stitched together at 450 mm centers. In this manner the standard 600 mm cover width can be adjusted to 429 mm or 257 mm, as indicated below.

For further information please do not hesitate to get in touch with our Technical Department at Tata Steel, E mail: roofdek@tatasteel.com

For pitched roof slopes where the support structure is set horizontal this will result in a point bearing contact. This must be avoided by the addition of a steel, or timber fillet, as indicated below, to ensure that full bearing support of 75 mm minimum is maintained.

For relatively thin timber packers, fasteners may be fixed through the deck and fillet into the support flange.

Deeper filet packers may be fixed independently to the support flange, with the deck then fixed solely to the timber. However, ensure that when attaching the timber the fastener has a countersunk head or is located within the open web. Care must be taken to ensure that the deck fasteners do not clash with any timber fasteners.

Fasteners into timber must be austenitic stainless steel, with an embedment of 40 mm minimum.

For further information please do not hesitate to get in touch with our Technical Department at Tata Steel, E mail: roofdek@tatasteel.com

Penetrations through decks affect the structural integrity, and therefore must be suitably braced, dependant on opening size.

1 – Small penetrations, through the bottom trough, can be accommodated without any additional support. Multiple penetrations must be spaced at 1500 mm minimum centres. Note, no part of the web, or crown, can be removed without additional support.

2 – Medium penetrations, up to one profile pitch, require a stiffening plate. The stiffening plate is generally installed above the deck, however for retro fitting of penetrations, where the roof build is installed, the stiffening plate can be installed under the deck.

Stiffening plate length must span over one full web each side of the penetration, with the plate width twice penetration dimension, ensuring that the cutout is central to the plate. D159 and deeper decks require a plate thickness of 3 mm, all other decks to be 2 mm. Stiffening plate requires fixing with Tec screws, or rivets, to fastener manufacturers recommendations.

3 – Larger penetrations, greater than one profile pitch, require full structural support. Deeper decks are capable of wider spans. A full structural frame support below the deck can be visually obtrusive. An alternative proposal would be to install a structural frame above the deck, ensuring that the support frame legs extend over two full webs either side. This however adds to cold bridgeing through the build construction, which must be considered.

Penetrations can be sealed with Dektite® closures, suitable for small diameter cable penetrations or large pipework up to 440 mm diameter.

Box penetrations up to 255 mm square can also be accommodated. Larger square or rectangular penetrations may be sealed with Dekstrip® expandable-edge flashing.

Dektite® closures are available in Grey EPDM or Red Silicone to suit operating temperatures:-

• EPDM will withstand constant temperatures at the roofline of -50°C to 115°C, and up to 150°C intermittently.
• Red Silicone will withstand constant temperatures at the roofline of -50°C to 200°C, and up to 250°C intermittently.

Dektite® Images courtesy of Deks

For further information please do not hesitate to get in touch with our Technical Department at Tata Steel
E mail: roofdek@tatasteel.com

Decks can be manufactured in lengths up to 12 metres. In general terms decks are more structurally efficient when in double span condition. There are however instances where short single span sheets are required:-

• Due to site limitation for access
• For spans over 6000 mm

Extended end laps can be used to create a double span condition whilst keeping the deck length to a minimum.

The deck must be fixed in every trough to support structure, with stitching screws to every web at each lap end.

The D159 deck indicated below requires 12 stitching screws in the webs at the lap ends.

Extended laps may be utilised for perforated decks, provided it is made clear in the fixing specification that the fixings go through plain material. However the extended overlap would block the perforations, wholly or partially, which will reduce the acoustic absorption.

For further information please do not hesitate to get in touch with our Technical Department at Tata Steel, E mail: roofdek@tatasteel.com

*** Tata Steel offer a free diaphragm design service when using RoofDek profiles – for more information, please call +44 (0)845 30 88 330 or email roofdek@tatasteel.com ***

Metal Decks installed in diaphragm, or stressed skin, design allows the horizontal side wind force to be transmitted through the deck into the support frame structure. This allows the designer to remove certain cross bracing, improving the internal aesthetic view of the roof.

Any diaphragm area must have a minimum of three braced walls. Where decks are required to resist diaphragm loads, in addition to standard fixing requirements the deck must be fixed around the full perimeter, as indicated in sketches below.

Deck directly fixed to main steelwork

Fixing to parallel members is not too difficult because these can be at the same level as the deck support beams.

Deck to support fastener positions (red arrows)

•   Each arrow may signify more than one fixing.
• Fastener specification to be determined by fastener manufacturer to suit support structure.
• Shot fired pins determined by manufacturer subject to minimum flange thickness (not suitable for timber supports)

Deck to parallel beam fastener positions (green arrows)

• Perimeter screw centers will be determined from diaphragm design calculations.

Side lap fastener (blue arrows)

• Stitching centers will be determined from diaphragm design calculations.
• Stitching fastener specification to be determined by fastener manufacturer.
• Alternative rivet specification to be determined by fastener manufacturer.

