COP:installation; secondary-fasteners

13.4 Secondary Fasteners 

Secondary fasteners are used to attach cladding sheets to one another, to transfer loads and to provide side lap sealing. They should be used in conjunction with primary fasteners to comply with the site wind design load. Because side and end laps are subject to shear loading caused by expansion and differential movement, the wind load and point load requirements will not be met unless secondary fasteners are used.

Laps on purlin spacings over 1.2 m should be stitched at mid-span with coarse thread stitching screws or self-sealing 4.8 mm aluminium blind rivets. This applies when fixing self-supporting ribbed metal profiled cladding, but not self-locking profiles.

Screw fasteners where the thread stops short of the head, ensuring that stripping out cannot happen, are available for side lap fastening.

13.4.1 Rivets 

The rivets used in fixing roof and wall cladding are 'blind' or 'pop' rivets that can only be fastened from one side. Sealed type rivets are recommended.

Zinc-plated steel rivets should not be used for roof or wall cladding in New Zealand because they do not meet the durability requirements of the NZBC. Similarly, rivets with a 3.2 mm diameter do not have the shear capacity required for fixing metal roof and wall cladding.

Rivets are subject to pull-over failure in tension because the head sizes are usually smaller than screws. It is necessary to use more rivets than screws to fasten the same joint because washers provide increased pull-over capacity for screws.

The correct drill size, 0.1 mm larger than the nominal rivet diameter, should be used when drilling metal for blind rivets.

13.4.1A Rivet Hole Size

RIVET DIAMETERDRILL SIZE
4.0 mm
4.8 mm
6.3 mm
4.1 mm
4.9 mm
6.4 mm

 

Rivets should not be spaced less than 3d or more than 20d to minimise distortion ; "d" represents the diameter of the rivet.

 

13.4.1C Minimum Strength Capacity: Blind Rivets

Minimum shear and axial tensile capacity (strength) of break mandrel blind rivets

CodeDrill
size
Nominal
diameter
Nominal
head
diameter
Minimum shear capacity
(strength)
(kN)
Minimum axial tensile capacity
(strength)
(kN)
 mmInchesmmmmAlMS/SA1MS/S
5
6
8
4.1
4.1
6.5
5/32
3/16
1/4
4.0
4.8
6.3
8.0
9.5
12.7
1.3
1.6
3.2
2.5
3.7
6.48
3.0
4.4
7.8
1.62
2.3
4.26
3.24
4.63
8.56
3.8
5.5
9.72
AI = aluminium alloy 5154, 5056 and 5754, with carbon steel mandrel.
M = nickel-copper alloy (Monel) with carbon steel mandrel.
S/S = 300 series stainless steel with carbon or stainless steel mandrel.

The values in 13.4.1C Minimum Strength Capacity: Blind Rivets should be multiplied by ø, the reduction factor = 0.65 for shear and 0.50 for tilting and bearing. 

13.4.2 Monel Rivets 

Monel Rivets have been used to fix galvanised steel flashings for many years. In theory, corrosion of the galvanised steel could be expected, because zinc and monel are far apart on the galvanic scale. This does not often happen in reality because of the natural passivation of monel in normal environments, and because the area of the rivet (the cathode) is small compared to the sheeting (the anode).
The copper content of monel is low, and being alloyed with nickel, it gives good performance in most environments. In an aggressive marine or industrial environment, a barrier coating is needed to avoid any likelihood of corrosion.
N.B. Using monel rivets with Zinc/Aluminium coating may compromise the manufacturer's warranty.

13.4.3 Aluminium Rivets 

Aluminium Rivets are compatible with zinc or AZ coated steel but have a shear performance of less than half that of monel or stainless steel rivets. They are available in open and sealed types.
Because of the high shear expansion forces developed in roof cladding and flashings, the minimum diameter of aluminium rivets used in roof or wall cladding must be 4 mm ( 5/32 ).

 

13.4.4 Stainless Steel Rivets 

Stainless Steel rivets have been used historically in a similar manner to monel, but in aggressive marine or industrial environments, a separation barrier should be used between dissimilar metals.

 

13.4.5 Spacing 

Spacing and size of rivets is related to the likely stress imposed at the connection, so as the strength of the rivet increases, the spacing may also increase.

The following maximum spacings should not be exceeded when sealing and joining gutters or cappings.

13.4.5A Rivet Spacing

DiameterSpacing
4 mm (5/32)
4.8 mm (3/16)
50 mm
60 mm
N.B. This equates to approximately 12d; "d" = diameter of the rivet

13.4.6 Fastener Frequency 

The fastener frequency for fixing roof and wall cladding is determined by the wind uplift load for the particular site and the gauge of the cladding. In low wind load areas, the frequency may be determined by a sufficient number to prevent the cladding from 'flutter' or noise associated with the relative movement of the cladding against the structure.

A uniform linear fastening pattern should be used to resist wind uplift loads; however, additional staggered fastening may be necessary to avoid noise and flutter.

Nailing and screwing patterns are shown in 3.14.4 Fastener Patterns, and the pullover values in 3.12.2 Pull-over Values.

The primary fastener frequency should not exceed 600 mm, and where this is not possible a secondary fastener should be used at closer spacings. Where fixing is not provided for primary fasteners, secondary fasteners such as stitching screws, type 17 and 4.8 mm aluminium rivets can be used, providing their number and frequency equates with the equivalent primary fastening for the wind design load.