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Deck Fastener Installation

24 January 2012 No Comment

Fasteners are a vital component to ensure the safe installation of decking. The correct fixing as specified by Tata Steel RoofDek should be used, or the specification determined 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 are available in carbon steel or austenitic stainless steel, and must resist all pullout, pullover and shear forces. For compliance with non fragility requirements, fasteners to be positioned 50 mm minimum in from deck sheet end.

Alternatively:-
1 – Shot fired pins may be installed into hot rolled steel sections, subject to minimum flange
requirements, as specified by fastener manufacturer.

2 – Side lap stitching screws may be replaced with rivets.

When designing to Eurocode 3 for steel decks, or Eurocode 9 for aluminium decks, a fixing adequacy check is required.

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

Fastener manufacturers can confirm pullout, pullover and shear values for their products, and provide information on the correct torque setting for screw guns, to ensure that the screw is not overdriven or underdriven.

Decks are fixed to the support structure from above the deck, which does leave the drill point, or fastener end protruding below the deck. Where required, to minimize the visual impact, fastener manufacturers can supply colour coated caps to cover the screw end. Any attempt to cut off 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 screw performance. Removal of the drill point protrusion is only permissible when the support structure steel flange is 10 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.

The table below indicates generic recommendations, which must be checked against specific project requirements. Especially for high wind suction loads, which may require more than one screw per trough, and side lap centres when the deck acts in diaphragm.

Profiles Primary Fasteners (End Support)  Primary Fasteners (Intermediate Support) Secondary Side Lap Fastners(*)
D32 Every Trough Every Alternate Trough Not Essential
D35 Every Trough Every Trough Not Essential
D46 Every Trough Every Trough 450 mm Centres
D60 Every Trough Every Trough 450 mm Centres
D100 Every Trough Every Trough 450 mm Centres
D135 Every Trough Every Trough 450 mm Centres
D159 Every Trough Every Trough 450 mm Centres
D200 Every Trough Every Trough 450 mm Centres
D210 (**) Every Trough Every Trough 350 mm Centres

(*)   Fixing centres for side laps may be required to be reduced, subject to diaphragm design.
(**)  D210 requires alignment cleats to prevent profile spread.

Where the deck is required to provide a vapour barrier, screws must be supplied with a sealing washer. If a separate vapour control membrane is installed above the deck, screws can be installed without washers.
Self drilling screws are primarily used, suitable up to specific support flange thickness specified by fastener manufacturers. Where flange thickness is greater, self tapping screws are required, installed into a pre drilled pilot hole.

Aluminium decking

The fastener frequency is the same as for steel decks, however to prevent bi-metallic corrosion, austenitic stainless steel screws must be installed. A simple site test to determine the grade of screw is to check with a magnet, as stainless steel screws are non magnetic.

Tubular support structure

Decks can be installed above tubular structure, however this requires special consideration.
1 – The bearing support will be minimal, thus increasing the bending and reaction moment over
the support.
2 – Location of screws is more difficult, any deviation from the apex position could cause the
screw to twist out of alignment.
3 – The sheet end will be visible, any deviation in panel length tolerance may indicate staggered
sheet ends. Manufacturing tolerances on length allow + 10 mm / – 5 mm up to 3000 mm, and
+  20 mm / – 5 mm for lengths over 3000 mm.

Therefore for tubular structure it is recommended that a flat plate is added to provide a normal 100 mm minimum flat bearing support.

Timber support structure

Screw installation into timber taken from BS 5268:Part 2: 2000

Timber may be coated with preservatives that could have an adverse effect on carbon steel screws; therefore it is recommended that austenitic stainless steel screws are specified for installation into timber, ensuring a minimum embedment and minimum edge distances as indicated below.

The screw diameter may be taken as the screw root diameter.

Diaphragm Installation
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. Perimeter screw centers and side lap stitching centers will be determined from diaphragm design calculations.

Deck directly fixed to main steelwork
Fixing to parallel members is not too difficult because these are at the same level as the deck support beams.

Deck to support fastener positions (red arrows). Each arrow may signify more than one fixing.
Deck to parallel beam fastener positions (green arrows).
Side lap fastener (blue arrows).

Deck fixed to purlins
When the deck is on purlins, shear connectors are needed to enable the connection to parallel rafters.

Deck to support fastener positions (red arrows). Each arrow may signify more than one fixing.
Deck to parallel beam fastener positions (green arrows).
Side lap fasteners (blue arrows).
Further information is available from fastener manufacturers, or Tata Steel structural technical department on roofdek@tatasteel.com

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