Deck fixed to purlins

When the deck is on purlins, shear connector brackets are needed to enable the connection to parallel rafters.

Deck to support fastener positions (red arrows)

• Each arrow may signify more than one fixing.
• Fastener specification to be determined by fastener manufacturer to suit support structure.
• Shot fired pins determined by manufacturer subject to minimum flange thickness (not suitable for timber supports)

Deck to parallel beam fastener positions (green arrows)

• Perimeter screw centers will be determined from diaphragm design calculations.

Side lap fasteners (blue arrows)

• Stitching centers will be determined from diaphragm design calculations.
• Stitching fastener specification to be determined by fastener manufacturer.
• Alternative rivet specification to be determined by fastener manufacturer.

Diaphragm Design

Tata Steel can provide diaphragm design calculations, carried out in accordance with BS 5950: Part 9: 1994 Structural use of steelwork in building.  Part 9 Code of practice for stressed skin design are subject to the limitations and conditions specified therein. Prior to any diaphragm calculation the deck must be checked for normal loads to include:- live, dead, wind suction and pressure, snow drift loads, all stated as kN/m2.
Information for diaphragm calculations comprise:-

• A dimensioned drawing indicating main support, or purlin, positions.
• An indication where braced walls are positioned, a minimum of three braced walls are required.
• The wind load acting at the perimeter, stated as a line load in kN/m.

Responsibility

BS 5950 Part 9. 1.0.2 Overall stability.

The designer responsible for the overall stability of the structure should ensure the compatibility of design and details of parts and components, including stressed skin shear diaphragms. There should be no doubt as to where the responsibility for overall stability lies when some or all of the design and details are not made by the same designer.

The provider of the diaphragm calculations is not responsible for the overall stability of the structure. Responsibility for overall stability lies with the Structural Engineer responsible for the project.

The building structure design must consider how the load will be transferred between the walls and the roof deck and how the load will transfer to foundations though braced wall elements. It is essential that any diaphragm area be surrounded by at least three braced walls, that the deck is fixed down at all four edges of the diaphragm area and that edge members are continuous. Where there is one unbraced and three braced walls supporting the diaphragm area, this gives rise to a cantilever diaphragm.
Openings

Openings in decks must be trimmed or fully supported according to Tata Steel deck penetration rules. Openings totaling more than 3% of the area in each panel should not be permitted unless they conform to BS 5950: Part 9, clause 8.3. Openings of less than 3% of the area in each panel may be permitted without special calculation provided the total number of fasteners in each panel is not reduced.

Diaphragm deflection

An overall deflection is given, however no criteria are applied as to whether this is satisfactory or not. It is up to the building designer to define an acceptable level of deflection of the diaphragm.

Pitched roof with a ridge

A full structural connection must be made between the deck sheets on each side of a ridge. Where the deck sheets are perpendicular to the ridge, a suitable connection would be a 1.6 mm galvanized steel flashing securely fixed to both sheets fixed at the frequency specified for fixings at the supports but not less than every trough of the deck.

The fixings should pass through the deck into the 1.6 mm steel flashing. Where the deck sheets are parallel, a suitable connection would be a 1.2 mm galvanised steel flashing securely fixed to both sheets fixed at the frequency specified for seam fixings.

Roofs of irregular shape

For convenience, the software assumes that a building is rectangular with one diaphragm area. The real building might consist of more than one diaphragm area as in the case shown below. In this case the calculations must be run for each diaphragm area separately. When a roof which is required to act as a diaphragm has an irregular plan form and / or parts of the roof at different levels it is necessary to divide the roof into zones so that each part of the building is stabilised by a rectangular diaphragm zone to resist horizontal forces applied in both the longitudinal and transverse directions.

In the design calculations each zone is then treated as a separate diaphragm. The diagram below shows a typical roof plan of such a building in which the roof is divided into zones A, B, C, and D. Zones A and C act as conventional diaphragms spanning between vertical bracings to resist both North / South and East / West loads.

Zone D resists North / South loads only and acts as a cantilever off zone A. Zone B contains a relatively high proportion of roof lights but is in fact surplus to requirements and need not be designed as a diaphragm.

For further information please do not hesitate to get in touch with our Technical Department at Tata Steel, E mail: roofdek@tatasteel.com

The increasing complexity of roof decking projects makes any new software aimed at easing the process very welcome indeed. Earlier this year Tata Steel introduced RoofDek Analysis Software, allowing structural engineers and specifiers to conduct full deck analysis to Eurocodes 3 and 9. The company has now launched a new online tool to further improve its service.

The Tata Steel RoofDek Quick Selector is a web based structural analysis tool. It allows a user to rapidly select the right structural roof deck for a project on a PC or tablet, including iPad.

“The program does two things simultaneously. It provides an optimised RoofDek specification, and it also allows the user to choose an alternative RoofDek profile,” explained Adrian Wallwork, Tata Steel’s Product Manager – Structural Decking Products.

“The Selector page opens with pre-populated span and load figures as an example. Users can then modify these to their own requirements to find the right decking profile for a given project.

“A suitable decking profile for both single and double span applications is then displayed.

“The pre-filled information is suitable for a general roof deck application, but of course may not be suitable for any particular project.

“The user can therefore edit all the input data to suits the project; a steel or aluminium deck, perforated or not perforated and the support type. Deflection limits under imposed and wind load can be set separately.

“Loads can then be input. Unfactored loads are required, as the program applies load factors to them. Separate provision is made for live load, imposed dead, wind suction, wind pressure, general snow, snow drift and construction line load.

“Each time new information is added, it is evaluated and the deck selection may change accordingly. The user can also try different decks from the dropdowns at any point if required.

“The whole exercise is carried out on one screen and the page may be printed out as record of the design. Calculations are carried in accordance with Eurocode 3.”

The RoofDek Quick Selector augments Tata Steel’s suite of roof decking software, which includes RoofDek Analysis Software.

Please click here to find out more about RoofDek Quick Selector.

Fasteners are fixed from above the deck into suitable support structure. It is not possible to install fasteners from below the deck. This unfortunately does leave the drill point and end of the fastener visible from below. Any attempt to remove the protruding end could result in damage to the deck coating but more importantly could cause elongation of the hole, which then will seriously impair the stability and fastener performance.

Removal of the drill point protrusion is only permissible when the support structure steel flange is 6 mm or thicker. Therefore it is not possible to remove the protruding end of stitching screws through the deck side lap. Stitching screws, or rivets, for the side lap could be installed from the underside of the deck. However this has access problems, leading to longer installation times, which will increase costs.

Where decks are installed above thin gauge purlins approximately 6 mm threaded shank should protrude to ensure correct location. Fastener manufacturers can confirm minimum requirements to suit their product.

For correct specification of fasteners consult supplying manufacturer. To minimise the visual effect, colour coated caps, to match the deck colour, can be inserted over the protruding ends.

Should the visibility of fasteners be an issue, this should be determined at the initial design concept stage by designing supporting steelwork of square or rectangular hollow sections which will ensure fastener non visibility.

For further information please do not hesitate to get in touch with our Technical Department at Tata Steel, E mail: roofdek@tatasteel.com

End laps for decks or trays  must be positioned above suitable support structure.
Decks installed above purlins require sheet ends to overlap 100 mm minimum, ensuring that the fasteners are positioned 50 mm from the sheet edge, as indicated in Fig 1:

Where decks are installed above main structure providing wider support, end laps may be butted, as indicated in Fig.2. Fasteners positioned 50 mm from sheet edge and 25 mm from support flange edge provide a minimum end bearing of 75 mm. Therefore the minimum support flange must be 150 mm.

Trays will not overlap and therefore are butted over a suitable support, ensuring 75 mm minimum bearing. Butted laps, as Fig. 2 require a minimum support flange of 150 mm.

However, there are manufacture tolerance on length, which should be considered:-
· Up to 3000 mm = + 10 mm / – 5 mm
· Over 3000 mm = + 20 mm / – 5 mm

Therefore based on the upper tolerance, a minimum support flange of 170 mm is required as indicated in Fig 3.
Butted laps are difficult to seal, therefore a separate vapour control membrane should be specified.

Extended end laps can be used to create a double span condition whilst keeping the deck length to a minimum. For further information see How To Create Double Span Condition From Short Sheets.

Overlapping end laps can easily be sealed if the deck is required to act as a vapour control / air permeability layer.

However where decks are butt jointed, especially where allowance is made for manufacturing tolerances, the deck cannot be adequately sealed, and therefore a separate vapour control membrane is required.

For further information please do not hesitate to get in touch with our Technical Department at Tata Steel, E mail: roofdek@tatasteel.com

Malcolm Chapman, Technical Support Engineer at Tata Steel, discusses the installation of solar panels to structural roof decking and trays. This guide uses the SOL-F system from SFS intec as an example.

SOL-F overview

Secure Anchor Posts For Fastening Solar PV Panels Directly to roof decking applications with Single Ply Membranes.

SOL-F system comprising:- distance profile, upper and lower clamping flange, fasteners and optional top flange plate. (PV panels and rail system supplied by others).

• A coring tool is available with depth stop for quick and accurate installation preparation.
• A calculation tool is available for easy structural engineering calculations of the necessary fastening points.
• Suitable for use with new build, or retrofit, on Tata Steel RoofDek products.
• Suitable for Tata Steel structural liner tray products, installed to timber or ply support.

Installation sequence

Special Requirement for RoofDek D35

For further information please do not hesitate to get in touch with our Technical Department at Tata Steel, E mail: roofdek@tatasteel.com or SFS Intec, E mail: Uk.leeds@sfsintec.biz